Article

Large historical growth in global terrestrial gross primary production

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Abstract

Growth in terrestrial gross primary production (GPP)—the amount of carbon dioxide that is ‘fixed’ into organic material through the photosynthesis of land plants—may provide a negative feedback for climate change1, 2. It remains uncertain, however, to what extent biogeochemical processes can suppress global GPP growth3. As a consequence, modelling estimates of terrestrial carbon storage, and of feedbacks between the carbon cycle and climate, remain poorly constrained4. Here we present a global, measurement-based estimate of GPP growth during the twentieth century that is based on long-term atmospheric carbonyl sulfide (COS) records, derived from ice-core, firn and ambient air samples5. We interpret these records using a model that simulates changes in COS concentration according to changes in its sources and sinks—including a large sink that is related to GPP. We find that the observation-based COS record is most consistent with simulations of climate and the carbon cycle that assume large GPP growth during the twentieth century (31% ± 5% growth; mean ± 95% confidence interval). Although this COS analysis does not directly constrain models of future GPP growth, it does provide a global-scale benchmark for historical carbon-cycle simulations.

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... Yet, robust tools for investigating these processes at a large scale are scarce (2). Recent studies suggest that carbonyl sulfide (COS) could provide an improved constraint on terrestrial photosynthesis (gross primary production, GPP) (2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12). COS is the major long-lived sulfurbearing gas in the atmosphere and the main supplier of sulfur to the stratospheric sulfate aerosol layer (13), which exerts a cooling effect on the Earth's surface and regulates stratospheric ozone chemistry (14). ...
... Contrary to CO 2 , COS undergoes rapid and irreversible hydrolysis mainly by the enzyme carbonic-anhydrase (6,7). Thus, COS can be used as a proxy for the one-way flux of CO 2 removal from the atmosphere by terrestrial photosynthesis (2,(8)(9)(10)(11). However, the large uncertainties in estimating the COS sources weaken this approach (10)(11)(12)15). ...
... Thus, COS can be used as a proxy for the one-way flux of CO 2 removal from the atmosphere by terrestrial photosynthesis (2,(8)(9)(10)(11). However, the large uncertainties in estimating the COS sources weaken this approach (10)(11)(12)15). Tropospheric COS has two main sources: the oceans and anthropogenic emissions, and one main sink-terrestrial plant uptake (8,(10)(11)(12)(13). ...
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Robust estimates for the rates and trends in terrestrial gross primary production (GPP; plant CO 2 uptake) are needed. Carbonyl sulfide (COS) is the major long-lived sulfur-bearing gas in the atmosphere and a promising proxy for GPP. Large uncertainties in estimating the relative magnitude of the COS sources and sinks limit this approach. Sulfur isotope measurements ( ³⁴ S/ ³² S; δ ³⁴ S) have been suggested as a useful tool to constrain COS sources. Yet such measurements are currently scarce for the atmosphere and absent for the marine source and the plant sink, which are two main fluxes. Here we present sulfur isotopes measurements of marine and atmospheric COS, and of plant-uptake fractionation experiments. These measurements resulted in a complete data-based tropospheric COS isotopic mass balance, which allows improved partition of the sources. We found an isotopic (δ ³⁴ S ± SE) value of 13.9 ± 0.1‰ for the troposphere, with an isotopic seasonal cycle driven by plant uptake. This seasonality agrees with a fractionation of −1.9 ± 0.3‰ which we measured in plant-chamber experiments. Air samples with strong anthropogenic influence indicated an anthropogenic COS isotopic value of 8 ± 1‰. Samples of seawater-equilibrated-air indicate that the marine COS source has an isotopic value of 14.7 ± 1‰. Using our data-based mass balance, we constrained the relative contribution of the two main tropospheric COS sources resulting in 40 ± 17% for the anthropogenic source and 60 ± 20% for the oceanic source. This constraint is important for a better understanding of the global COS budget and its improved use for GPP determination.
... It has been suggested that a CO 2 -induced long-term increase in global photosynthesis, a process known as CO 2 fertilization, is responsible for a large proportion of the current terrestrial carbon sink [4][5][6][7] . The estimated magnitude of the historic increase in photosynthesis as result of increasing atmospheric CO 2 concentrations, however, differs by an order of magnitude between long-term proxies and terrestrial biosphere models [7][8][9][10][11][12][13] . Here we quantify the historic effect of CO 2 on global photosynthesis by identifying an emergent constraint [14][15][16] that combines terrestrial biosphere models with global carbon budget estimates. ...
... Global photosynthesis cannot be observed directly, however, and must instead be either predicted by terrestrial biosphere models (TBMs) or inferred from proxies 18 . The multiple long-term proxies from which changes in global photosynthesis are derived include satellite-based estimates 8,9 , ice-core records of carbonyl sulfide 13 and herbarium samples of deuterium isotopomers 12 , along with information gleaned from the seasonal cycle of atmospheric CO 2 11 . Despite the importance of photosynthesis, however, and the multiple proxies that exist, there is no consensus regarding the expected historic global change due to increasing CO 2 levels [7][8][9][10][11][12][13] . ...
... The multiple long-term proxies from which changes in global photosynthesis are derived include satellite-based estimates 8,9 , ice-core records of carbonyl sulfide 13 and herbarium samples of deuterium isotopomers 12 , along with information gleaned from the seasonal cycle of atmospheric CO 2 11 . Despite the importance of photosynthesis, however, and the multiple proxies that exist, there is no consensus regarding the expected historic global change due to increasing CO 2 levels [7][8][9][10][11][12][13] . ...
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The global terrestrial carbon sink is increasing1,2,3, offsetting roughly a third of anthropogenic CO2 released into the atmosphere each decade¹, and thus serving to slow⁴ the growth of atmospheric CO2. It has been suggested that a CO2-induced long-term increase in global photosynthesis, a process known as CO2 fertilization, is responsible for a large proportion of the current terrestrial carbon sink4,5,6,7. The estimated magnitude of the historic increase in photosynthesis as result of increasing atmospheric CO2 concentrations, however, differs by an order of magnitude between long-term proxies and terrestrial biosphere models7,8,9,10,11,12,13. Here we quantify the historic effect of CO2 on global photosynthesis by identifying an emergent constraint14,15,16 that combines terrestrial biosphere models with global carbon budget estimates. Our analysis suggests that CO2 fertilization increased global annual photosynthesis by 11.85 ± 1.4%, or 13.98 ± 1.63 petagrams carbon (mean ± 95% confidence interval) between 1981 and 2020. Our results help resolve conflicting estimates of the historic sensitivity of global photosynthesis to CO2, and highlight the large impact anthropogenic emissions have had on ecosystems worldwide.
... 9-12 Tropospheric COS has two main sources -the oceans and anthropogenic emissions, and one main sink -terrestrial plant uptake. 6,9,10,13 Smaller sources include biomass burning, soil emissions, wetlands, volcanoes, and smaller sinks include OH destruction, stratospheric destruction, and soil uptake. 11 ...
... 31,32 Thus our initial assessment for seawater-emitted COS isotopic signal, may not be representative of the global oceans. Based on all of the noted above, we created a measurement-based isotopic mass balance for tropospheric COS (Eq. 2 and on Monte-Carlo simulations, was suggested by Campbell et al. 10 These optimized values lead to a relative contribution of 30±5% and 70±16% for the anthropogenic and oceanic sources respectively, which agrees with our independent estimate. ...
... From Eq. 2 and Eq. 3 above, one can derive Eq. 4.Using Eq. 4 and with our measured values, we calculated that the relative contribution of the anthropogenic source to the atmosphere is 26±11% and that of the oceanic source is 74±23%. Previous studies demonstrated large uncertainties in the global COS budget with up to fourfold on the anthropogenic source and up to 20-fold on the oceanic source.10,[16][17][18][19]21 However, optimized values of 830±150 GgS/Yr for the oceanic source and 350±40 GgS/Yr for the industrial source, based ...
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Carbonyl sulfide (COS) is the major long-lived sulfur bearing gas in the atmosphere and a promising proxy for terrestrial gross primary production (GPP; CO 2 uptake). However, large uncertainties in estimating the relative magnitude of the COS sources and sinks limit this approach. Isotopic measurements have been suggested as a novel tool to constrain COS sources, yet such measurements are currently scarce. Here we present, for the first time, a complete data-based tropospheric COS isotopic mass balance, which allows improved partition of the sources. We found an isotopic (δ ³⁴ S±SE) value of 13.9±0.1‰ (versus V-CDT standard) for the troposphere, with an isotopic seasonal cycle driven by plant uptake. This seasonality agrees with a fractionation of -1.9±0.3‰ which we measured in plant-chamber experiments. Anthropogenic-influenced air samples indicated an anthropogenic COS isotopic signal of 8±1‰. Samples of seawater-equilibrated-air indicate that marine COS emissions have an isotopic signal of 13±0.4‰. Using our new data-based mass balance, we constrained the relative contribution of the two main tropospheric COS sources resulting in 26±11% for the anthropogenic source and 74±23% for the oceanic source. This constraint is important for a better understanding of the global COS budget and its improved use for GPP determination.
... Thirdly, we evaluated the response of gross primary productivity (GPP) to elevated CO 2 to assess the response of plant productivity to changing resource availability (i.e., CO 2 ) and historical perturbation C fluxes. For this, we used observationbased estimates (Ehlers et al., 2015;Campbell et al., 2017). Fourthly, we evaluated large-scale patterns of vegetation and soil N : P ratios as well as the N and P openness and turnover rates on the ecosystem level to assess spatial variation in nutrient limitation and the underlying drivers. ...
... We compare the simulated response of plant productivity to increasing CO 2 during the historical period (i.e., CO 2 fertilization effect Eco 2 ) with observation-based estimates for C 3 plants from the historical change in deuterium isotopomers in leaf herbarium samples (Ehlers et al., 2015). For global (C 3 and C 4 ) vegetation we compare to indirect evidence from carbonyl sulfide (COS) atmospheric ice-core observations (Campbell et al., 2017). The CO 2 fertilization effect is defined here by the GPP ratio (E CO 2 ): ...
... where GPP 296 indicates pre-industrial GPP (g C m −2 yr −1 ) under a CO 2 concentration of 296 ppm and GPP 396 under a current CO 2 concentration of 396 ppm. Those CO 2 concentrations of 296 and 396 ppm correspond to tropospheric mixing ratios of CO 2 in the years ∼ 1900 and 2013, respectively, similar to values used for estimating the response of GPP to a ∼ 100 ppm CO 2 increase in Ehlers et al. (2015) and Campbell et al. (2017). Modeled E CO 2 by ORCHIDEE-CNP of natural biomes ranges between 1.0 and 1.3 for most regions (Fig. 5a), slightly lower than global E CO 2 derived from COS of 1.26-1.36 ...
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The availability of phosphorus (P) and nitrogen (N) constrains the ability of ecosystems to use resources such as light, water and carbon. In turn, nutrients impact the distribution of productivity, ecosystem carbon turnovers and their net exchange of CO2 with the atmosphere in response to variation of environmental conditions in both space and time. In this study, we evaluated the performance of the global version of the land surface model ORCHIDEE-CNP (v1.2), which explicitly simulates N and P biogeochemistry in terrestrial ecosystems coupled with carbon, water and energy transfers. We used data from remote sensing, ground-based measurement networks and ecological databases. Components of the N and P cycle at different levels of aggregation (from local to global) are in good agreement with data-driven estimates. When integrated for the period 1850 to 2017 forced with variable climate, rising CO2 and land use change, we show that ORCHIDEE-CNP underestimates the land carbon sink in the Northern Hemisphere (NH) during recent decades despite an a priori realistic gross primary productivity (GPP) response to rising CO2. This result suggests either that processes other than CO2 fertilization, which are omitted in ORCHIDEE-CNP such as changes in biomass turnover, are predominant drivers of the northern land sink and/or that the model parameterizations produce emerging nutrient limitations on biomass growth that are too strict in northern areas. In line with the latter, we identified biases in the simulated large-scale patterns of leaf and soil stoichiometry as well as plant P use efficiency, pointing towards P limitations that are too severe towards the poles. Based on our analysis of ecosystem resource use efficiencies and nutrient cycling, we propose ways to address the model biases by giving priority to better representing processes of soil organic P mineralization and soil inorganic P transformation, followed by refining the biomass production efficiency under increasing atmospheric CO2, phenology dynamics and canopy light absorption.
... The contributions of CO2 and climate in the DGVMs were estimated analogously as trends in the differences (S1 -S0) and (S2 -S1). We further quantified 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 A c c e p t e d M a n u s c r i p t 6 the contribution of each driver to GPP by running simulations in which only one driver was allowed to change, and calculated the differences (Sdriver -Snought). These differences were also used to quantify the sensitivity of GPP to different drivers, as follows: ...
... We used these inferred changes to investigate the time course of LUE (for details, see section 2.4.4). Carbonyl sulfide (COS) provides another proxy for changes in GPP, relying on the fact that COS and CO2 enter plants by the same diffusion pathway while the uptake is irreversible for COS.Campbell et al. (2017) used long-term atmospheric COS trends derived from measurements in ice cores, firn and ambient air to infer changes in GPP. ...
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Gross primary production (GPP) by terrestrial ecosystems is the largest flux in the global carbon cycle, and its continuing increase in response to environmental changes is key to land ecosystems’ capacity to offset anthropogenic CO 2 emissions. However, the CO 2 - and climate-sensitivities of GPP vary among models. We applied the ‘P model’—a parameter-sparse and extensively tested light use efficiency (LUE) model, driven by CO 2 , climate and remotely sensed greenness data—at 29 sites with multi-year eddy-covariance flux measurements. Observed (both positive and negative) GPP trends at these sites were predicted, albeit with some bias. Increasing LUE (due to rising atmospheric CO 2 concentration) and green vegetation cover were the primary controls of modelled GPP trends across sites. Global GPP simulated by the same model increased by 0.46 ± 0.09 Pg C yr –2 during 1982–2016. This increase falls in the mid-range rate of simulated increase by the TRENDY v8 ensemble of state-of-the-art ecosystem models. The modelled LUE increase during 1900–2013 was 15%, similar to a published estimate based on deuterium isotopomers. Rising CO 2 was the largest contributor to the modelled GPP increase. Greening, which may in part be caused by rising CO 2 , ranked second but dominated the modelled GPP change over large areas, including semi-arid vegetation on all continents. Warming caused a small net reduction in modelled global GPP, but dominated the modelled GPP increase in high northern latitudes. These findings strengthen the evidence that rising LUE due to rising CO 2 level and increased green vegetation cover (fAPAR) are the main causes of increasing GPP, and thereby, the terrestrial carbon sink.
