I. Baker

Colorado State University, Fort Collins, Colorado, United States

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Publications (104)141.05 Total impact

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    ABSTRACT: Climate change is leading to a disproportionately large warming in the high northern latitudes, but the magnitude and sign of the future carbon balance of the Arctic are highly uncertain. Using 40 terrestrial biosphere models for Alaska, we provide a baseline of terrestrial carbon cycle structural and parametric uncertainty, defined as the multi-model standard deviation (σ) against the mean (x\bar) for each quantity. Mean annual uncertainty (σ/x\bar) was largest for net ecosystem exchange (NEE) (-0.01± 0.19 kg C m-2 yr-1), then net primary production (NPP) (0.14 ± 0.33 kg C m-2 yr-1), autotrophic respiration (Ra) (0.09 ± 0.20 kg C m-2 yr-1), gross primary production (GPP) (0.22 ± 0.50 kg C m-2 yr-1), ecosystem respiration (Re) (0.23 ± 0.38 kg C m-2 yr-1), CH4 flux (2.52 ± 4.02 g CH4 m-2 yr-1), heterotrophic respiration (Rh) (0.14 ± 0.20 kg C m-2 yr-1), and soil carbon (14.0± 9.2 kg C m-2). The spatial patterns in regional carbon stocks and fluxes varied widely with some models showing NEE for Alaska as a strong carbon sink, others as a strong carbon source, while still others as carbon neutral. Additionally, a feedback (i.e., sensitivity) analysis was conducted of 20th century NEE to CO2 fertilization (β) and climate (γ), which showed that uncertainty in γ was 2x larger than that of β, with neither indicating that the Alaskan Arctic is shifting towards a certain net carbon sink or source. Finally, AmeriFlux data are used at two sites in the Alaskan Arctic to evaluate the regional patterns; observed seasonal NEE was captured within multi-model uncertainty. This assessment of carbon cycle uncertainties may be used as a baseline for the improvement of experimental and modeling activities, as well as a reference for future trajectories in carbon cycling with climate change in the Alaskan Arctic.
    01/2014; 11(2).
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    ABSTRACT: Climate change is leading to a disproportionately large warming in the high northern latitudes, but the magni-tude and sign of the future carbon balance of the Arctic are highly uncertain. Using 40 terrestrial biosphere models for the Alaskan Arctic from four recent model intercomparison projects – NACP (North American Carbon Program) site and regional syntheses, TRENDY (Trends in net land atmosphere carbon exchanges), and WETCHIMP (Wetland and Wetland CH 4 Inter-comparison of Models Project) – we provide a baseline of terrestrial carbon cycle uncertainty, defined as the multi-model standard deviation (σ) for each quantity that follows. Mean annual absolute uncertainty was largest for soil carbon (14.0 ± 9.2 kg C m −2), then gross primary pro-duction (GPP) (0.22 ± 0.50 kg C m −2 yr −1), ecosystem res-piration (Re) (0.23 ± 0.38 kg C m −2 yr −1), net primary pro-duction (NPP) (0.14 ± 0.33 kg C m −2 yr −1), autotrophic res-piration (Ra) (0.09 ± 0.20 kg C m −2 yr −1), heterotrophic res-piration (Rh) (0.14 ± 0.20 kg C m −2 yr −1), net ecosystem ex-change (NEE) (−0.01 ± 0.19 kg C m −2 yr −1), and CH 4 flux (2.52 ± 4.02 g CH 4 m −2 yr −1). There were no consistent spa-tial patterns in the larger Alaskan Arctic and boreal regional carbon stocks and fluxes, with some models showing NEE for Alaska as a strong carbon sink, others as a strong car-bon source, while still others as carbon neutral. Finally, AmeriFlux data are used at two sites in the Alaskan Arc-tic to evaluate the regional patterns; observed seasonal NEE was captured within multi-model uncertainty. This assess-ment of carbon cycle uncertainties may be used as a base-line for the improvement of experimental and modeling ac-tivities, as well as a reference for future trajectories in car-bon cycling with climate change in the Alaskan Arctic and larger boreal region.
