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Path-dependent reductions in CO2 emission budgets caused by permafrost carbon release

DOI:10.1038/s41561-018-0227-0
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T. Gasser
T. Gasser
T. Gasser
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M. Kechiar
M. Kechiar
M. Kechiar
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P. Ciais
P. Ciais
P. Ciais
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Eleanor J. Burke at Met Office
Eleanor J. Burke
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Abstract and Figures

Emission budgets are defined as the cumulative amount of anthropogenic CO2 emission compatible with a global temperature-change target. The simplicity of the concept has made it attractive to policy-makers, yet it relies on a linear approximation of the global carbon–climate system’s response to anthropogenic CO2 emissions. Here we investigate how emission budgets are impacted by the inclusion of CO2 and CH4 emissions caused by permafrost thaw, a non-linear and tipping process of the Earth system. We use the compact Earth system model OSCAR v2.2.1, in which parameterizations of permafrost thaw, soil organic matter decomposition and CO2 and CH4 emission were introduced based on four complex land surface models that specifically represent high-latitude processes. We found that permafrost carbon release makes emission budgets path dependent (that is, budgets also depend on the pathway followed to reach the target). The median remaining budget for the 2 °C target reduces by 8% (1–25%) if the target is avoided and net negative emissions prove feasible, by 13% (2–34%) if they do not prove feasible, by 16% (3–44%) if the target is overshot by 0.5 °C and by 25% (5–63%) if it is overshot by 1 °C. (Uncertainties are the minimum-to-maximum range across the permafrost models and scenarios.) For the 1.5 °C target, reductions in the median remaining budget range from ~10% to more than 100%. We conclude that the world is closer to exceeding the budget for the long-term target of the Paris Climate Agreement than previously thought. © 2018, The Author(s), under exclusive licence to Springer Nature Limited.
Illustration of the three budget-calculation approaches used in this study Exceedance budgets (red) are the amount of CO2 that can be emitted before exceeding a given temperature target. Avoidance budgets (blue) are the amount of CO2 that can be emitted while staying below the target. Capture budgets (yellow) are the amount of CO2 that needs be captured when the target temperature is overshot by a given level. Capture budgets are combined with avoidance budgets to give the net overshoot budgets. Methods and Supplementary Fig. 1 give technical details on how these budgets are calculated. texc is the time at which the target is exceeded (for exceedance budgets), tavo is when temperature peaks at the targeted level (for avoidance budgets), tpeak is when temperature peaks at a given overshooting level above the targeted level (for overshoot budgets) and tcap is when temperature returns below the targeted level (for capture budgets and hence also overshoot budgets).
Illustration of the three budget-calculation approaches used in this study Exceedance budgets (red) are the amount of CO2 that can be emitted before exceeding a given temperature target. Avoidance budgets (blue) are the amount of CO2 that can be emitted while staying below the target. Capture budgets (yellow) are the amount of CO2 that needs be captured when the target temperature is overshot by a given level. Capture budgets are combined with avoidance budgets to give the net overshoot budgets. Methods and Supplementary Fig. 1 give technical details on how these budgets are calculated. texc is the time at which the target is exceeded (for exceedance budgets), tavo is when temperature peaks at the targeted level (for avoidance budgets), tpeak is when temperature peaks at a given overshooting level above the targeted level (for overshoot budgets) and tcap is when temperature returns below the targeted level (for capture budgets and hence also overshoot budgets).
… 
Change in emission budgets caused by permafrost carbon release The different budget-calculation approaches include different levels of overshoot and the avoidance budgets based on two subgroups of scenarios: NetNegEm0 and NetNegEm+. The uncertainty bars show the full range and the symbols show the average, across all permafrost models and scenarios. a, Absolute reductions in emission budgets. b, Relative reductions in the remaining budgets, calculated by assuming 2,240 Gt CO2 (ref. ⁴⁴) was emitted between 1870 (the preindustrial reference year) and 2017 (Methods). To better isolate the effect of permafrost carbon in b, we present values under a constant present-day non-CO2 radiative forcing (other non-CO2 backgrounds are available in Supplementary Table 1). The grey hashed area represents a permafrost-induced reduction of 100% or more, which would imply that the budget was already exceeded in 2017.