... GPP estimated using this approach is significantly higher than in previous studies (117). Another signal is the long-term trend in COS resolved from firn data, which appears to require a large increase in GPP since 1900 (126). The COS lifetime is proportional to CO 2 /(GPP·LRU) where LRU (leaf relative uptake) is a measure of the relative uptake efficiency of COS versus CO 2 by vegetation (122). ...
... The large changes in the seasonal cycle show that significant changes have occurred in net photosynthetic uptake of CO 2 in the summer (31) and in fall respiration (37). The carbon isotopic data help clarify that large-scale changes in water-use efficiency have accompanied the rise in CO 2 (74) and the COS data support a large increase in GPP (126). These large changes, which were partly unanticipated by ecologists (e.g., 128), have played a major role in motivating research into the terrestrial carbon cycle in recent years. ...
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The past century has been a time of unparalleled changes in global climate and global biogeochemistry. At the forefront of the study of these changes are regular time-series observations at remote stations of atmospheric CO 2 , isotopes of CO 2 , and related species, such as O 2 and carbonyl sulfide (COS). These records now span many decades and contain a wide spectrum of signals, from seasonal cycles to long-term trends. These signals are variously related to carbon sources and sinks, rates of photosynthesis and respiration of both land and oceanic ecosystems, and rates of air-sea exchange, providing unique insights into natural biogeochemical cycles and their ongoing changes. This review provides a broad overview of these records, focusing on what they have taught us about large-scale global biogeochemical change.
... Soil-water content will vary locally in response to variations in precipitation patterns, permafrost thawing, and evaporative demands, with wet areas of Earth likely to become even wetter (17,18), leading in the end to wetland expansion (19). Ultimately, substrate availability may also change in response to the observed increase in primary productivity, mainly driven by CO 2 fertilization (20) or by increased bioavailability in carbon-rich soils and permafrost (21). Because of the multiple feedbacks between climate and the drivers of CH 4 emissions, wetlands have the potential to substantially amplify human-induced climate change and are therefore ecosystems of major concern for prediction of future climate trajectories. ...
... Our emission estimates that ignore the contribution of new wetland areas are in the lower range of previous findings (18)(19)(20)(21)(22), whereas our estimates corrected by wetland expansion but ignoring adaptation are lower than ones reported by Zhang et al. (19) and rather similar by the values reported by Shindell et al. (38) (Fig. 3 and fig. S11). ...
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Wetlands are a major source of methane (CH 4 ) and contribute between 30 and 40% to the total CH 4 emissions. Wetland CH 4 emissions depend on temperature, water table depth, and both the quantity and quality of organic matter. Global warming will affect these three drivers of methanogenesis, raising questions about the feedbacks between natural methane production and climate change. Until present the large-scale response of wetland CH 4 emissions to climate has been investigated with land-surface models that have produced contrasting results. Here, we produce a novel global estimate of wetland methane emissions based on atmospheric inverse modeling of CH 4 fluxes and observed temperature and precipitation. Our data-driven model suggests that by 2100, current emissions may increase by 50% to 80%, which is within the range of 50% and 150% reported in previous studies. This finding highlights the importance of limiting global warming below 2°C to avoid substantial climate feedbacks driven by methane emissions from natural wetlands.
... [COS] a is the background atmospheric COS mixing ratio considered here to be a constant (500 ppt); g T_COS , g B_COS , g S_COS , and g I_COS are respectively the total, boundary layer, stomatal, and internal conductances to COS (mol COS m −2 s −1 ); and g B_W and g S_W are respectively the boundary layer and stomatal conductances to water vapour (mol H 2 O m −2 s −1 ). Note that in this work [COS] a is held constant when computing the COS fluxes, contrary to Berry et al. (2013) and Campbell et al. (2017), where [COS] a is dynamic and taken from the previous time step's PCTM (Parameterized Chemical Transport Model) value. The uncertainty introduced by this simplification is evaluated in the Discussion section. ...
Article
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Land surface modellers need measurable proxies to constrain the quantity of carbon dioxide (CO2) assimilated by continental plants through photosynthesis, known as gross primary production (GPP). Carbonyl sulfide (COS), which is taken up by leaves through their stomates and then hydrolysed by photosynthetic enzymes, is a candidate GPP proxy. A former study with the ORCHIDEE land surface model used a fixed ratio of COS uptake to CO2 uptake normalised to respective ambient concentrations for each vegetation type (leaf relative uptake, LRU) to compute vegetation COS fluxes from GPP. The LRU approach is known to have limited accuracy since the LRU ratio changes with variables such as photosynthetically active radiation (PAR): while CO2 uptake slows under low light, COS uptake is not light limited. However, the LRU approach has been popular for COS–GPP proxy studies because of its ease of application and apparent low contribution to uncertainty for regional-scale applications. In this study we refined the COS–GPP relationship and implemented in ORCHIDEE a mechanistic model that describes COS uptake by continental vegetation. We compared the simulated COS fluxes against measured hourly COS fluxes at two sites and studied the model behaviour and links with environmental drivers. We performed simulations at a global scale, and we estimated the global COS uptake by vegetation to be −756 Gg S yr−1, in the middle range of former studies (−490 to −1335 Gg S yr−1). Based on monthly mean fluxes simulated by the mechanistic approach in ORCHIDEE, we derived new LRU values for the different vegetation types, ranging between 0.92 and 1.72, close to recently published averages for observed values of 1.21 for C4 and 1.68 for C3 plants. We transported the COS using the monthly vegetation COS fluxes derived from both the mechanistic and the LRU approaches, and we evaluated the simulated COS concentrations at NOAA sites. Although the mechanistic approach was more appropriate when comparing to high-temporal-resolution COS flux measurements, both approaches gave similar results when transporting with monthly COS fluxes and evaluating COS concentrations at stations. In our study, uncertainties between these two approaches are of secondary importance compared to the uncertainties in the COS global budget, which are currently a limiting factor to the potential of COS concentrations to constrain GPP simulated by land surface models on the global scale.
... Long-term atmospheric carbonyl sulfide (COS) records also show the large historical growth of GPP. The COS records, derived from ice-core and ambient air samples, imply a 31% increase in GPP during the twentieth century [3]. ...
... The terrestrial plant COS uptake estimate has a broad range from 400 to 1360 Gg S y -1 (Campbell et al., 2017). Recently the inversion modeling study by Ma et al. (2021) pointed to missing sources in the tropics and missing sinks at high latitudes. ...
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The seasonality and interannual variability of terrestrial carbonyl sulfide (COS) flux are poorly constrained. We present the first easy-to-use parameterization for net COS forest sink based on the longest eddy covariance record from a boreal pine forest, covering 32 months over 5 years. Fluxes from hourly to yearly scales are reported, with the aim of revealing controlling factors and the level of interannual variability. The parameterization is based on the photosynthetically active radiation, vapor pressure deficit, air temperature, and leaf area index. The spring recovery of the flux after the winter dormancy period was mostly governed by air temperature, and the onset of the uptake varied by 2 weeks. For the first time, we report a significant reduction of ecosystem-scale COS flux under large water vapor pressure deficit in summer. The maximum monthly and weekly median COS uptake varied 26 and 20 % between years, respectively. The timing of the latter varied by 6 weeks. The fraction of the nocturnal uptake remained below 21 % of the total COS uptake. We observed the growing season (April–August) average net uptake of COS totaling −58.0 gS ha−1 with 37 % interannual variability. The long-term flux observations were scaled up to evergreen needleleaf forests (ENFs) in the whole boreal region by the Simple Biosphere Model Version 4 (SiB4). The observations were reparameterized by using SiB4 meteorological drivers and phenology. The total COS uptake by boreal ENF was in line with a missing COS sink at high latitudes pointed out in earlier studies.
... LUCC is regarded as one of the most important driven factors influencing terrestrial gross primary productivity (GPP) (Ma et al., 2019;Li et al., 2020), a quantic index of the capacity of vegetation fixed carbon (Zhao et al., 2006;Campbell et al., 2017;He et al., 2018). For example, previous studies have reported that LUCC has a significant impact on vegetation phenology and carbon flux, which have a further influence on vegetation GPP (Li et al., 2020;Melnikova and Sasai, 2020). ...
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Large-scale Ecosystem Restoration Projects (ESPs) have been launched to restore vegetation and increase carbon stocks across China. Whether these ESPs could mitigate the loss of carbon emission due to vegetation degradation caused by human disturbances, such as urban expansion, remains unclear. In this study, we analysed the major human-driven land use and land cover change (LUCC) and evaluated their impacts on gross primary productivity (GPP) dynamics in Southwest China during 2001–2015. Results showed that afforestation, agricultural reclamation, urban expansion and grass planting were the major LUCC. Afforestation accounted for approximately 52% of the LUCC area and greatly contributed to the GPP increase, particularly the multiyear accumulative GPP (5.26 Tg C) in the whole area. Urban expansion only accounted for 20% of the LUCC area and led to the decrease of multiyear accumulative GPP (2.52 Tg C) in the whole area. In terms of legacy effect, afforestation mitigated the GPP decrease caused by urban expansion. However, the urban expansion rate (~15.01% per year) was much faster than that of afforestation (~0.13% per year). Therefore, urban expansion might offset more GPP increase from afforestation in the future. Hence, an effective regulation of urban expansion whilst strengthening conservation efforts is urgently needed to enhance vegetation cover and C stock in Southwest China.
... First, CarbonSink+ attempts to take into account the influences of changing atmospheric composition on the forest carbon uptake over the time scales relevant for afforestation/reforestation activities. This is motivated by the notable changes in the global terrestrial carbon sink observed during the recent decades (Sarmiento et al. 2010, Ballantyne et al. 2012, Campbell et al. 2017, Ciais et al. 2019. Second, CarbonSink+ considers the facts that in addition to acting as a carbon sink, forests perturb the climate system by changing the surface albedo and atmospheric aerosol loading (e.g. ...
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M. 2020: CarbonSink+-Accounting for multiple climate feedbacks from forests. Boreal Env. Res. 25: 145-159. Forests cool the climate system by acting as a sink for carbon dioxide (CO 2) and by enhancing the atmospheric aerosol load, whereas the simultaneous decrease of the surface albedo tends to have a warming effect. Here, we present the concept of CarbonSink+, which considers these combined effects. Using the boreal forest environment as an illustrative example, we estimated that accounting for the CarbonSink+ enhances the forest CO 2 uptake by 10-50% due to the combined effects of CO 2 fertilization and aerosol-induced diffuse radiation enhancement on photosynthesis. We further estimated that with affor-estation or reforestation, i.e., replacing grasslands with forests in a boreal environment, the radiative cooling due to forest aerosols cancels most of the radiative warming due to decreased surface albedos. These two forcing components have, however, relatively large uncertainty ranges, resulting in large uncertainties in the overall effect of CarbonSink+. We discuss shortly the potential future changes in the strength of CarbonSink+ in the boreal region, resulting from changes in atmospheric composition and climate.
... Cooler temperatures could reduce photosynthetic carbon uptake if warming leads to higher productivity; or cooler temperatures could increase uptake if heat stress on forests is reduced. While elevated CO 2 can increase photosynthesis and productivity (57,58), other factors can dampen or eliminate this effect, including nutrient limitation (59) and drought (60). Even if CO 2 fertilization increases carbon uptake without increasing mineral nutrient demand, it could cause changes in the tissue stoichiometry of primary producers (61) that could be detrimental to herbivores (62,63). ...
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As the effects of anthropogenic climate change become more severe, several approaches for deliberate climate intervention to reduce or stabilize Earth’s surface temperature have been proposed. Solar radiation modification (SRM) is one potential approach to partially counteract anthropogenic warming by reflecting a small proportion of the incoming solar radiation to increase Earth’s albedo. While climate science research has focused on the predicted climate effects of SRM, almost no studies have investigated the impacts that SRM would have on ecological systems. The impacts and risks posed by SRM would vary by implementation scenario, anthropogenic climate effects, geographic region, and by ecosystem, community, population, and organism. Complex interactions among Earth’s climate system and living systems would further affect SRM impacts and risks. We focus here on stratospheric aerosol intervention (SAI), a well-studied and relatively feasible SRM scheme that is likely to have a large impact on Earth’s surface temperature. We outline current gaps in knowledge about both helpful and harmful predicted effects of SAI on ecological systems. Desired ecological outcomes might also inform development of future SAI implementation scenarios. In addition to filling these knowledge gaps, increased collaboration between ecologists and climate scientists would identify a common set of SAI research goals and improve the communication about potential SAI impacts and risks with the public. Without this collaboration, forecasts of SAI impacts will overlook potential effects on biodiversity and ecosystem services for humanity.
... The lower carbon cost of belowground resource acquisition under fertilization allows for increased aboveground growth and productivity. The extent to which this increased observable productivity [Campbell et al., 2017] translates to a greater land carbon sink remains a topic of investigation [Xia et al., 2017], and the multitude of processes involved suggest a possible divergence between NPP and NEP. First, the increased aboveground and decreased belowground detritus production are unlikely to simply offset one another in terms of soil C inputs, as the differences in chemical composition and physical protection from decomposer and invertebrate communities allow belowground detritus to persist longer in the soil than aboveground detritus [Adamczyk et al., 2019;Clemmensen et al., 2013;Godbold et al., 2006]. ...