    Biogeosciences 01/2014; 11(18):4271-4288. · 3.75 Impact Factor
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    ABSTRACT: Significant changes in the water cycle are expected under current global environmental change. Robust assessment of these changes at global scales is confounded by shortcomings in the observed record. Modeled assessments yield conflicting results which are linked to differences in model structure and simulation protocol. Here we compare simulated runoff from six terrestrial biosphere models (TBMs), five reanalysis products, and one gridded surface station product with observations from a network of stream gauges in the contiguous United States (CONUS) from 2001 to 2005. We evaluate the consistency of simulated runoff with stream gauge data at the CONUS and water resource region scale, as well as examining similarity across TBMs and reanalysis products at the grid cell scale. Mean runoff across all simulated products and regions varies widely (range: 71-356 mm yr-1) relative to observed continental-scale runoff (209 mm yr-1). Across all 12 products only two are within 10% of the observed value and only four exhibit Nash-Sutcliffe efficiency values in excess of 0.8. Region-level mismatch exhibits a weak pattern of overestimation in western and underestimation in eastern regions; although two products are systematically biased across all regions. In contrast, bias in a temporal sense, within region by water year, is highly consistent. Although gridded composite TBM and reanalysis runoff show some regional similarities for 2001-2005 with CONUS means, individual product values are highly variable. To further constrain simulated runoff and to link model-observation mismatch to model structural characteristics would require watershed-level simulation studies coupled with river routing schemes, standardized forcing data, and explicit consideration of water cycle management.
    12/2013; 11(1).
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    ABSTRACT: Surface ecophysiology at five sites in tropical South America across vegetation and moisture gradients is investigated. From the moist northwest (Manaus) to the relatively dry southeast (Pé de Gigante, state of São Paulo) simulated seasonal cycles of latent and sensible heat, and carbon flux produced with the Simple Biosphere Model (SiB3) are confronted with observational data. In the northwest, abundant moisture is available, suggesting that the ecosystem is light-limited. In these wettest regions, Bowen ratio is consistently low, with little or no annual cycle. Carbon flux shows little or no annual cycle as well; efflux and uptake are determined by high-frequency variability in light and moisture availability. Moving downgradient in annual precipitation amount, dry season length is more clearly defined. In these regions, a dry season sink of carbon is observed and simulated. This sink is the result of the combination of increased photosynthetic production due to higher light levels, and decreased respiratory efflux due to soil drying. The differential response time of photosynthetic and respiratory processes produce observed annual cycles of net carbon flux. In drier regions, moisture and carbon fluxes are in-phase; there is carbon uptake during seasonal rains and efflux during the dry season. At the driest site, there is also a large annual cycle in latent and sensible heat flux.
    Agricultural and Forest Meteorology 12/2013; · 3.89 Impact Factor
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    ABSTRACT: [1] Carbonyl sulfide (COS) is an atmospheric trace gas that participates in some key reactions of the carbon cycle and thus holds great promise for studies of carbon cycle processes. Global monitoring networks and atmospheric sampling programs provide concurrent data on COS and CO2 concentrations in the free troposphere and atmospheric boundary layer over vegetated areas. Here we present a modeling framework for interpreting these data and illustrate what COS measurements might tell us about carbon cycle processes. We implemented mechanistic and empirical descriptions of leaf and soil COS uptake into a global carbon cycle model (SiB 3) to obtain new estimates of the COS land flux. We then introduced these revised boundary conditions to an atmospheric transport model (Parameterized Chemical Transport Model) to simulate the variations in the concentration of COS and CO2 in the global atmosphere. To balance the threefold increase in the global vegetation sink relative to the previous baseline estimate, we propose a new ocean COS source. Using a simple inversion approach, we optimized the latitudinal distribution of this ocean source and found that it is concentrated in the tropics. The new model is capable of reproducing the seasonal variation in atmospheric concentration at most background atmospheric sites. The model also reproduces the observed large vertical gradients in COS between the boundary layer and free troposphere. Using a simulation experiment, we demonstrate that comparing drawdown of CO2 with COS could provide additional constraints on differential responses of photosynthesis and respiration to environmental forcing. The separation of these two distinct processes is essential to understand the carbon cycle components for improved prediction of future responses of the terrestrial biosphere to changing environmental conditions.