Change in emission budgets caused by permafrost carbon release The different budget-calculation approaches include different levels of overshoot and the avoidance budgets based on two subgroups of scenarios: NetNegEm0 and NetNegEm+. The uncertainty bars show the full range and the symbols show the average, across all permafrost models and scenarios. a, Absolute reductions in emission budgets. b, Relative reductions in the remaining budgets, calculated by assuming 2,240 Gt CO2 (ref. ⁴⁴) was emitted between 1870 (the preindustrial reference year) and 2017 (Methods). To better isolate the effect of permafrost carbon in b, we present values under a constant present-day non-CO2 radiative forcing (other non-CO2 backgrounds are available in Supplementary Table 1). The grey hashed area represents a permafrost-induced reduction of 100% or more, which would imply that the budget was already exceeded in 2017.
… 
Path dependency of the permafrost-induced budget reductions Reductions in avoidance and overshoot budgets are compared to reductions in exceedance budgets (reference case of 100%). Each type of budget represents a different archetype of pathway, and they are roughly sorted by the increasing intensity of the permafrost effect. The uncertainty bars show the full range of our results. Values >100% mean that the emission budgets are more largely reduced than in the exceedance case. If the permafrost effect was path independent, all the points would be close to 100%.
Path dependency of the permafrost-induced budget reductions Reductions in avoidance and overshoot budgets are compared to reductions in exceedance budgets (reference case of 100%). Each type of budget represents a different archetype of pathway, and they are roughly sorted by the increasing intensity of the permafrost effect. The uncertainty bars show the full range of our results. Values >100% mean that the emission budgets are more largely reduced than in the exceedance case. If the permafrost effect was path independent, all the points would be close to 100%.
… 
Contribution of CH4 released by permafrost thaw to the budget reductions a,b, Absolute (a) and relative (b) terms with regard to the values shown in Fig. 2a. Uncertainty bars show the full range and symbols show the average, across all permafrost models and scenarios.
Contribution of CH4 released by permafrost thaw to the budget reductions a,b, Absolute (a) and relative (b) terms with regard to the values shown in Fig. 2a. Uncertainty bars show the full range and symbols show the average, across all permafrost models and scenarios.
… 
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Articles
https://doi.org/10.1038/s41561-018-0227-0
1International Institute for Applied Systems Analysis (IIASA), Laxenburg, Austria. 2École Polytechnique, Palaiseau, France. 3Laboratoire des Sciences du
Climat et de l’Environnement, LSCE/IPSL, Université Paris-Saclay, CEA – CNRS – UVSQ, Gif-sur-Yvette, France. 4Met Office Hadley Centre, Exeter, UK.
5Max Planck Institut für Meteorologie, Hamburg, Germany. 6Climate and Environmental Physics, Physics Institute, University of Bern, Bern, Switzerland.
7Oeschger Centre for Climate Change Research, University of Bern, Bern, Switzerland. *e-mail: gasser@iiasa.ac.at
Sometimes called ‘carbon budgets’, cumulative anthropogenic
CO2 emission budgets compatible with a given global mean
warming target have been evaluated in many ways1–10. Yet, only
a handful of the studies11–13 made (incomplete) preliminary attempts
to account for permafrost thaw. The additional emission of CO2 and
CH4 caused by this natural process triggered by warming in the high
latitudes13,14 will diminish the budget of CO2 humankind can emit
to keep below a certain level of global warming. Permafrost carbon
release is also an irreversible process over the course of a few centu-
ries13,14, and may thus be considered a ‘tipping’ element of the Earth’s
carbon–climate system15, which puts the linear approximation of
the emission budget framework1,4,5,16,17 to the test.
To quantify the impact of permafrost carbon release on emission
budgets, we use an Earth system model of reduced complexity whose
processes are parameterized to faithfully emulate more complex
models. OSCAR v2.2.1 (a minor update of v2.2 (ref. 18)) was run in its
default configuration, which is comparable to the median of its prob-
abilistic set-up. Therefore, all our results are for about a 50% chance
of meeting the temperature targets. OSCAR is extended here with a
new permafrost carbon module that emulates four state-of-the-art
land surface models: JSBACH (Methods), ORCHIDEE-MICT
(ref. 19), and two versions of JULES (refs 20,21). These complex models
were developed specifically to represent high-latitude processes, in
particular soil thermic and biogeochemistry mechanisms that con-
trol carbon sequestration and emission. In this new emulator, the
permafrost carbon in two high-latitude regions is represented as
an initially frozen pool that thaws as global temperature increases.