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Net primary productivity (NPP) and net ecosystem production (NEP) are often used interchangeably, as their difference, heterotrophic respiration (soil heterotrophic CO2 efflux, RSH = NPP−NEP), is assumed a near‐fixed fraction of NPP. Here, we show, using a range‐wide replicated experimental study in loblolly pine (Pinus taeda) plantations that RSH responds differently than NPP to fertilization and drought treatments, leading to the divergent responses of NPP and NEP. Across the natural range of the species, the moderate responses of NPP (+11%) and RSH (−7%) to fertilization combined such that NEP increased nearly threefold in ambient control and 43% under drought treatment. A 13% decline in RSH under drought led to a 26% increase in NEP while NPP was unaltered. Such drought benefit for carbon sequestration was nearly twofold in control, but disappeared under fertilization. Carbon sequestration efficiency, NEP:NPP, varied twofold among sites, and increased up to threefold under both drought and fertilization.
... According to the Farquhar's study and other related research [52,53], terrestrial plants have shown a significant natural ability to adapt to increased CO2-concentration over the last century. The development of more advanced TBMs taking into account the plant coordination of photosynthesis [54] will probably give a better estimation of plant's potential for CO2-removal in the near future. ...
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In recent years, a great deal of attention has been paid by the scientific community to improving the efficiency of photosynthetic carbon assimilation, plant growth and biomass production in order to achieve a higher crop productivity. Therefore, the primary carboxylase enzyme of the photosynthetic process Rubisco has received considerable attention focused on many aspects of the enzyme function including protein structure, protein engineering and assembly, enzyme activation and kinetics. Based on its fundamental role in carbon assimilation Rubisco is also targeted by the CO2-fertilization effect, which is the increased rate of photosynthesis due to increasing atmospheric CO2-concentration. The aim of this review is to provide a framework, as complete as possible, of the mechanism of the RuBP carboxylation/hydration reaction including description of chemical events occurring at the enzyme “activating” and “catalytic” sites (which involve Broensted acid-base reactions) and the functioning of the complex molecular machine. Important research results achieved over the last few years providing substantial advancement in understanding the enzyme functioning will be discussed.
... Additionally, drylands are the largest source of interannual variability in global carbon sink and play a key role in future variability of vegetation productivity at global scale (Yao et al., 2020). Consequently, a better understanding of the evolution of drylands can help reducing the large uncertainties (Schlund et al., 2020) in the projected increase of gross and net primary production forced by both warming and increasing CO 2 (Anav et al., 2015;Campbell et al., 2017;Cai and Prentice, 2020). ...
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One of the possible consequences of projected global warming is the progressive enlargement of drylands. This study investigates to what extent population and land-use (forests, pastures, and croplands) are likely to be in areas turning arid in the 21st century. The first part of the study focuses on the climatological enlargement of arid areas at global, macro-regional, and high-resolution (0.44°) scales. To do so we analysed a large ensemble of CORDEX climate simulations, combined three indicators (FAO-UNEP aridity index, Köppen-Geiger climate classification, and Holdridge life zones), and quantified the areas turning from climatologically not arid into climatologically arid (and vice-versa) from recent past (1981–2010) to four projected global warming levels (GWLs) from 1.5°C to 4°C. In the second part, we used population and land-use projections to analyze their exposure to progressive shifts to drier or wetter climate. Both types of projections follow five socio-economic scenarios (SSPs from SSP1 to SSP5). We present results for the viable combinations between SSPs and GWLs. Depending on GWL, the projected drying patterns show regional differences but, overall, the negative consequences of climate change are clear. Already at 1.5°C warming, approximately 2 million km2 (1.4% of global land) are likely to become arid; at 2°C this area corresponds to 2.6 million km2 (2.7%), at 3°C to 5.2 million km2 (3.5%), and at 4°C to 6.8 million km2 (4.5%), an area that can be ranked the seventh largest country in the World. Such drying is particular strong over South America and southern Europe. In the worst-case scenario (SSP3, regional rivalry, at 4°C), approximately 500 million people will live in areas shifting towards arid climate. Forest areas are likely to be more affected in South America, pastures in Africa, and croplands in the Northern Hemisphere. For land-use, the worst-case scenarios are SSP3 and SSP5 (fossil-fuel based future): at GWL 4°C, about 0.5 million km2 of forests and 1.2 million km2 of both pastures and croplands are likely to be in areas shifting to arid climate.
... Indeed, ∼70% of the chronologies in this study that showed decreasing g s were broadleaf deciduous tree species. Our data using tree ring isotopes provide strong support of studies using carbonyl sulfide (13,42,57), satellite data (58), and seasonal C a patterns (59) that show global increases in A net as a result of increasing C a (13,42), and build upon recent observations showing widespread stimulated A net resulting in increased iWUE in the United States (9). Moreover, the rates at which iWUE increased were highest in those chronologies with reduced g s (Fig. 4C), followed by those with constant g s (Fig. 4B), and the lowest in those chronologies with increased g s (Fig. 4A). ...
Article
We conducted a meta-analysis of carbon and oxygen isotopes from tree ring chronologies representing 34 species across 10 biomes to better understand the environmental drivers and physiological mechanisms leading to historical changes in tree intrinsic water use efficiency (iWUE), or the ratio of net photosynthesis ( A net ) to stomatal conductance ( g s ), over the last century. We show a ∼40% increase in tree iWUE globally since 1901, coinciding with a ∼34% increase in atmospheric CO 2 (C a ), although mean iWUE, and the rates of increase, varied across biomes and leaf and wood functional types. While C a was a dominant environmental driver of iWUE, the effects of increasing C a were modulated either positively or negatively by climate, including vapor pressure deficit (VPD), temperature, and precipitation, and by leaf and wood functional types. A dual carbon–oxygen isotope approach revealed that increases in A net dominated the observed increased iWUE in ∼83% of examined cases, supporting recent reports of global increases in A net , whereas reductions in g s occurred in the remaining ∼17%. This meta-analysis provides a strong process-based framework for predicting changes in tree carbon gain and water loss across biomes and across wood and leaf functional types, and the interactions between C a and other environmental factors have important implications for the coupled carbon–hydrologic cycles under future climate. Our results furthermore challenge the idea of widespread reductions in g s as the major driver of increasing tree iWUE and will better inform Earth system models regarding the role of trees in the global carbon and water cycles.
... Physiology A number of independent lines of indirect evidenceice-core OCS (Campbell et al., 2017) and O 18 , glucose isotopomers (Ehlers et al., 2015), satellite ET (Cheng et al., 2017), and flux-partitioned eddy-covariance (Fernández-Martínezet al., 2017)provide high confidence that terrestrial GPP has increased concurrently with iCO 2 . Estimates of the GPP increase disagree by a factor of 1.7 (β app = 0.95-1.6, ...
Article
Atmospheric carbon dioxide concentration ([CO2]) is increasing, which increases leaf‐scale photosynthesis and intrinsic water‐use efficiency. These direct responses have the potential to increase plant growth, vegetation biomass, and soil organic matter; transferring carbon from the atmosphere into terrestrial ecosystems (a carbon sink). A substantial global terrestrial carbon sink would slow the rate of [CO2] increase and thus climate change. However, ecosystem CO2‐responses are complex or confounded by concurrent changes in multiple agents of global change and evidence for a [CO2]‐driven terrestrial carbon sink can appear contradictory. Here we synthesise theory and broad, multi‐disciplinary evidence for the effects of increasing [CO2] (iCO2) on the global terrestrial carbon sink. Evidence suggests a substantial increase in global photosynthesis since pre‐industry. Established theory, supported by experiments, indicates that iCO2 is likely responsible for about half of the increase. Global carbon budgeting, atmospheric data, and forest inventories indicate a historical carbon sink, and these apparent iCO2‐responses are high in comparison with experiments and theory. Plant mortality and soil carbon iCO2‐responses are highly uncertain. In conclusion, a range of evidence supports a positive terrestrial carbon sink in response to iCO2, albeit with uncertain magnitude and strong suggestion of a role for additional agents of global change.
... The importance of tropical rainforests for storing carbon and maintaining biodiversity is well known. The largest component of the global carbon cycle is the terrestrial gross primary productivity, which sequesters CO 2 in organic matter by photosynthetic activity (e.g., Campbell et al. 2017). Approximately one-third of this carbon fixing takes place in the large expanses of rainforest ecosystems in the Amazon Basin of South America, the Congo Basin of equatorial Africa, and Southeast Asia, and in smaller forests scattered in Central America, Madagascar, coastal India, and elsewhere (Saugier et al. 2001). ...
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Trends in Congo Basin rainfall in six observational datasets are examined on monthly and annual time scales to motivate an investigation of changes in the atmospheric hydrodynamics. Annual-mean Congo Basin rainfall declines over the 1979–2017 time period in all datasets, with strongest agreement and statistical significance from March through August. The trends are less pronounced over the 1998–2017 time period, especially for boreal spring, with although the boreal summer season continues to dry. Decadal-scale differences in the atmospheric hydrodynamics are examined in three reanalyses to improve our physical understanding of the precipitation trends and add confidence. During much of the spring and fall, changes in the atmospheric circulation reflect regional processes and feedbacks. During the warm season in each hemisphere, Congo Basin precipitation is supported when the circulation about continental thermal lows converges with cross-equatorial flow from the winter hemisphere. The drying trend during these seasons is associated with poleward shifts of the continental thermal lows, which weakens this convergence. Rainfall anomalies are not directly related to local surface warming, and they do not involve modifications of moisture transport from the Atlantic or Indian Oceans. For boreal summer, the drying is related to amplified warming over the Sahara. In austral summer, a southward shift of the thermal low is part of a large-scale, zonal mean pattern shifting the subtropical highs poleward.
... G ross primary productivity (GPP) has been globally stimulated by rising anthropogenic [CO 2 ] 1,2 and Earth system models (ESM) suggest this effect could continue to ca. 2070 3 . While global-scale studies infer a moderate historical fertilization effect 1,4 , evidence for rising CO 2 stimulating productivity at the ecosystem scale in mature forests has proven elusive [5][6][7] . This incongruity has limited accurate constraints of the fertilization effect in ESM 3 , which is critical to predicting terrestrial carbon feedbacks that may continue to mitigate anthropogenic emissions 8,9 . ...
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Plant–mycorrhizal interactions mediate plant nitrogen (N) limitation and can inform model projections of the duration and strength of the effect of increasing CO 2 on plant growth. We present dendrochronological evidence of a positive, but context-dependent fertilization response of Quercus rubra L . to increasing ambient CO 2 (iCO 2 ) along a natural soil nutrient gradient in a mature temperate forest. We investigated this heterogeneous response by linking metagenomic measurements of ectomycorrhizal (ECM) fungal N-foraging traits and dendrochronological models of plant uptake of inorganic N and N bound in soil organic matter (N-SOM). N-SOM putatively enhanced tree growth under conditions of low inorganic N availability, soil conditions where ECM fungal communities possessed greater genomic potential to decay SOM and obtain N-SOM. These trees were fertilized by 38 years of iCO 2 . In contrast, trees occupying inorganic N rich soils hosted ECM fungal communities with reduced SOM decay capacity and exhibited neutral growth responses to iCO 2 . This study elucidates how the distribution of N-foraging traits among ECM fungal communities govern tree access to N-SOM and subsequent growth responses to iCO 2 .
... In addition, the SIF total -based GPP estimates can be applied to the study of feedbacks related to anthropogenic activities (Zhu et al., 2016) or CO 2 fertilization (Smith et al., 2016) on the terrestrial vegetation. Although it is not possible to fully validate our annual mean GPP estimate, our estimates in the magnitude are in line with previous studies (Beer et al., 2010;Anav et al., 2015;Campbell et al., 2017;Joiner et al., 2018) and this good agreement supports the usefulness of a consistent SIF-based GPP model after mitigating canopy structure effects. However, it should be noted that our aim is not to provide a definitive estimate of global annual GPP from SIF; rather, we intend to investigate whether simple and consistent SIF-based GPP models can be obtained. ...
Article
Quantifying global photosynthesis remains a challenge due to a lack of accurate remote sensing proxies. Solar-induced chlorophyll fluorescence (SIF) has been shown to be a good indicator of photosynthetic activity across various spatial scales. However, a global and spatially challenging estimate of terrestrial gross primary production (GPP) based on satellite SIF remains unresolved due to the confounding effects of species-specific physical and physiological traits and external factors, such as canopy structure or photosynthetic pathway (C 3 or C 4). Here we analyze an ensemble of far-red SIF data from OCO-2 satellite and ground observations at multiple sites, using the spectral invariant theory to reduce the effects of canopy structure and to retrieve a structure-corrected total canopy SIF emission (SIF total). We find that the relationships between observed canopy-leaving SIF and ecosystem GPP vary significantly among biomes. In contrast, the relationships between SIF total and GPP converge around two unique models, one for C 3 and one for C 4 plants. We show that the two single empirical models can be used to globally scale satellite SIF observations to terrestrial GPP. We obtain an independent estimate of global terrestrial GPP of 129.56 ± 6.54 PgC/year for the 2015-2017 period, which is consistent with the state-of-the-art data-and process-oriented models. The new GPP product shows improved sensitivity to previously undetected 'hotspots' of productivity, being able to resolve the double-peak in GPP due to rotational cropping systems. We suggest that the direct scheme to estimate GPP presented here, which is based on satellite SIF, may open up new possibilities to resolve the dynamics of global terrestrial GPP across space and time.
... In addition, estimates of GPP lit and Rs lit have varied among studies (see Supplementary Fig. 8 and refs. 15,36 ), reflecting methodological and technological differences, but uncertainty in these estimates have remained high (Supplementary Tables 1, 3); see also ref. 37 . We highlight that more recent GPP estimates have tended towards higher estimates but still with high uncertainty. ...