    Journal of Geophysical Research: Biogeosciences 06/2013; 118(2). · 3.02 Impact Factor
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    ABSTRACT: There is a continued need for models to improve consistency and agreement with observations [Friedlingstein et al., 2006], both overall and under more frequent extreme climatic events related to global environmental change such as drought [Trenberth et al., 2007]. Past validation studies of terrestrial biosphere models have focused only on few models and sites, typically in close proximity and primarily in forested biomes [e.g., Amthor et al., 2001; Delpierre et al., 2009; Grant et al., 2005; Hanson et al., 2004; Granier et al., 2007; Ichii et al., 2009; Ito, 2008; Siqueira et al., 2006; Zhou et al., 2008]. Furthermore, assessing model‐data agreement relative to drought requires, in addition to high‐quality observedCO2 exchange data, a reliable drought metric as well as a natural experiment across sites and drought conditions.
    Journal of Geophysical Research 02/2013; 115. · 3.17 Impact Factor
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    ABSTRACT: Export Date: 25 July 2013, Source: Scopus, Article in Press
    Agricultural and Forest Meteorology; 01/2013
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    Biogeosciences 01/2013; 10(11):6893-6909. · 3.75 Impact Factor
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    ABSTRACT: This data set provides standardized output variables for gross primary productivity (GPP), net ecosystem exchange (NEE), leaf area index (LAI), ecosystem respiration (Re), latent heat flux (LE), and sensible heat flux (H) from 24 terrestrial biosphere models for 47 eddy covariance flux tower sites in North America. Each model used standardized input data for each flux tower site (i.e., gap-filled, locally observed weather; land use history; and other site specific data) and followed standard model setup and spinup procedures. The files also contain gap-filled observations and total uncertainty estimates. The data set was compiled for the North American Carbon Program (NACP) Site-Level Synthesis for use in model inter-comparison and assessment of how well the models simulate carbon processes across vegetation types and environmental conditions in North America. There is one compressed (.zip) file with this data set. When expanded, the .zip file contains model output data for one variable at one site. The model output and observations are available at the native half-hourly time step, or in daily, monthly, and annual aggregations, in comma-separated text (.csv) format.
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    ABSTRACT: Correct representations of root functioning, such as root water uptake and hydraulic redistribution, are critically important for modeling the responses of vegetation to droughts and seasonal changes in soil moisture content. However, these processes are poorly represented in global land surface models. In this study, we incorporated two root functions: a root water uptake function which assumes root water uptake efficiency varies with rooting depth, and a hydraulic redistribution function into a global land surface model, CABLE. The water uptake function developed by Lai and Katul (2000) was also compared with the default one (see Wang et al., 2010) that assumes that efficiency of water uptake per unit root length is constant. Using eddy flux measurements of CO2 and water vapor fluxes at three sites experiencing different patterns of seasonal changes in soil water content, we showed that the two root functions significantly improved the agreement between the simulated fluxes of net ecosystem exchange and latent heat flux and soil moisture dynamics with those observed during the dry season while having little impact on the model simulation during the wet seasons at all three sites. Sensitivity analysis showed that varying several model parameters influencing soil water dynamics in CABLE did not significantly affect the model's performance. We conclude that these root functions represent a valuable improvement for land surface modeling and should be implemented into CABLE and other land surface models for studying carbon and water dynamics where rainfall varies seasonally or interannually.
    Journal of Geophysical Research 10/2012; · 3.17 Impact Factor
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    ABSTRACT: Understanding of carbon exchange between terrestrial ecosystems and the atmosphere can be improved through direct observations and experiments, as well as through modeling activities. Terrestrial biosphere models (TBMs) have become an integral tool for extrapolating local observations and understanding to much larger terrestrial regions. Although models vary in their specific goals and approaches, their central role within carbon cycle science is to provide a better understanding of the mechanisms currently controlling carbon exchange. Recently, the North American Carbon Program (NACP) organized several interim-synthesis activities to evaluate and inter-compare models and observations at local to continental scales for the years 2000–2005. Here, we compare the results from the TBMs collected as part of the regional and continental interim-synthesis (RCIS) activities. The primary objective of this work is to synthesize and compare the 19 participating TBMs to assess current understanding of the terrestrial carbon cycle in North America. Thus, the RCIS focuses on model simulations available from analyses that have been completed by ongoing NACP projects and other recently published studies. The TBM flux estimates are compared and evaluated over different spatial (1° × 1° and spatially aggregated to different regions) and temporal (monthly and annually) scales. The range in model estimates of net ecosystem productivity (NEP) for North America is much narrower than estimates of productivity or respiration, with estimates of NEP varying between −0.7 and 2.2 PgC yr−1, while gross primary productivity and heterotrophic respiration vary between 12.2 and 32.9 PgC yr−1 and 5.6 and 13.2 PgC yr−1, respectively. The range in estimates from the models appears to be driven by a combination of factors, including the representation of photosynthesis, the source and of environmental driver data and the temporal variability of those data, as well as whether nutrient limitation is considered in soil carbon decomposition. The disagreement in current estimates of carbon flux across North America, including whether North America is a net biospheric carbon source or sink, highlights the need for further analysis through the use of model runs following a common simulation protocol, in order to isolate the influences of model formulation, structure, and assumptions on flux estimates.Highlights► Evaluating model results provides assessment of understanding of the terrestrial carbon cycle. ► The models differ substantially in their estimates of net ecosystem productivity. ► Much of the variability in modeled respiration is likely driven by variability in GPP. ► Models can predict plausible values for net ecosystem productivity, but for the wrong reasons. ► Details studies are needed to understand how model formulation and choices impact model performance.
    Ecological Modelling 07/2012; 232:144-157. · 2.07 Impact Factor
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    ABSTRACT: Accurately simulating gross primary productivity (GPP) in terrestrial ecosystem models is critical because errors in simulated GPP propagate through the model to introduce additional errors in simulated biomass and other fluxes. We evaluated simulated, daily average GPP from 26 models against estimated GPP at 39 eddy covariance flux tower sites across the United States and Canada. None of the models in this study match estimated GPP within observed uncertainty. On average, models overestimate GPP in winter, spring, and fall, and underestimate GPP in summer. Models overpredicted GPP under dry conditions and for temperatures below 0�C. Improvements in simulated soil moisture and ecosystem response to drought or humidity stress will improve simulated GPP under dry conditions. Adding a low-temperature response to shut down GPP for temperatures below 0�C will reduce the positive bias in winter, spring, and fall and improve simulated phenology. The negative bias in summer and poor overall performance resulted from mismatches between simulated and observed light use efficiency (LUE). Improving simulated GPP requires better leaf-to-canopy scaling and better values of model parameters that control the maximum potential GPP, such as ɛmax (LUE), Vcmax (unstressed Rubisco catalytic capacity) or Jmax (the maximum electron transport rate).
    Journal of Geophysical Research 01/2012; 117:G03010. · 3.17 Impact Factor
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    ABSTRACT: Understanding of carbon exchange between terrestrial ecosystems and the atmosphere can be improved through direct observations and experiments, as well as through modeling activities. Terrestrial biosphere models (TBMs) have become an integral tool for extrapolating local observations and understanding to much larger terrestrial regions. Although models vary in their specific goals and approaches, their central role within carbon cycle science is to provide a better understanding of the mechanisms currently controlling carbon exchange. Recently, the North American Carbon Program (NACP) organized several interim-synthesis activities to evaluate and inter-compare models and observations at local to continental scales for the years 2000 to 2005. Here, we compare the results from the TBMs collected as part of the regional and continental interim-synthesis (RCIS) activities. The primary objective of this work is to synthesize and compare the 19 participating TBMs to assess current understanding of the terrestrial carbon cycle in North America.
    Ecological Modelling - ECOL MODEL. 01/2012; 232.
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    ABSTRACT: Interannual variability in biosphere-atmosphere exchange of CO2 is driven by a diverse range of biotic and abiotic factors. Replicating this variability thus represents the ‘acid test’ for terrestrial biosphere models. Although such models are commonly used to project responses to both normal and anomalous variability in climate, they are rarely tested explicitly against inter-annual variability in observations. Herein, using standardized data from the North American Carbon Program, we assess the performance of 16 terrestrial biosphere models and 3 remote sensing products against long-term measurements of biosphere-atmosphere CO2 exchange made with eddy-covariance flux towers at 11 forested sites in North America. Instead of focusing on model-data agreement we take a systematic, variability-oriented approach and show that although the models tend to reproduce the mean magnitude of the observed annual flux variability, they fail to reproduce the timing. Large biases in modeled annual means are evident for all models. Observed interannual variability is found to commonly be on the order of magnitude of the mean fluxes. None of the models consistently reproduce observed interannual variability within measurement uncertainty. Underrepresentation of variability in spring phenology, soil thaw and snowpack melting, and difficulties in reproducing the lagged response to extreme climatic events are identified as systematic errors, common to all models included in this study.