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The terrestrial carbon cycle is a major source of uncertainty in climate projections. Its dominant fluxes, gross primary productivity (GPP), and respiration (in particular soil respiration, R S ), are typically estimated from independent satellite-driven models and upscaled in situ measurements, respectively. We combine carbon-cycle flux estimates and partitioning coefficients to show that historical estimates of global GPP and R S are irreconcilable. When we estimate GPP based on R S measurements and some assumptions about R S :GPP ratios, we found the resulted global GPP values (bootstrap mean $${149}_{-23}^{+29}$$ 149 − 23 + 29 Pg C yr ⁻¹ ) are significantly higher than most GPP estimates reported in the literature ( $${113}_{-18}^{+18}$$ 113 − 18 + 18 Pg C yr ⁻¹ ). Similarly, historical GPP estimates imply a soil respiration flux (Rs GPP , bootstrap mean of $${68}_{-8}^{+10}$$ 68 − 8 + 10 Pg C yr ⁻¹ ) statistically inconsistent with most published R S values ( $${87}_{-8}^{+9}$$ 87 − 8 + 9 Pg C yr ⁻¹ ), although recent, higher, GPP estimates are narrowing this gap. Furthermore, global R S :GPP ratios are inconsistent with spatial averages of this ratio calculated from individual sites as well as CMIP6 model results. This discrepancy has implications for our understanding of carbon turnover times and the terrestrial sensitivity to climate change. Future efforts should reconcile the discrepancies associated with calculations for GPP and Rs to improve estimates of the global carbon budget.
... This flux, known as gross primary productivity, is the largest carbon dioxide flux between the atmosphere and the land surface, and accounts for approximately 120 billion tonnes of carbon per year [1]. Global terrestrial gross primary productivity varies with environmental conditions, and, according to an analysis of atmospheric carbonyl sulfide records, increased by 31% over the twentieth century [2]. Air temperature plays a key role in the process of photosynthetic carbon dioxide assimilation [3]. ...
... Specifically, 63 to 69% of C3 canopy photosynthesis originates from light-saturated conditions for all seasons using the ERA5land forcing. Together, these features explain the high β CO2 predicted by our framework and also lead to a good agreement with the overall trend in EC-inferred GPP and supported by multiple independent studies (39)(40)(41)(42). ...
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Significance The magnitude of the CO 2 fertilization effect on terrestrial photosynthesis is uncertain because it is not directly observed and is subject to confounding effects of climatic variability. We apply three well-established eco-evolutionary optimality theories of gas exchange and photosynthesis, constraining the main processes of CO 2 fertilization using measurable variables. Using this framework, we provide robust observationally inferred evidence that a strong CO 2 fertilization effect is detectable in globally distributed eddy covariance networks. Applying our method to upscale photosynthesis globally, we find that the magnitude of the CO 2 fertilization effect is comparable to its in situ counterpart but highlight the potential for substantial underestimation of this effect in tropical forests for many reflectance-based satellite photosynthesis products.
... The terrestrial plant COS uptake estimate has a broad range, from 400 to 1360 Gg S yr −1 (Campbell et al., 2017;Remaud et al., 2021;Hu et al., 2021). Recently, the inversion modeling study by Ma et al. (2021) pointed to missing sources in the tropics and missing sinks at high latitudes. ...
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The seasonality and interannual variability of terrestrial carbonyl sulfide (COS) fluxes are poorly constrained. We present the first easy-to-use parameterization for the net COS forest sink based on the longest existing eddy covariance record from a boreal pine forest, covering 32 months over 5 years. Fluxes from hourly to yearly scales are reported, with the aim of revealing controlling factors and the level of interannual variability. The parameterization is based on the photosynthetically active radiation, vapor pressure deficit, air temperature, and leaf area index. Wavelet analysis of the ecosystem fluxes confirmed earlier findings from branch-level fluxes at the same site and revealed a 3 h lag between COS uptake and air temperature maxima at the daily scale, whereas no lag between radiation and COS flux was found. The spring recovery of the flux after the winter dormancy period was mostly governed by air temperature, and the onset of the uptake varied by 2 weeks. For the first time, we report a significant reduction in ecosystem-scale COS uptake under a large water vapor pressure deficit in summer. The maximum monthly and weekly median COS uptake varied by 26 % and 20 % between years, respectively. The timing of the latter varied by 6 weeks. The fraction of the nocturnal uptake remained below 21 % of the total COS uptake. We observed the growing season (April–August) average net flux of COS totaling −58.0 g S ha−1 with 37 % interannual variability. The long-term flux observations were scaled up to evergreen needleleaf forests (ENFs) in the whole boreal region using the Simple Biosphere Model Version 4 (SiB4). The observations were closely simulated using SiB4 meteorological drivers and phenology. The total COS uptake by boreal ENFs was in line with a missing COS sink at high latitudes pointed out in earlier studies.
... Our LRU set is derived from Whelan et al. (2018) and uses, for C 3 plants, the median value of 53 LRU data and, for C 4 plants, the median value of 4 LRU data. This simplification is supported by Hilton et al. (2017) and Campbell et al. (2017), who showed that the uncertainty on the LRU parameter is of a second-order importance compared to the uncertainties on the GPP and the other COS fluxes. Moreover, Maignan et al. (2020) showed that using a mechanistic model or its LRU equivalent model (i.e., with a constant LRU per PFT in ORCHIDEE LSM) for the plant uptake leads to similar results when transporting the COS fluxes with LMDz and comparing the COS concentrations at stations of the NOAA network. ...
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Carbonyl sulfide (COS), a trace gas showing striking similarity to CO2 in terms of biochemical diffusion pathway into leaves, has been recognized as a promising indicator of the plant gross primary production (GPP), the amount of carbon dioxide that is absorbed through photosynthesis by terrestrial ecosystems. However, large uncertainties about the other components of its atmospheric budget prevent us from directly relating the atmospheric COS measurements to GPP. The largest uncertainty comes from the closure of its atmospheric budget, with a source component missing. Here, we explore the benefit of assimilating both COS and CO2 measurements into the LMDz atmospheric transport model to obtain consistent information on GPP, plant respiration and COS budget. To this end, we develop an analytical inverse system that optimizes biospheric fluxes for the 15 plant functional types (PFTs) defined in the ORCHIDEE global land surface model. Plant uptake of COS is parameterized as a linear function of GPP and of the leaf relative uptake (LRU), which is the ratio of COS to CO2 deposition velocities in plants. A possible scenario for the period 2008–2019 leads to a global biospheric sink of 800 GgS yr−1, with higher absorption in the high latitudes and higher oceanic emissions between 400 and 600 GgS yr−1 most of which is located in the tropics. As for the CO2 budget, the inverse system increases GPP in the high latitudes by a few GtC yr−1 without modifying the respiration compared to the ORCHIDEE fluxes used as a prior. In contrast, in the tropics the system tends to weaken both respiration and GPP. The optimized components of the COS and CO2 budgets have been evaluated against independent measurements over North America, the Pacific Ocean, at three sites in Japan and at one site in France. Overall, the posterior COS concentrations are in better agreement with the COS retrievals at 250 hPa from the MIPAS satellite and with airborne measurements made over North America and the Pacific Ocean. The system seems to have rightly corrected the underestimated GPP over the high latitudes. However, the change in seasonality of GPP in the tropics disagrees with solar-induced fluorescence (SIF) data. The decline in biospheric sink in the Amazon driven by the inversion also disagrees with MIPAS COS retrievals at 250 hPa, highlighting the lack of observational constraints in this region. Moreover, the comparison with the surface measurements in Japan and France suggests misplaced sources in the prior anthropogenic inventory, emphasizing the need for an improved inventory to better partition oceanic and continental sources in Asia and Europe.
... Furthermore, during low-GPP years, there is a shift away from oceanic and toward continental source regions, which is most evident in ecoregions such as SAH and NGP. Particularly for 12 Ann. N.Y. ...
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The land biosphere is a crucial component of the Earth system that interacts with the atmosphere in a complex manner through manifold feedback processes. These relationships are bidirectional, as climate affects our terrestrial ecosystems, which, in turn, influence climate. Great progress has been made in understanding the local interactions between the terrestrial biosphere and climate, but influences from remote regions through energy and water influxes to downwind ecosystems remain less explored. Using a Lagrangian trajectory model driven by atmospheric reanalysis data, we show how heat and moisture advection affect gross carbon production at interannual scales and in different ecoregions across the globe. For water‐limited regions, results show a detrimental effect on ecosystem productivity during periods of enhanced heat and reduced moisture advection. These periods are typically associated with winds that disproportionately come from continental source regions, as well as positive sensible heat flux and negative latent heat flux anomalies in those upwind locations. Our results underline the vulnerability of ecosystems to the occurrence of upwind climatic extremes and highlight the importance of the latter for the spatiotemporal propagation of ecosystem disturbances.
... Recently, Schimel et al. (2015) concluded that increasing CO 2 -concentrations in the atmosphere likely act as a significant negative feedback in the global carbon cycle by absorbing up to 30% of CO 2 -emissions caused by the combustion of fossil fuels. Similar results were obtained by Campbell et al. (2017), who documented, using a variety of sophisticated methods that currently ca. 31% of anthropogenic CO 2 -emissions are re-cycled by the more rapidly growing land vegetation (see Bastin et al. 2019a, b and for further discussion of this topic). ...
Article
One century ago, the German chemist and botanist Wilhelm Pfeffer (1845–1920) died, shortly after finishing his last lecture at the University of Leipzig. Pfeffer was, together with Julius Sachs (1832–1897), the founder of modern plant physiology. In contrast to Sachs, Pfeffer’s work was exclusively based on the principles of physics and chemistry, so that with his publications, notably the ca. 1.600 pages-long Handbuch der Pflanzenphysiologie (2. ed., Vol. I/II; 1897/1904), experimental plant research was founded. Here we summarize Pfeffer’s life and work with special emphasis on his experiments on osmosis, plant growth in light vs. darkness, gravitropism, cell physiology, photosynthesis and leaf movements. We document that Pfeffer was the first to construct/establish constant temperature rooms (growth chambers) for seed plants. Moreover, he pioneered in outlining the carbon-cycle in the biosphere, and described the effect of carbon dioxide (CO2)-enhancement on assimilation and plant productivity. Wilhelm Pfeffer pointed out that, at ca. 0.03 vol% CO2 (in 1900), photosynthesis is sub-optimal. Accordingly, due to human activities, anthropogenic CO2 released into the atmosphere promotes plant growth and crop yield. We have reproduced Pfeffer’s classical experiments on the role of CO2 with respect to plant development, and document that exhaled air of a human (ca. 4 vol% CO2) strongly promotes growth. We conclude that Pfeffer not only acted as a key figure in the establishment of experimental plant physiology. He was also the discoverer of the phenomenon of CO2-mediated global greening and promotion of crop productivity, today known as the “CO2-fertilization-effect”. These topics are discussed with reference to climate change and the most recent findings in this area of applied plant research.
... Constraints on these fluxes at larger spatial scales are therefore needed to improve terrestrial biosphere models to better simulate the responses of photosynthesis and stomatal gas exchange to a changing climate. Recently, COS has been shown to be valuable for understanding changes in plant uptake, e.g., the inhibition of photosynthesis during a heat wave , the growth of the terrestrial gross primary production (GPP) during the twentieth century (Campbell et al., 2017), the regional-scale partitioning of net ecosystem exchange (NEE) into GPP and ecosystem respiration (Hu et al., 2021), and changes in transpiration (Berkelhammer et al., 2020;Wehr et al., 2017). To further advance COS as a constraint on the carbon and water cycles in models requires an accurate representation and evaluation of COS biosphere fluxes in models. ...
Article
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The uptake of carbonyl sulfide (COS) by terrestrial plants is linked to photosynthetic uptake of CO2 as these gases partly share the same uptake pathway. Applying COS as a photosynthesis tracer in models requires an accurate representation of biosphere COS fluxes, but these models have not been extensively evaluated against field observations of COS fluxes. In this paper, the COS flux as simulated by the Simple Biosphere Model, version 4 (SiB4), is updated with the latest mechanistic insights and evaluated with site observations from different biomes: one evergreen needleleaf forest, two deciduous broadleaf forests, three grasslands, and two crop fields spread over Europe and North America. We improved SiB4 in several ways to improve its representation of COS. To account for the effect of atmospheric COS mole fractions on COS biosphere uptake, we replaced the fixed atmospheric COS mole fraction boundary condition originally used in SiB4 with spatially and temporally varying COS mole fraction fields. Seasonal amplitudes of COS mole fractions are ∼50–200 ppt at the investigated sites with a minimum mole fraction in the late growing season. Incorporating seasonal variability into the model reduces COS uptake rates in the late growing season, allowing better agreement with observations. We also replaced the empirical soil COS uptake model in SiB4 with a mechanistic model that represents both uptake and production of COS in soils, which improves the match with observations over agricultural fields and fertilized grassland soils. The improved version of SiB4 was capable of simulating the diurnal and seasonal variation in COS fluxes in the boreal, temperate, and Mediterranean region. Nonetheless, the daytime vegetation COS flux is underestimated on average by 8±27 %, albeit with large variability across sites. On a global scale, our model modifications decreased the modeled COS terrestrial biosphere sink from 922 Gg S yr−1 in the original SiB4 to 753 Gg S yr−1 in the updated version. The largest decrease in fluxes was driven by lower atmospheric COS mole fractions over regions with high productivity, which highlights the importance of accounting for variations in atmospheric COS mole fractions. The change to a different soil model, on the other hand, had a relatively small effect on the global biosphere COS sink. The secondary role of the modeled soil component in the global COS budget supports the use of COS as a global photosynthesis tracer. A more accurate representation of COS uptake in SiB4 should allow for improved application of atmospheric COS as a tracer of local- to global-scale terrestrial photosynthesis.
... Model-based reconstructions indicate significant changes in gross uptake of carbon by the vegetated land surface over the 20 th Century. From 1901 to 2010 GPP has increased globally ( Fig. 1) by 10.5 Pg C per annum (uncertainty range: +8.2 to +12.4 Pg C per annum), in qualitative agreement with long-term atmospheric records of carbonyl sulfide 13 , and equivalent to 9% of current satellite-era global GPP of 119 Pg C per annum 2 . Increases in GPP are centered on the tropics as well as forested regions of the USA and Eurasia. ...