    Global Change Biology 01/2012; · 8.22 Impact Factor
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    ABSTRACT: Global carbon and water cycles are inextricably linked through photosynthesis and the nexus between these two biogeochemical cycles is stomatal conductance. It is estimated that approximately 35,000 Pg H20 and 360 Pg C pass through stomates annually. Thus, understanding how stomatal conductance responds to concomitant increases in atmospheric CO2, surface temperatures, and atmospheric water vapor is critical for constraining climate predictions. First, we use a global dataset of atmospheric CO2 isotopes (13CO2) to demonstrate that over the growth season source 13CO2 to the atmosphere varies in response to stomatal conductance. We then test two commonly-used stomatal conductance functions, based on relative humidity and vapor pressure deficit (VPD). We determine that the VPD-based function is better at predicting stomatal conductance on a global scale. Lastly, we use an ensemble of model simulations to predict stomatal conductance in response to increasing temperatures. What we discovered, is that relative humidity will remain constant, but vapor pressure deficit will increase significantly. Therefore future increases in vapor pressure deficit may greatly curtail the amount of carbon assimilated and water transpired by the terrestrial biosphere, resulting in a positive climate-carbon feedback.
    AGU Fall Meeting Abstracts. 12/2011;
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    ABSTRACT: It has been shown that there is a correlation between carbon uptake and diffuse fraction of radiation. This correlation can be direct, as higher diffuse fraction can result in greater penetration of radiation into the canopy. There are indirect effects as well, as greater diffuse fraction (and decreased overall insolation) may be associated with decreased temperature and vapor pressure deficit (VPD), both of which are associated with an increase in photosynthesis. In this study, we evaluate conditions at two eddy covariance flux towers, located within 20 km of each other, in the Tapajos River National Forest, Brazil. One tower is frequently underneath a regularly-occurring cloud band and the other is not, providing a situation where sites with similar vegetation experience different forcing regimes. While there are slight differences between the sites in terms of vegetation and topography, we do not find a systematic difference in ecosystem behavior at the sites on days when the meteorology is comparable at the 2 towers. We compare within- and between-site carbon flux during clear and cloudy days, and evaluate carbon flux relationship to variability in radiation, temperature, and moisture regimes. Using a mixture of observational data and model analysis, we describe the contrast in diurnal cycles of energy, moisture and carbon flux imposed by systematic differences in atmospheric forcing regime.
    AGU Fall Meeting Abstracts. 12/2011;
  • I. Shiach, I. T. Baker, A. Denning
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    ABSTRACT: The response of terrestrial fluxes of energy, water, and carbon to drought is evaluated. Major droughts should be clearly evident in reanalyzed precipitation data, although this is not always the case. With reduced precipitation we can expect a suppression in Gross Primary Photosynthesis (GPP) if physiological stress is sufficient, with atttendant changes in energy partitioning due to stomatal closure. There may also be a response in respiratory release of CO2 with temperature increase. This study aimed to investigate the behavior of the terrestrial biosphere using the Simple Biosphere Model (SiB3) during and following times of drought and to identify any model responses inconsistent with observational relationships. The Standardized Precipitation Index (SPI) was evaluated from 1983 to 2006 in order to evaluate historical drought maps, and to facilitate a qualitative analysis of modeled drought behavior. Standardized and raw anomaly maps were produced for modeled physiological variables (GPP, transpiration, respiration, heat fluxes, carbon flux, and stress factors) in order to determine general response patterns for comparison with observations. The SiB model was determined to be generally accurate in its representation of significant drought, with regard to perturbations in Bowen ratio, GPP, and CO2 respiration. However, model response was heterogeneous, and did not always respond in a manner consistent with published descriptions of drought.
    AGU Fall Meeting Abstracts. 12/2011;
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    ABSTRACT: Since terrestrial carbon fluxes cannot be measured directly on regional and global scales, land surface models are an important tool in improving estimates of carbon sources and sinks. One common limitation in biosphere models is requiring the use of remotely sensed data to represent vegetation phenology; however, prognostic phenology models are being developed to predict the phonological timing and leaf state of both natural vegetation and crops (Stockli et al., 2008; Lokupitiya et al., 2009; Stockli et al., 2011). Simulating phenology rather than relying on data products removes the uncertainty due to satellite retrievals, allows the short yet highly productive growing season of crops to be more accurately simulated, and enables predictive capabilities. The Simple Biosphere Model (SiB) has been modified to include prognostic phenology for twenty different plant functional types, including maize, soybean and wheat. Predicting the phenology will alter carbon fluxes regionally and globally on diurnal to seasonal timescales, and this study will discuss the impact of prognostic phenology on the resulting simulated net ecosystem exchange of carbon dioxide.
    AGU Fall Meeting Abstracts. 12/2011;
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    ABSTRACT: Understanding biospheric CO2 fluxes is paramount if climate studies are to be able to analyze the response of terrestrial ecosystems to climate change and monitor fossil fuel emissions reductions. Carbonyl sulfide (COS) may be a useful tracer to provide a constraint on photosynthesis [gross primary production (GPP)]. Here we simulate both COS and CO2 using the Stochastic Time-Inverted Lagrangian Transport (STILT) model coupled with various biospheric fluxes, such as fluxes estimated from the Vegetation Photosynthesis and Respiration Model (VPRM), CarbonTracker, and from the Carnegie-Ames-Stanford Approach (CASA) model. The STILT model is driven by Weather Research and Forecast (WRF) meteorological fields. The WRF-STILT system is compared with the STILT driven by the ECMWF (European Center for Medium range Weather Forecasting) meteorology for the North American domain. This study uses measurements of COS and CO2 in 2008 from the NOAA/ESRL tall tower and aircraft air sampling networks, with ~ 6,000 observations in total. Biospheric COS fluxes will be estimated from a GPP-based model coupled with the GPP estimates from above mentioned biosphere models. Soil uptakes of COS are derived from a biosphere model (SiB) that assimilates the soil moisture and temperature. Estimation of other COS fluxes, such as anthropogenic, biomass burning are based on existing analyses of temporal and spatial variations. Empirical boundary curtains are built based on observations at the NOAA/ESRL marine boundary layer stations and from aircraft vertical profiles, and are utilized as the lateral boundary conditions for COS and CO2 for North America. Comparison of the simulations for both COS and CO2 using different biospheric fluxes provides an opportunity to assess the performance of both the biospheric models and the representation of atmospheric transport. In addition, we will estimate the carbon fluxes for North America from a joint inversion for COS and CO2 in a Bayesian synthesis framework, in which the GPP and Respiration are separately optimized for each vegetation type.
    AGU Fall Meeting Abstracts. 12/2011;
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    ABSTRACT: Carbonyl sulfide (OCS) is an atmospheric trace gas that is taken up by leaves in parallel with photosynthetic uptake of CO2 (GPP). The latter is an important component of carbon cycle models and accurate measurements of photosynthesis are needed to test these models. However, it is difficult to obtain this information from atmospheric measurements of CO2 as air depleted of CO2 by GPP mixes very near the surface with air enriched in CO2 by respiration. Gradients in CO2, thus reflect the net sum of these two processes. In contrast, there is essentially no source of OCS in terrestrial ecosystems. The primary source of OCS is the oceans, thus mixing between this source and the photosynthetic sink occurs in the more remote atmosphere. This has led several researchers to postulate that gradients or time dependent changes in the concentration of OCS in air masses in contact with continental surfaces might be used to infer the rate of GPP of these surfaces. Several recent studies support this inference. However, it is important to note that these observations occur in a global context. The magnitude, spatial distribution and temporal dynamics of OCS sources and sinks are still poorly defined. This presentation will review recent efforts to improve our representation of the global cycle of OCS. We summarize new physiological measurements of exchange of OCS and CO2 by leaves including a wide range of species and environmental conditions, and report a new parameterization for OCS uptake for SiB3. We use these new simulations of surface exchanges in an atmospheric transport model, PCTM to simulate the concentration of OCS at points and times corresponding to flask measurements. While there remain important uncertainties about soil uptake and the ocean source, these simulations agree fairly well with observations.
    AGU Fall Meeting Abstracts. 12/2011;