Article
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Terrestrial vegetation removes CO2 from the atmosphere; an important climate regulation service that slows global warming. This 119 Pg C per annum transfer of CO2 into plants—gross primary productivity (GPP)—is the largest land carbon flux globally. While understanding past and anticipated future GPP changes is necessary to support carbon management, the factors driving long-term changes in GPP are largely unknown. Here we show that 1901 to 2010 changes in GPP have been dominated by anthropogenic activity. Our dual constraint attribution approach provides three insights into the spatiotemporal patterns of GPP change. First, anthropogenic controls on GPP change have increased from 57% (1901 decade) to 94% (2001 decade) of the vegetated land surface. Second, CO2 fertilization and nitro gen deposition are the most important drivers of change, 19.8 and 11.1 Pg C per annum (2001 decade) respectively, especially in the tropics and industrialized areas since the 1970’s. Third, changes in climate have functioned as fertilization to enhance GPP (1.4 Pg C per annum in the 2001 decade). These findings suggest that, from a land carbon balance perspective, the Anthropocene began over 100 years ago and that global change drivers have allowed GPP uptake to keep pace with anthropogenic emissions.
... The results of our study have important implications for how soil C storage in temperate forests may respond to environmental change. For example, rising atmospheric CO 2 stimulates plant growth (Campbell et al., 2017), and ectomycorrhizal plants may increase their investment in organic N acquisition by ECM mutualists to maintain this growth (Terrer et al., 2016). Our observation that certain ECM fungi may decrease soil C when acquiring N from SOM (Figure 4a,b) suggests elevated CO 2 could decrease soil C storage in temperate forests by increasing organic N acquisition. ...
Article
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Interactions between soil nitrogen (N) availability, fungal community composition, and soil organic matter (SOM) regulate soil carbon (C) dynamics in many forest ecosystems, but context dependency in these relationships has precluded general predictive theory. We found that ectomycorrhizal (ECM) fungi with peroxidases decreased with increasing inorganic N availability across a natural inorganic N gradient in northern temperate forests, whereas ligninolytic fungal saprotrophs exhibited no response. Lignin‐derived SOM and soil C were negatively correlated with ECM fungi with peroxidases and were positively correlated with inorganic N availability, suggesting decay of lignin‐derived SOM by these ECM fungi reduced soil C storage. The correlations we observed link SOM decay in temperate forests to tradeoffs in tree N nutrition and ECM composition, and we propose SOM varies along a single continuum across temperate and boreal ecosystems depending upon how tree allocation to functionally distinct ECM taxa and environmental stress covary with soil N availability. Ectomycorrhizal fungi with peroxidases decline with increasing soil inorganic nitrogen availability. Lignin‐derived soil organic matter and total soil carbon are negatively correlated with ectomycorrhizal fungi with peroxidases, causing soil carbon storage to increase with increasing soil inorganic nitrogen. Naturally high soil inorganic nitrogen availability in temperate forests promotes soil carbon storage by reducing the decay of lignin‐derived soil organic matter by ectomycorrhizal fungi with peroxidases.
... Trends in the tropical forest carbon cycle in the early Anthropocene have been uncertain (Malhi et al. 2014, Malhi 2017. Several lines of evidence, from both empirical and modeled data, have shown that elevated temperatures could facilitate vegetation growth globally, including in tropical regions (Lewis et al. 2009, Campbell et al. 2017, Huang et al. 2018b), but see Brienen et al. (2015) and Lewis et al. (2011). Our Landsat time-series analyses revealed a similar enhanced growing trend in the summer EVI for both evergreen and deciduous forests (Figs. ...
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Tropical dry forests (TDFs) have experienced pronounced droughts and increased temperatures for the last century. To assess whether these climatic shifts have influenced dry forest vegetation and ecosystem functioning, we integrated ground observations from a Costa Rican long‐term forest dynamics monitoring plot with remotely sensed measures of forest productivity and canopy functioning from a diverse set of satellite data. Previously reported long‐term climate data (1921–1997) show a reduction in annual rainfall, but since 1980 there has been no directional change in mean annual precipitation. However, the 2015 El Niño Southern Oscillation (ENSO)‐induced drought was unprecedented. Temperatures have increased by 1.1°C since 1931. However, the Landsat wet season (1987–2017) Enhanced Vegetation Index (EVI) (canopy greenness) and the dry season (1985–2017) fraction of non‐photosynthetic canopy cover all indicate that TDFs have become more deciduous but also more productive during the wet season. These changes are consistent with a shift in the functional composition observed in the long‐term plot as more drought‐deciduous tree species have increased in abundance. Nonetheless, more continuous 16‐d MODIS (the Moderate Resolution Imaging Spectroradiometer) measures of the EVI over the past 17 yr (2001–2017) showed no change in the total annual forest productivity. Further, while the 2015 ENSO event temporarily reduced forest EVI, it did not cause a longer‐term impact on forest productivity. Instead, high spatial resolution Worldview‐2 satellite imagery showed that forest phenology shifted in the subsequent years even though the region returned to normal precipitation. Our results indicate that while the species composition of TDFs may be sensitive to the long‐term trend of gradually increasing temperatures and aridity, the annual forest functioning has so far been resilient to long‐term drying and a large episodic extreme drought event. This study demonstrates the feasibility of synthesizing satellite images of different characteristics to study the vegetation dynamics of a long‐term forest dynamics plot. Our synoptically sensed results show that the longer‐term changing climate has been and is currently shifting the ecological functioning, and also provide a baseline to assess the impacts of an extreme drought year on TDFs.
... The novel benchmarks proposed here may also provide new targets for evaluating LSMs' performance, as the metrics could be used in the objective function of any data assimilation technique to rigorously account for the information contained in TRW datasets. The value of tree-ring records for LSM verification might be further enhanced by (i) developing new, unbiased networks, such as the European biomass network, to both complement and identify biases in the ITRDB; (ii) adding their stable isotope ratios to verification benchmarks that may be simulated by isotope-enabled LSMs (Levesque et al., 2019;Barichivich et al., 2021); and (iii) combining their use with high-frequency but short-term eddy covariance measurements (Pappas et al., 2020;Teets et al., 2018), experimental data from plant growth under pre-industrial CO 2 concentrations (Temme et al., 2015), and proxies of atmospheric composition (Campbell et al., 2017). ...
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The search for a long-term benchmark for land-surface models (LSMs) has brought tree-ring data to the attention of the land-surface modelling community, as tree-ring data have recorded growth well before human-induced environmental changes became important. We propose and evaluate an improved conceptual framework of when and how tree-ring data may, despite their sampling biases, be used as century-long hindcasting targets for evaluating LSMs. Four complementary benchmarks – size-related diameter growth, diameter increment of mature trees, diameter increment of young trees, and the response of tree growth to extreme events – were simulated using the ORCHIDEE version r5698 LSM and were verified against observations from 11 sites in the independent, unbiased European biomass network datasets. The potential for big-tree selection bias in the International Tree-Ring Data Bank (ITRDB) was investigated by subsampling the 11 sites from European biomass network. We find that in about 95 % of the test cases, using ITRDB data would result in the same conclusions as using the European biomass network when the LSM is benchmarked against the annual radial growth during extreme climate years. The ITRDB data can be used with 70 % confidence when benchmarked against the annual radial growth of mature trees or the size-related trend in annual radial growth. Care should be taken when using the ITRDB data to benchmark the annual radial growth of young trees, as only 50 % of the test cases were consistent with the results from the European biomass network. The proposed maximum tree diameter and annual growth increment benchmarks may enable the use of ITRDB data for large-scale validation of the LSM-simulated response of forest ecosystems to the transition from pre-industrial to present-day environmental conditions over the past century. The results also suggest ways in which tree-ring width observations may be collected and/or reprocessed to provide long-term validation tests for land-surface models.
... Net primary productivity (NPP) has been globally stimulated by rising anthropogenic [CO2] (Campbell et al. 2017) and coupled climate-biogeochemical (CCB) models suggest this effect could continue to ~2070 (Wenzel et al. 2016). This global biogeochemical feedback could determine the accumulation of anthropogenic CO2 in the atmosphere (Arneth et al. 2010), however, current model projections of NPP under elevated CO2 (eCO2) vary widely, from +12 to 60% (Wenzel et al. 2016). ...
Thesis
This dissertation investigates the capacity for ectomycorrhizal fungi to obtain Nitrogen (N), organically bound in soil organic matter (N-SOM). In Chapter 1, I delineate the gene families involved in the decay of SOM, and study their distribution across the ~ 85 independent evolutionary lineages of ECM fungi. I provide evidence that the polyphyletic nature of the ECM lifestyle has resulted in considerable variation in their genetic potential to obtain N-SOM. In addition, I describe several untested physiological conditions that limit our understanding of the contribution of N-SOM to plant growth. In chapters 2 and 3, I study ECM communities arrayed across a natural soil fertility gradient in Northern Michigan using a standardized tree host (Quercus rubra L.). I develop and test a whole-plant resource allocation framework that explicitly considers the composition and function of ECM communities and their net effect on plant uptake of organic and inorganic forms of N at the ecosystem scale. In Chapter 2, I employ a trait-based shotgun metagenome enabled approach to study variation in the genomic potential of ECM communities to obtain N-SOM. Foremost, I gathered support for the hypothesis that soil inorganic N availability acts as an environmental filter structuring the assembly of ECM communities and their trait distributions. Specifically, I document that the community weighted mean (CWM) genomic decay potential of ECM communities is inversely correlated with soil inorganic N availability. Furthermore, I tested the hypothesis that Q. rubra inhabiting low inorganic N soils, obtain greater quantities of N-SOM than do Q. rubra occupying inorganic N rich soils, due to physiological variation in the attributes of their ECM symbionts. I scaled CWM gene counts by the number of ECM infected root-tips present on individual root-systems to document that Q. rubra inhabiting low inorganic N soils host greater composite quantities of ECM genes involved in decay. Chapter 3 incorporates dendrochronological tree core data, Bayesian plant growth modeling approaches and molecular characterization of ECM communities and associated foraging morphologies. I show that N-SOM is likely to bolster net primary productivity (NPP) in soils where inorganic N is relatively scarce due to compositional and functional variation of associated ECM communities. Moreover, I compile dendrochronological evidence that trees inhabiting low inorganic N soils, exhibit a positive response to nearly 40 years of increasing ambient [CO2]. Integrating functional attributes of ECM communities, provides supports for the hypothesized importance of organic N in the global fertilizing effect of CO2 on NPP. Because ECM fungi that can degrade N-SOM carry a high carbon cost to their plant host, my results highlight potential tradeoffs in the role of the ECM symbiosis across soil inorganic N gradients. By documenting that N-SOM is unlikely to ubiquitously contribute to plant growth, my dissertation provides unique support for theory of optimal plant N foraging; I suggest that shifts in the functional attributes of ECM communities represent a mechanistic basis for plant flexibility in nutrient foraging strategies. Together, my analyses offers unprecedented molecular insight into the physiology of ECM communities and extends a functional biogeographic perspective that clarifies widespread observations of consistent patterns of ECM community turnover. Finally, my work adds further mechanistic evidence that the plant CO2 fertilization response is predicated upon the capacity of their ECM symbionts to obtain N-SOM, and clarifies the heterogeneous response of ECM forests to eCO2.
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Carbonyl sulfide (COS) was measured in firn air collected during seven different field campaigns carried out at four different sites in Greenland and Antarctica between 2001 and 2015. A Bayesian probabilistic statistical model is used to conduct multisite inversions and to reconstruct separate atmospheric histories for Greenland and Antarctica. The firn air inversions cover most of the 20th century over Greenland and extend back to the 19th century over Antarctica. The derived atmospheric histories are consistent with independent surface air time series data from the corresponding sites and the Antarctic ice core COS records during periods of overlap. Atmospheric COS levels began to increase over preindustrial levels starting in the 19th century, and the increase continued for much of the 20th century. Atmospheric COS peaked at higher than present‐day levels around 1975 CE over Greenland and around 1987 CE over Antarctica. An atmosphere/surface ocean box model is used to investigate the possible causes of observed variability. The results suggest that changes in the magnitude and location of anthropogenic sources have had a strong influence on the observed atmospheric COS variability.
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Carbonyl sulfide (OCS) provides a proxy for measuring photosynthesis and is the primary background source of stratospheric aerosols. OCS emissions due to biomass burning are a variable and substantial (over 10%) part of the current OCS budget. OCS emission ratios from open burning fires, coupled with 1997–2016 data from the Global Fire Emissions Database (GFED4), yield OCS biomass burning emissions with a global average annual flux of 60 ± 37 Gg(S) year⁻¹. A global box model suggests these emissions are more consistent with observations from global atmospheric composition monitoring networks than fluxes derived from previous synthesis papers. Even after considering the uncertainty in emission factor observations for each category of emissions and the interannual variation in total burned dry matter, the total OCS emissions from open burning are insufficient to account for the large imbalance between current estimates of global OCS sources and sinks.
Article
Carbonyl sulfide (COS) has emerged as a multi‐scale tracer for terrestrial photosynthesis. To infer ecosystem‐scale photosynthesis from COS fluxes often requires knowledge of leaf relative uptake (LRU), the concentration‐normalized ratio between leaf COS uptake and photosynthesis. However, current mechanistic understanding of LRU variability remains inadequate for deriving robust COS‐based estimates of photosynthesis. We derive a set of closed‐form equations to describe LRU responses to light, humidity, and CO2 based on the Ball–Berry stomatal conductance model and the biochemical model of photosynthesis. This framework reproduces observed LRU responses: decreasing LRU with increasing light or decreasing humidity; it also predicts that LRU increases with ambient CO2. By fitting the LRU equations to flux measurements on a C3 reed (Typha latifolia), we obtain physiological parameters that control LRU variability, including an estimate of the Ball–Berry slope of 7.1 without using transpiration measurements. Sensitivity tests reveal that LRU is more sensitive to photosynthetic capacity than to the Ball–Berry slope, indicating stomatal response to photosynthesis. This study presents a simple framework for interpreting observed LRU variability and upscaling LRU. The stoma‐regulated LRU response to CO2 suggests that COS may offer a unique window into long‐term stomatal acclimation to elevated CO2.
Article
Carbonyl sulfide (COS) is the most abundant and long-lived sulfur-containing gas in the atmosphere. Soil is the main sink of COS in the atmosphere and uptake is dominated by soil microorganisms; however, biochemical research has not yet been conducted on fungal COS degradation. COS hydrolase (COSase) was purified from Trichoderma harzianum strain THIF08, which degrades COS at concentrations higher than 10,000 parts per million by volume from atmospheric concentrations, and its gene cos (492 bp) was cloned. The recombinant protein purified from Escherichia coli expressing the cos gene converted COS to H2S. The deduced amino acid sequence of COSase (163 amino acids) was assigned to clade D in the phylogenetic tree of the β-carbonic anhydrase (β-CA) family, to which prokaryotic COSase and its structurally related enzymes belong. However, the COSase of strain THIF08 differed from the previously known prokaryotic COSase and its related enzymes due to its low reactivity to CO2 and inability to hydrolyze CS2. Sequence comparisons of the active site amino acids of clade D β-CA family enzymes suggested that various Ascomycota, particularly Sordariomycetes and Eurotiomycetes, possess similar enzymes to the COSase of strain THIF08 with >80% identity. These fungal COSase were phylogenetically distant to prokaryotic clade D β-CA family enzymes. These results suggest that various ascomycetes containing COSase contribute to the uptake of COS by soil.
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This study examined the overall performance of the climate models in Phase 6 of the Coupled Model Intercomparison Project (CMIP6) in simulating the key energy and water fluxes over land. For this purpose, this study selected multiple land flux products as reference data sets and assessed the global spatial means, patterns, trends, seasonal cycles, and regional mean estimates of the sensible heat (SH), latent heat (LH), net radiation (RN), runoff (RF), and precipitation (PR) simulated by 32 CMIP6 models in recent decades. The global (Antarctica, Greenland, and hot deserts are not included) mean SH, LH, RN, RF, and PR simulated by the CMIP6 models are 37.55 ± 4.81 W m⁻², 49.88 ± 5.31 W m⁻², 89.10 ± 4.45 W m⁻², 351.31 ± 95.28 mm yr⁻¹, and 948.35 ± 88.77 mm yr⁻¹, respectively. The ensemble median of CMIP6 simulations (CMIP6‐MED) can provide robust estimates of global and regional land fluxes, which are within the ranges given by the reference data sets, and highly consistent spatiotemporal patterns of these fluxes. The comparison of CMIP6‐MED with the first preferred reference data sets shows that CMIP6‐MED generally overestimates the water and energy fluxes over land, except for the simulated RF and PR in the Amazon region. The most disagreements between CMIP6‐MED and the reference data sets occur in South America (particularly the Amazon region) and the Tibetan Plateau. Finally, the sources of model biases are discussed. It is suggested that current land flux products should be widely used to optimize the structures and parameters of climate models in future work.
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Cities are implementing additional urban green as a means to capture CO2 and become more carbon neutral. However, cities are complex systems where anthropogenic and natural components of the CO2 budget interact with each other, and the ability to measure the efficacy of such measures is still not properly addressed. There is still a high degree of uncertainty in determining the contribution of the vegetation signal, which furthermore confounds the use of CO2 mole fraction measurements for inferring anthropogenic emissions of CO2. Carbonyl sulfide (OCS) is a tracer of photosynthesis which can aid in constraining the biosphere signal. This study explores the potential of using OCS to track the urban biosphere signal. We used the Sulfur Transport and dEposition Model (STEM) to simulate the OCS concentrations and the Carnegie Ames Stanford Approach ecosystem model to simulate global CO2 fluxes over the Bay Area of San Francisco during March 2015. Two observation towers provided measurements of OCS and CO2: The Sutro tower in San Francisco (upwind from the area of study providing background observations), and a tower located at Sandia National Laboratories in Livermore (downwind of the highly urbanized San Francisco region). Our results show that the STEM model works better under stable marine influence, and that the boundary layer height and entrainment are driving the diurnal changes in OCS and CO2 at the downwind Sandia site. However, the STEM model needs to better represent the transport and boundary layer variability, and improved estimates of gross primary productivity for characterizing the urban biosphere signal are needed.
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Plain Language Summary Carbonyl sulfide is an atmospheric gas that can be used to estimate how much carbon plants assimilate during photosynthesis. One of the most important regions for understanding the carbon cycle is the Amazon rainforest. In order to determine how much carbon is being taken up in the Amazon, we use models of the chemistry and movement of the atmosphere to estimate the uptake of carbonyl sulfide and compare our model output to satellite observations. Our research shows that estimates of Amazonian GPP using carbonyl sulfide uptake are consistent with ecosystem models and other types of observations. This builds confidence in our understanding of carbon uptake in the Amazon, which has major implications for climate predictions.
Article
Photosynthesis is a keystone process for the Earth system. The emergence of photosynthesis transformed Earth’s geologic, geochemical, and biologic evolution, and today, virtually all life on Earth depends on this process as a direct or indirect food source. Photosynthesis controls a fundamental link between the global carbon, water, and energy cycles, which underlies central scientific mysteries of the Earth system. In particular, post-industrial growth in global photosynthesis is responsible for one of the largest and most uncertain feedbacks to anthropogenic climate change. Despite its importance, photosynthesis cannot be measured directly at scales larger than the leaf. Historically, measurements of CO₂ gas exchange are suitable for leaf chambers, but at larger scales, this technique is confounded by CO₂ emissions from soils. Theories of global photosynthesis are largely in the realm of computer simulations. Thus, measurement technology limits our ability to pursue questions that are essential to understanding the processes governing the Earth system and impacting our future. To confront this key scientific challenge, the workshop "Next-Generation Approach for Detecting Climate–Carbon Feedbacks: Space-Based Integration of OCS, CO₂, and SIF" assembled a multi-disciplinary team to conceive a new integrated technique for measuring photosynthesis at regional to global scales. The participants merged perspectives from the fields of ecology, biogeochemistry, atmospheric chemistry, and space science to focus on how a rapidly emerging technique with carbonyl sulfide sensors (OCS or COS) could be integrated with existing CO₂ and satellite observations of solar-induced chlorophyll fluorescence (SIF) platforms. The workshop discussions leveraged recent findings from atmospheric OCS observations and plant gas exchange studies that reveal a robust relationship between regional variation in photosynthesis and atmospheric variation in OCS. Plant leaves consume atmospheric OCS gas through a one-way hydration sink, which is controlled by stomatal conductance that is also a primary control on photosynthesis. Atmospheric OCS observations, such as the satellite detection of a massive depletion in OCS over the Amazon, can then provide a measurement-based estimate of photosynthesis. These OCS findings were analyzed within the context of recent breakthroughs from spaceborne SIF analysis. SIF platforms record the electromagnetic energy released from plant leaves during photosynthesis. Strong correlations between SIF and photosynthesis suggest an alternative means of assessing global photosynthesis from space. The key result of this workshop is that these alternative methods fill critical, yet different, methodological gaps, suggesting the need for a unified, space-based, photosynthesis observation platform. First, the highly complementary temporal and spatial scales of SIF analysis provide instantaneous, spatially resolved data and OCS provides spatially and temporally integrated data. Second, the independent photosynthesis processes that need to be constrained include the biochemical SIF constraint and stomatal conductance OCS constraint. Third, the Amazon basin is identified as an ideal domain where the temporally integrated OCS analysis could confront cloud contamination problems of alternative approaches. These outcomes has been used to develop a roadmap for near-, mid-, and long-term activities to achieve this vision for a unified global photosynthesis observing system. Proof-of-concept studies, including an airborne field experiment in the Amazon and an observing system simulation experiment, will provide critical evidence for the proposed satellite observations. The workshop team will collaborate on perspective articles in diverse disciplinary journals and develop a research coordinating network to communicate this new approach to the broad community of scientists and technologists who would be impacted by enabling a large-scale understanding of global photosynthesis.
Article
The increasing amount of measurement data on land-atmosphere flux has made it possible to assess the interannual and longer processes that are driven by environmental change and disturbance of terrestrial ecosystems. In this study, I used a global dataset of carbon dioxide (CO2) fluxes at eddy-covariance tower sites (FLUXNET2015) to investigate long-term trends of net ecosystem CO2 exchange (NEE), gross primary production (GPP), ecosystem respiration (RE), and related variables. From 118 sites with records of at least 5-years duration, I extracted 1198 site-years of data for use in my analyses. Applying moderate screening by data quality, I found that 58% of the sites showed increasing trends as net CO2 sinks, in which median slopes of annual NEE of -1.4 and -4.1 g C m⁻² y⁻² were obtained by linear regression and Sen’s slope estimator. Both GPP and RE showed increasing trends at different slopes; their slopes were positively correlated among sites. Across-site variation of NEE trends was analyzed by generalized linear mixed modeling; the best statistical model used temperature, stand age, and biome type as explanatory variables. The trend of increasing CO2 sinks differed among biome types, from almost none in grassland and savanna to steep slopes in deciduous broad-leaved forest sites. The flux trends derived from terrestrial model simulations showed that the increasing sink trend also prevails over the land. The global model simulations implied that the increasing land sink is primarily attributable to elevated CO2 concentration. These results demonstrate the usefulness of flux measurement datasets, especially in conjunction with models, to deepen our understanding of long-term terrestrial ecosystem processes.
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As anthropogenic carbon emissions continue, it is important to predict how vegetation will change due to the warming climate, increasing atmospheric CO2, and changing precipitation patterns. Some projections show decreases in vegetation either regionally in the tropics (e.g., Falloon et al., 2012; Port et al., 2012) or globally (e.g., Bastin et al., 2019) due to high temperatures and drying, and there is large uncertainty surrounding the response of global vegetation to climate change (Arora et al., 2020; Friedlingstein et al., 2006; Walker et al., 2020). Here we use simulations with the NCAR Community Earth System Model (CESM) with dynamic vegetation to explore how increasing CO2 may affect vegetation and, through feedbacks, the climate. We find that globally, warming only decreases vegetation, but when we include the effects of CO2 on plant physiology, vegetated area increases by ∼23% relative to the preindustrial simulation. This greening occurs even without significant increases in precipitation, as high CO2 increases plant water use efficiency, allowing for more vegetation in arid and hot regions. Our results show that CO2‐induced vegetation changes affect climate by amplifying high latitude warming, cooling some areas of the tropics, and shifting global precipitation patterns. Our results highlight the importance of including climate‐vegetation feedbacks in Earth system modeling.
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Plain Language Summary Accurate estimates of marine carbonyl sulfide (OCS) sources are critical for both modeling stratospheric aerosol concentrations, as OCS is an important precursor to stratospheric sulphate aerosol particles, and for estimating gross primary production, as OCS is readily consumed by the terrestrial biosphere. Despite the importance of OCS to both stratospheric aerosol chemistry and as an effective proxy for CO2 plant uptake, considerable uncertainty remains in the sources and sinks of OCS. A large source of this uncertainty arises in the marine sources, dominated by the oxidation of marine sulfur gases. Here, we examine the global production of OCS from the oxidation of a marine biologically produced molecule, dimethyl sulfide (DMS). We show that the multi‐generational production of OCS proceeds through the oxidation of stable, water‐soluble intermediates. Using a global chemical transport model, we find that OCS production is largest in the tropics, where cloud loss of hydroperoxymethyl thioformate, the primary precursor to OCS in the DMS oxidation mechanism, is at a minimum.
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Leaf phenology is key for regulating total growing season mass and energy fluxes. Long-term temporal trends towards earlier leaf unfolding are observed across Northern Hemisphere forests. Phenological dates also vary between years, whereby end-of-season (EOS) dates correlate positively with start-of-season (SOS) dates and negatively with growing season total net CO 2 assimilation ( A net ). These associations have been interpreted as the effect of a constrained leaf longevity or of premature carbon (C) sink saturation - with far-reaching consequences for long-term phenology projections under climate change and rising CO 2 . Here, we use multi-decadal ground and remote-sensing observations to show that the relationships between A net and EOS are opposite at the interannual and the decadal time scales. A decadal trend towards later EOS persists in parallel with a trend towards increasing A net ‐ in spite of the negative A net ‐EOS relationship at the interannual scale. This indicates that acclimation of phenology has enabled plants to transcend a constrained leaf longevity or premature C sink saturation over the course of several decades, leading to a more effective use of available light and a sustained extension of the vegetation CO 2 uptake season over time.
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Carbonyl sulfide (OCS) is a non‐hygroscopic trace species in the free troposphere and a large sulfur reservoir maintained by both direct oceanic, geologic, biogenic, and anthropogenic emissions and the oxidation of other sulfur‐containing source species. It is the largest source of sulfur transported to the stratosphere during volcanically quiescent periods. Data from 22 ground‐based globally dispersed stations are used to derive trends in total and partial column OCS. Middle infrared spectral data are recorded by solar‐viewing Fourier transform interferometers that are operated as part of the Network for the Detection of Atmospheric Composition Change between 1986 and 2020. Vertical information in the retrieved profiles provides analysis of discreet altitudinal regions. Trends are found to have well‐defined inflection points. In two linear trend time periods ∼2002 to 2008 and ∼2008 to 2016 tropospheric trends range from ∼0.0 to (1.55 ± 0.30%/yr) in contrast to the prior period where all tropospheric trends are negative. Regression analyses show strongest correlation in the free troposphere with anthropogenic emissions. Stratospheric trends in the period ∼2008 to 2016 are positive up to (1.93 ± 0.26%/yr) except notably low latitude stations that have negative stratospheric trends. Since ∼2016, all stations show a free tropospheric decrease to 2020. Stratospheric OCS is regressed with simultaneously measured N2O to derive a trend accounting for dynamical variability. Stratospheric lifetimes are derived and range from (54.1 ± 9.7)yr in the sub‐tropics to (103.4 ± 18.3)yr in Antarctica. These unique long‐term measurements provide new and critical constraints on the global OCS budget.
Article
China announced its national goal to reach the peak of carbon emission by 2030 and achieve carbon neutrality by 2060, during the General Assembly of the United Nations in September 2020. In this context, the potential of the carbon sink in China’s terrestrial ecosystems to mitigate anthropogenic carbon emissions has attracted unprecedented attention from scientific communities, policy makers and the public. Here, we reviewed the assessments on China’s terrestrial ecosystem carbon sink, with focus on the principles, frameworks and methods of terrestrial ecosystem carbon sink estimates, as well as the recent progress and existing problems. Looking forward, we identified critical issues for improving the accuracy and precision of China’s terrestrial ecosystem carbon sink, in order to serve the more realistic policy making in pathways to achieve carbon neutrality for China.
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Fossil fuel combustion, land use change and other human activities have increased the atmospheric carbon dioxide (CO2) abundance by about 50% since the beginning of the industrial age. The atmospheric CO2 growth rates would have been much larger if natural sinks in the land biosphere and ocean had not removed over half of this anthropogenic CO2. As these CO2 emissions grew, uptake by the ocean increased in response to increases in atmospheric CO2 partial pressure (pCO2). On land, gross primary production also increased, but the dynamics of other key aspects of the land carbon cycle varied regionally. Over the past three decades, CO2 uptake by intact tropical humid forests declined, but these changes are offset by increased uptake across mid‐ and high‐latitudes. While there have been substantial improvements in our ability to study the carbon cycle, measurement and modeling gaps still limit our understanding of the processes driving its evolution. Continued ship‐based observations combined with expanded deployments of autonomous platforms are needed to quantify ocean‐atmosphere fluxes and interior ocean carbon storage on policy‐relevant spatial and temporal scales. There is also an urgent need for more comprehensive measurements of stocks, fluxes and atmospheric CO2 in humid tropical forests and across the Arctic and boreal regions, which are experiencing rapid change. Here, we review our understanding of the atmosphere, ocean, and land carbon cycles and their interactions, identify emerging measurement and modeling capabilities and gaps and the need for a sustainable, operational framework to ensure a scientific basis for carbon management.
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Terrestrial biosphere models (TBMs) play a key role in the detection and attribution of carbon cycle processes at local to global scales and in projections of the coupled carbon‐climate system. TBM evaluation commonly involves direct comparison to eddy‐covariance flux measurements. We use atmospheric CO2 mole fraction ([CO2]) measured in situ from aircraft and tower, in addition to flux‐measurements from summer 2016 to evaluate the Carnegie‐Ames‐Stanford‐Approach (CASA) TBM. WRF‐Chem is used to simulate [CO2] using biogenic CO2 fluxes from a CASA parameter‐based ensemble and CarbonTracker version 2017 (CT2017) in addition to transport and CO2 boundary condition ensembles. The resulting “super ensemble” of modeled [CO2] demonstrates that the biosphere introduces the majority of uncertainty to the simulations. Both aircraft and tower [CO2] data show that the CASA ensemble net ecosystem exchange (NEE) of CO2 is biased high (NEE too positive) and identify the maximum light use efficiency Emax a key parameter that drives the spread of the CASA ensemble in summer 2016. These findings are verified with flux‐measurements. The direct comparison of the CASA flux ensemble with flux‐measurements confirms missing sink processes in CASA. Separating the daytime and nighttime flux, we discover that the underestimated net uptake results from missing sink processes that result in overestimation of respiration. NEE biases are smaller in the CT2017 posterior biogenic fluxes, which assimilate observed [CO2]. Flux tower analyses reveal an unrealistic overestimation of nighttime respiration in CT2017 which we attribute to limited flexibility in the inversion strategy.
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Carbonyl sulfide (COS) measurements are one of the emerging tools to better quantify gross primary production (GPP), the largest flux in the global carbon cycle. COS is a gas with a similar structure to CO2; COS uptake is thought to be a proxy for GPP. However, soils are a potential source or sink of COS. This study presents a framework for understanding soil–COS interactions. Excluding wetlands, most of the few observations of isolated soils that have been made show small uptake of atmospheric COS. Recently, a series of studies at an agricultural site in the central United States found soil COS production under hot conditions an order of magnitude greater than fluxes at other sites. To investigate the extent of this phenomenon, soils were collected from five new sites and incubated in a variety of soil moisture and temperature states. We found that soils from a desert, an oak savannah, a deciduous forest, and a rainforest exhibited small COS fluxes, behavior resembling previous studies. However, soil from an agricultural site in Illinois, > 800 km away from the initial central US study site, demonstrated comparably large soil fluxes under similar conditions. These new data suggest that, for the most part, soil COS interaction is negligible compared to plant uptake of COS. We present a model that anticipates the large agricultural soil fluxes so that they may be taken into account. While COS air-monitoring data are consistent with the dominance of plant uptake, improved interpretation of these data should incorporate the soil flux parameterizations suggested here.
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The global budget of tropospheric carbonyl sulfide (OCS) is believed to be at equilibrium because background air concentrations have remained roughly stable over at least the last decade. Since the uptakes of OCS by leaves (associated to photosynthesis) and soils have been revised significantly upwards recently, an equilibrated budget can only be obtained with a compensatory source of OCS. It has been assumed that the missing source of OCS comes from the low latitude ocean, following the incident solar flux. The present work uses parameterizations of major production and removal processes of organic compounds in the NEMO-PISCES Ocean General Circulation and Biogeochemistry Model to assess the marine source of OCS. In addition, the OCS photo-production rates computed with the NEMO-PISCES model were evaluated independently using UV absorption coefficient of chromophoric dissolved organic matter (derived from satellite ocean color) and apparent quantum yields available in the literature. Our simulations show global direct marine emissions of COS in the range of 573–3997 Gg S yr−1, depending mostly on the quantification of the absorption rate of chromophoric dissolved organic matter. The high estimates on that range are unlikely, as they correspond to a formulation that most likely overestimate photo-production process. Low and medium (813 Gg S yr−1) estimates derived from the NEMO-PISCES model are however consistent spatially and temporally with the suggested missing source of Berry et al. (2013), allowing thus to close the global budget of OCS given the recent estimates of leaf and soil OCS uptakes.
Article
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Global environmental change is rapidly altering the dynamics of terrestrial vegetation, with consequences for the functioning of the Earth system and provision of ecosystem services. Yet how global vegetation is responding to the changing environment is not well established. Here we use three long-term satellite leaf area index (LAI) records and ten global ecosystem models to investigate four key drivers of LAI trends during 1982-2009. We show a persistent and widespread increase of growing season integrated LAI (greening) over 25% to 50% of the global vegetated area, whereas less than 4% of the globe shows decreasing LAI (browning). Factorial simulations with multiple global ecosystem models suggest that CO2 fertilization effects explain 70% of the observed greening trend, followed by nitrogen deposition (9%), climate change (8%) and land cover change (LCC) (4%). CO2 fertilization effects explain most of the greening trends in the tropics, whereas climate change resulted in greening of the high latitudes and the Tibetan Plateau. LCC contributed most to the regional greening observed in southeast China and the eastern United States. The regional effects of unexplained factors suggest that the next generation of ecosystem models will need to explore the impacts of forest demography, differences in regional management intensities for cropland and pastures, and other emerging productivity constraints such as phosphorus availability.
Article
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Terrestrial vegetation currently absorbs approximately a third of anthropogenic CO2 emissions, mitigating the rise of atmospheric CO2. However, terrestrial net primary production is highly sensitive to atmospheric CO2 levels and associated climatic changes. In C3 plants, which dominate terrestrial vegetation, net photosynthesis depends on the ratio between photorespiration and gross photosynthesis. This metabolic flux ratio depends strongly on CO2 levels, but changes in this ratio over the past CO2 rise have not been analyzed experimentally. Combining CO2 manipulation experiments and deuterium NMR, we first establish that the intramolecular deuterium distribution (deuterium isotopomers) of photosynthetic C3 glucose contains a signal of the photorespiration/photosynthesis ratio. By tracing this isotopomer signal in herbarium samples of natural C3 vascular plant species, crops, and a Sphagnum moss species, we detect a consistent reduction in the photorespiration/photosynthesis ratio in response to the ∼100-ppm CO2 increase between ∼1900 and 2013. No difference was detected in the isotopomer trends between beet sugar samples covering the 20th century and CO2 manipulation experiments, suggesting that photosynthetic metabolism in sugar beet has not acclimated to increasing CO2 over >100 y. This provides observational evidence that the reduction of the photorespiration/photosynthesis ratio was ca. 25%. The Sphagnum results are consistent with the observed positive correlations between peat accumulation rates and photosynthetic rates over the Northern Hemisphere. Our results establish that isotopomers of plant archives contain metabolic information covering centuries. Our data provide direct quantitative information on the "CO2 fertilization" effect over decades, thus addressing a major uncertainty in Earth system models.
Article
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Atmospheric mass balance analyses suggest that terrestrial carbon (C) storage is increasing, partially abating the atmospheric [CO2] growth rate1, although the continued strength of this important ecosystem service remains uncertain2–6. Some evidence suggests that these increases will persist owing to positive responses of vegetation growth (net primary productivity; NPP) to rising atmospheric [CO2] (that is, ‘CO2 fertilization’)5–8. Here, we present a new satellite-derived global terrestrial NPP data set9–11, which shows a significant increase in NPP from 1982 to 2011. However, comparison against Earth system model (ESM) NPP estimates reveals a significant divergence, with satellite-derivedincreases (2.8 � 1.50%)less than half of ESM-derived increases (7.6�1.67%) over the 30-year period. By isolating the CO2 fertilization e�ect in each NPP time series and comparing it against a synthesis of available free-air CO2 enrichment data12–15, we provide evidence that much of the discrepancy may be due to an over-sensitivity of ESMs to atmospheric [CO2], potentially reflecting an under-representation of climatic feedbacks16–20 and/or a lack of representation of nutrient constraints21–25. Our understanding of CO2 fertilization e�ects on NPP needs rapid improvement to enable more accurate projections of future C cycle–climate feedbacks; we contend that better integration of modelling, satellite and experimental approaches o�ers a promising way forward.
Article
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Carbonyl sulfide (OCS), the most abundant sulfur gas in the atmosphere, has a summer minimum associated with uptake by vegetation and soils, closely correlated with CO2. We report the first direct measurements to our knowledge of the ecosystem flux of OCS throughout an annual cycle, at a mixed temperate forest. The forest took up OCS during most of the growing season with an overall uptake of 1.36 ± 0.01 mol OCS per ha (43.5 ± 0.5 g S per ha, 95% confidence intervals) for the year. Daytime fluxes accounted for 72% of total uptake. Both soils and incompletely closed stomata in the canopy contributed to nighttime fluxes. Unexpected net OCS emission occurred during the warmest weeks in summer. Many requirements necessary to use fluxes of OCS as a simple estimate of photosynthesis were not met because OCS fluxes did not have a constant relationship with photosynthesis throughout an entire day or over the entire year. However, OCS fluxes provide a direct measure of ecosystem-scale stomatal conductance and mesophyll function, without relying on measures of soil evaporation or leaf temperature, and reveal previously unseen heterogeneity of forest canopy processes. Observations of OCS flux provide powerful, independent means to test and refine land surface and carbon cycle models at the ecosystem scale.
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Rahmstorf et al ’s (2012) conclusion that observed climate change is comparable to projections, and in some cases exceeds projections, allows further inferences if we can quantify changing climate forcings and compare those with projections. The largest climate forcing is caused by well-mixed long-lived greenhouse gases. Here we illustrate trends of these gases and their climate forcings, and we discuss implications. We focus on quantities that are accurately measured, and we include comparison with fixed scenarios, which helps reduce common misimpressions about how climate forcings are changing. Annual fossil fuel CO2 emissions have shot up in the past decade at about 3% yr⁻¹, double the rate of the prior three decades (figure 1). The growth rate falls above the range of the IPCC (2001) ‘Marker’ scenarios, although emissions are still within the entire range considered by the IPCC SRES (2000). The surge in emissions is due to increased coal use (blue curve in figure 1), which now accounts for more than 40% of fossil fuel CO2 emissions. Figure 1. CO2 annual emissions from fossil fuel use and cement manufacture, an update of figure 16 of Hansen (2003) using data of British Petroleum (BP 2012) concatenated with data of Boden et al (2012). The resulting annual increase of atmospheric CO2 (12-month running mean) has grown from less than 1 ppm yr⁻¹ in the early 1960s to an average ~2 ppm yr⁻¹ in the past decade (figure 2). Although CO2 measurements were not made at sufficient locations prior to the early 1980s to calculate the global mean change, the close match of global and Mauna Loa data for later years suggests that Mauna Loa data provide a good approximation of global change (figure 2), thus allowing a useful estimate of annual global change beginning with the initiation of Mauna Loa measurements in 1958 by Keeling et al (1973). Figure 2. Annual increase of CO2 based on data from the NOAA Earth System Research Laboratory (ESRL 2012). CO2 change and global temperature change are 12-month running means of differences for the same month of consecutive years. Nino index (Nino3.4 area) is 12-month running mean. Both temperature indices use data from Hansen et al (2010). Annual mean CO2 amount in 1958 was 315 ppm (Mauna Loa) and in 2012 was 394 ppm (Mauna Loa) and 393 ppm (Global). Interannual variability of CO2 growth is correlated with ENSO (El Nino Southern Oscillation) variations of tropical temperatures (figure 2). Ocean–atmosphere CO2 exchange is affected by ENSO (Chavez et al 1999), but ENSO seems to have a greater impact on atmospheric CO2 via the terrestrial carbon cycle through effects on the water cycle, temperature, and fire, as discussed in a large body of literature (referenced, e.g., by Schwalm et al 2011). In addition, volcanoes, such as the 1991 Mount Pinatubo eruption, slow the increase of atmospheric CO2 (Rothenberg et al 2012), at least in part because photosynthesis is enhanced by the increased proportion of diffuse sunlight (Gu et al 2003, Mercado et al 2009). Watson (1997) suggests that volcanic dust deposited on the ocean surface may also contribute to CO2 uptake by increasing ocean productivity. An important question is whether ocean and terrestrial carbon sinks will tend to saturate as human-made CO2 emissions continue. Piao et al (2008) and Zhao and Running (2010) suggest that there already may be a reduction of terrestrial carbon uptake, while Le Quéréet al (2007) and Schuster and Watson (2007) find evidence of decreased carbon uptake in the Southern Ocean and North Atlantic Ocean, respectively. However, others (Knorr 2009, Sarmiento et al 2010, Ballantyne et al 2012) either cast doubt on the reality of a reduced uptake strength or find evidence for increased uptake. An informative presentation of CO2 observations is the ratio of annual CO2 increase in the air divided by annual fossil fuel CO2 emissions (Keeling et al 1973), the ‘airborne fraction’ (figure 3, right scale). An alternative definition of airborne fraction includes in the denominator of this ratio an estimated net anthropogenic CO2 source from changes in land use, but this latter term is much more uncertain than the two terms involved in the Keeling et al (1973) definition. For example, analysis by Harris et al (2012) reveals a range as high as a factor of 2–4 in estimates of recent land use emissions; see also the discussion by Sarmiento et al (2010). However, note that the airborne fraction becomes smaller when estimated land use emissions are included, with the uptake fraction (one minus airborne fraction) typically greater than 0.5. Figure 3. Fossil fuel CO2 emissions (left scale) and airborne fraction, i.e., the ratio of observed atmospheric CO2 increase to fossil fuel CO2 emissions. Final three points are 5-, 3- and 1-year means. The simple Keeling airborne fraction, clearly, is not increasing (figure 3). Thus the net ocean plus terrestrial sink for carbon emissions has increased by a factor of 3–4 since 1958, accommodating the emissions increase by that factor. Remarkably, and we will argue importantly, the airborne fraction has declined since 2000 (figure 3) during a period without any large volcanic eruptions. The 7-year running mean of the airborne fraction had remained close to 60% up to 2000, except for the period affected by Pinatubo. The airborne fraction is affected by factors other than the efficiency of carbon sinks, most notably by changes in the rate of fossil fuel emissions (Gloor et al 2010). However, it is the dependence of the airborne fraction on fossil fuel emission rate that makes the post-2000 downturn of the airborne fraction particularly striking. The change of emission rate in 2000 from 1.5% yr⁻¹ to 3.1% yr⁻¹ (figure 1), other things being equal, would have caused a sharp increase of the airborne fraction (the simple reason being that a rapid source increase provides less time for carbon to be moved downward out of the ocean’s upper layers). A decrease in land use emissions during the past decade (Harris et al 2012) could contribute to the decreasing airborne fraction in figure 3, although Malhi (2010) presents evidence that tropical forest deforestation and regrowth are approximately in balance, within uncertainties. Land use change can be only a partial explanation for the decrease of the airborne fraction; something more than land use change seems to be occurring. We suggest that the huge post-2000 increase of uptake by the carbon sinks implied by figure 3 is related to the simultaneous sharp increase in coal use (figure 1). Increased coal use occurred primarily in China and India (Boden et al 2012; BP 2012; see graphs at www.columbia.edu/~mhs119/Emissions/Emis_moreFigs/). Satellite radiance measurements for July–December, months when desert dust does not dominate aerosol amount, yield an increase of aerosol optical depth in East Asia of about 4% yr⁻¹ during 2000–2006 (van Donkelaar et al 2008). Associated gaseous and particulate emissions increased rapidly after 2000 in China and India (Lu et al 2011, Tian et al 2010). Some decrease of the sulfur component of emissions occurred in China after 2006 as wide application of flue-gas desulfurization began to be initiated (Lu et al 2010), but this was largely offset by continuing emission increases from India (Lu et al 2011). We suggest that the surge of fossil fuel use, mainly coal, since 2000 is a basic cause of the large increase of carbon uptake by the combined terrestrial and ocean carbon sinks. One mechanism by which fossil fuel emissions increase carbon uptake is by fertilizing the biosphere via provision of nutrients essential for tissue building, especially nitrogen, which plays a critical role in controlling net primary productivity and is limited in many ecosystems (Gruber and Galloway 2008). Modeling (e.g., Thornton et al 2009) and field studies (Magnani et al 2007) confirm a major role of nitrogen deposition, working in concert with CO2 fertilization, in causing a large increase in net primary productivity of temperate and boreal forests. Sulfate aerosols from coal burning also might increase carbon uptake by increasing the proportion of diffuse insolation, as noted above for Pinatubo aerosols, even though the total solar radiation reaching the surface is reduced. Thus we see the decreased CO2 airborne fraction since 2000 as sharing some of the same causes as the decreased airborne fraction after the Pinatubo eruption (figure 3). CO2 fertilization is likely the major effect, as a plausible addition of 5 TgN yr⁻¹ from fossil fuels and net ecosystem productivity of 200 kgC kgN⁻¹ (Magnani et al 2007, 2008) yields an annual carbon drawdown of 1 GtC yr⁻¹, which is of the order of what is needed to explain the post-2000 anomaly in airborne CO2. However, an aerosol-induced increase of diffuse radiation might also contribute. Although tropospheric aerosol properties are not accurately monitored, there are suggestions of an upward trend of stratospheric background aerosols since 2000 (Hofmann et al 2009, Solomon et al 2011), which could be a consequence of more tropospheric aerosols at low latitudes where injection of tropospheric air into the stratosphere occurs (Holton et al 1995). We discuss climate implications of the reduced CO2 airborne fraction after presenting data for other greenhouse gases. Atmospheric CH4 is increasing more slowly than in IPCC scenarios (figure 4), which were defined more than a decade ago (IPCC 2001). However, after remaining nearly constant for several years, CH4 has increased during the past five years, pushing slightly above the level that was envisaged in the Alternative Scenario of Hansen et al (2000). Reduction of CH4, besides slowdown in CO2 growth in the twenty first century and a decline of CO2 in the twenty second century, is a principal requirement to achieve a low climate forcing that stabilizes climate, in part because CH4 also affects tropospheric ozone and stratospheric water vapor. The Alternative Scenario, defined in detail by Hansen and Sato (2004), keeps maximum global warming at ~1.5 °C relative to 1880–1920, under the assumption that fast-feedback climate sensitivity is ~3 °C for doubled CO2 (Hansen et al 2007). The Alternative Scenario allows CO2 to reach 475 ppm in 2100 before declining slowly; this scenario assumes that reductions of non-CO2 greenhouse gases and black carbon aerosols can be achieved sufficient to balance the warming effect of likely future decreases of reflective aerosols. Figure 4. Observed atmospheric CH4 amount and scenarios for twenty first century. Alternative scenario (Hansen et al 2000, Hansen and Sato 2004) yields maximum global warming ~1.5 °C above 1880–1920. Other scenarios are from IPCC (2001). Forcing on right hand scale is adjusted forcing, Fa, relative to values in 2000 (Hansen et al 2007). There are anthropogenic sources of CH4 that potentially could be reduced, indeed, the leveling off of CH4 amount during the past 20 years seems to have been caused by decreased venting in oil fields (Simpson et al 2012), but the feasibility of overall CH4 reduction also depends on limiting global warming itself, because of the potential for amplifying climate-CH4 feedbacks (Archer et al 2009, Koven et al 2011). Furthermore, reduction of atmospheric CH4 might become problematic if unconventional mining of gas, such as ‘hydro-fracking’, expands widely (Cipolla 2009), as discussed further below. The growth rate for the total climate forcing by well-mixed greenhouse gases has remained below the peak values reached in the 1970s and early 1980s, has been relatively stable for about 20 years, and is falling below IPCC (2001) scenarios (figure 5). However, the greenhouse gas forcing is growing faster than in the Alternative Scenario. MPTGs and OTGs in figure 5 are Montreal Protocol Trace Gases and Other Trace Gases (Hansen and Sato 2004). Figure 5. Five-year mean of the growth rate of climate forcing by well-mixed greenhouse gases, an update of figure 4 of Hansen and Sato (2004). Forcing calculations use equations of Hansen et al (2000). The moderate uncertainties in radiative calculations affect the scenarios and actual greenhouse gas results equally and thus do not alter the conclusion that the actual forcing falls below that of the IPCC scenarios. If greenhouse gases were the only climate forcing, we would be tempted to infer from Rahmstorf’s conclusion (that actual climate change has exceeded IPCC projections) and our conclusion (that actual greenhouse gas forcings are slightly smaller than IPCC scenarios) that actual climate sensitivity is on the high side of what has generally been assumed. Although that may be a valid inference, the evidence is weakened by the fact that other climate forcings are not negligible in comparison to the greenhouse gases and must be accounted for. Natural forcings, by changing solar irradiance and volcanic aerosols, are well-measured since the late 1970s and included in most IPCC (2007) climate simulations. The difficulty is human-made aerosols. Aerosols are readily detected in satellite observations, but determination of their climate forcing requires accurate knowledge of changes in aerosol amount, size distribution, absorption and vertical distribution on a global basis—as well as simultaneous data on changes in cloud properties to allow inference of the indirect aerosol forcing via induced cloud changes. Unfortunately, the first satellite mission capable of measuring the needed aerosol characteristics (Aerosol Polarimetry Sensor on the Glory satellite, (Mishchenko et al 2007)) suffered a launch failure and as yet there are no concrete plans for a replacement mission. The human-made aerosol climate forcing thus remains uncertain. IPCC (2007) concludes that aerosols are a negative (cooling) forcing, probably between -0.5 and -2.5 W m⁻². Hansen et al (2011), based mainly on analysis of Earth’s energy imbalance, derive an aerosol forcing -1.6 ± 0.3 W m⁻², consistent with an analysis of Murphy et al (2009) that suggests an aerosol forcing about -1.5 W m⁻² (see discussion in Hansen et al (2011)). This large negative aerosol forcing reduces the net climate forcing of the past century by about half (IPCC 2007; figure 1 of Hansen et al 2011). Coincidentally, this leaves net climate forcing comparable to the CO2 forcing alone. Reduction of the net human-made climate forcing by aerosols has been described as a ‘Faustian bargain’ (Hansen and Lacis 1990, Hansen 2009), because the aerosols constitute deleterious particulate air pollution. Reduction of the net climate forcing by half will continue only if we allow air pollution to build up to greater and greater amounts. More likely, humanity will demand and achieve a reduction of particulate air pollution, whereupon, because the CO2 from fossil fuel burning remains in the surface climate system for millennia, the ‘devil’s payment’ will be extracted from humanity via increased global warming. So is the new data we present here good news or bad news, and how does it alter the ‘Faustian bargain’? At first glance there seems to be some good news. First, if our interpretation of the data is correct, the surge of fossil fuel emissions, especially from coal burning, along with the increasing atmospheric CO2 level is ‘fertilizing’ the biosphere, and thus limiting the growth of atmospheric CO2. Also, despite the absence of accurate global aerosol measurements, it seems that the aerosol cooling effect is probably increasing based on evidence of aerosol increases in the Far East and increasing ‘background’ stratospheric aerosols. Both effects work to limit global warming and thus help explain why the rate of global warming seems to be less this decade than it has been during the prior quarter century. This data interpretation also helps explain why multiple warnings that some carbon sinks are ‘drying up’ and could even become carbon sources, e.g., boreal forests infested by pine bark beetles (Kurz et al 2008) and the Amazon rain forest suffering from drought (Lewis et al 2011), have not produced an obvious impact on atmospheric CO2. However, increased CO2 uptake does not necessarily mean that the biosphere is healthier or that the increased carbon uptake will continue indefinitely (Matson et al 2002, Galloway et al 2002, Heimann and Reichstein 2008, Gruber and Galloway 2008). Nor does it change the basic facts about the potential magnitude of the fossil fuel carbon source (figure 6) and the long lifetime of the CO2 in the surface carbon reservoirs (atmosphere, ocean, soil, biosphere) once the fossil fuels are burned (Archer 2005). Fertilization of the biosphere affects the distribution of the fossil fuel carbon among these reservoirs, at least on the short run, but it does not alter the fact that the fossil carbon will remain in these reservoirs for millennia. Figure 6. Fossil fuel CO2 emissions and carbon content (1 ppm atmospheric CO2~2.12 GtC). Historical emissions are from Boden et al (2012). Estimated reserves and potentially recoverable resources are based on energy content values of Energy Information Administration (EIA 2011), German Advisory Council (GAC 2011), and Global Energy Assessment (GEA 2012). We convert energy content to carbon content using emission factors of Table 4.2 of IPCC (2007) for coal, gas, and conventional oil, and, following IPCC, we use an emission factor of unconventional oil the same as that for coal. Humanity, so far, has burned only a small portion (purple area in figure 6) of total fossil fuel reserves and resources. Yet deleterious effects of warming are apparent (IPCC 2007), even though only about half of the warming due to gases now in the air has appeared, the remainder still ‘in the pipeline’ due to the inertia of the climate system (Hansen et al 2011). Already it seems difficult to avoid passing the ‘guardrail’ of no more than 2 °C global warming that was agreed in the Copenhagen Accord of the United Nations Framework Convention on Climate Change (UNFCCC 2010). And Hansen et al (2008), based primarily on paleoclimate data and evidence of deleterious climate impacts already at 385 ppm CO2, concluded that an appropriate initial target for CO2 was 350 ppm, which implied a global temperature limit, relative to 1880–1920 of about 1 °C. What is clear is that most of the remaining fossil fuels must be left in the ground if we are to avoid dangerous human-made interference with climate. The principal implication of our present analysis probably relates to the Faustian bargain. Increased short-term masking of greenhouse gas warming by fossil fuel particulate and nitrogen pollution represents a ‘doubling down’ of the Faustian bargain, an increase in the stakes. The more we allow the Faustian debt to build, the more unmanageable the eventual consequences will be. Yet globally there are plans to build more than 1000 coal-fired power plants (Yang and Cui 2012) and plans to develop some of the dirtiest oil sources on the planet (EIA 2011). These plans should be vigorously resisted. We are already in a deep hole—it is time to stop digging. Acknowledgments We thank ClimateWorks, Energy Foundation, Gerry Lenfest (Lenfest Foundation), Lee Wasserman (Rockefeller Family Foundation), and Stephen Toben (Flora Family Foundation) for research and communications support. References Archer D 2005 Fate of fossil fuel CO2 in geologic time J. Geophys. Res. 110 C09505 Archer D, Buffett B and Brovkin V 2009 Ocean methane hydrates as a slow tipping point in the global carbon cycle Proc. Natl Acad. 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