Thawed carbon is not immediately emitted: it is split between several
pools, each with its specific timescale of emission. We assume that
2.3% of the emission occurs as methane14 (Methods discusses the
uncertainty of this value), and this emitted CH4 is fully coupled to
the dynamical atmospheric chemistry of OSCAR. More details on
the protocol, the emulator and the models are provided in Methods.
We do not assume a priori that reductions in emission bud-
gets can simply be calculated as the cumulative permafrost carbon
release in a given scenario. Quite the opposite, we apply three spe-
cifically designed approaches to estimate emission budgets. The first
one is the ‘exceedance’ approach, in which the budget is a thresh-
old in terms of cumulative anthropogenic CO2 emissions above
which the temperature target is exceeded (with a given probability).
The second one is the ‘avoidance’ approach, in which the budget is
another—typically lower—cumulative emissions threshold below
which the target is avoided (also with a given probability). These two
approaches were used in the fifth International Panel on Climate
Change (IPCC) assessment report6,22. However, neither of these
considers the possibility of overshooting the target first, and then
returning below it afterwards. To investigate such a case, we adapted
the approach of MacDougall et al.12 to create ‘overshoot’ budgets.
Reductions in exceedance and avoidance budgets. With the
exceedance approach, budgets are calculated in any given scenario
as the maximum cumulative CO2 emissions before the point in
time when global temperature reaches the target level for the first
time. This is illustrated in Fig. 1 (Methods and example given in
Supplementary Fig. 1). Here our exceedance budgets are based on
the four extended representative concentration pathways (RCP)
emission scenarios23 and two idealized scenarios (Methods and
Supplementary Fig. 2).
When permafrost carbon is ignored, we estimated total exceed-
ance budgets of 2,350 (2,290–2,480) Gt CO2 for the 1.5 °C target and
Path-dependent reductions in CO2 emission
budgets caused by permafrost carbon release
T.Gasser 1*, M.Kechiar1,2, P.Ciais 3, E.J.Burke 4, T.Kleinen 5, D.Zhu 3, Y.Huang3,
A.Ekici6,7 and M.Obersteiner1
Emission budgets are defined as the cumulative amount of anthropogenic CO2 emission compatible with a global temperature-
change target. The simplicity of the concept has made it attractive to policy-makers, yet it relies on a linear approximation of
the global carbon–climate system’s response to anthropogenic CO2 emissions. Here we investigate how emission budgets are
impacted by the inclusion of CO2 and CH4 emissions caused by permafrost thaw, a non-linear and tipping process of the Earth
system. We use the compact Earth system model OSCAR v2.2.1, in which parameterizations of permafrost thaw, soil organic
matter decomposition and CO2 and CH4 emission were introduced based on four complex land surface models that specifically
represent high-latitude processes. We found that permafrost carbon release makes emission budgets path dependent (that is,
budgets also depend on the pathway followed to reach the target). The median remaining budget for the 2 °C target reduces by
8% (1–25%) if the target is avoided and net negative emissions prove feasible, by 13% (2–34%) if they do not prove feasible, by
16% (3–44%) if the target is overshot by 0.5 °C and by 25% (5–63%) if it is overshot by 1 °C. (Uncertainties are the minimum-
to-maximum range across the permafrost models and scenarios.) For the 1.5 °C target, reductions in the median remaining
budget range from ~10% to more than 100%. We conclude that the world is closer to exceeding the budget for the long-term
target of the Paris Climate Agreement than previously thought.
Corrected: Author Correction
NATURE GEOSCIENCE | VOL 11 | NOVEMBER 2018 | 830–835 | www.nature.com/naturegeoscience
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... Second, higher temperature also enhances decomposition more than productivity (Tang et al., 2022). Third, in high-latitude regions, the soil C decomposition rate is likely to increase under warmer climate and permafrost thaw conditions (Yokohata et al., 2020;Schneider von Deimling et al., 2015;Gasser et al., 2018;MacDougall and Knutti, 2016;Schuur et al., 2015). In the warming future, the estimation of CO 2 release under RCP 2.6 tends to be higher than the values estimated from other models (by 2100, 54.7-54.8 ...
... In addition, the C sink-source switch will occur before 2100 under RCP 4.5 and RCP 8.5. Except for the future decomposition increase, which is common among model predictions (Yokohata et al., 2020;Schneider Von Deimling et al., 2015;Gasser et al., 2018;Macdougall and Knutti, 2016;Schuur et al., 2015), these differences are mainly due to the suppressed NPP in this study. ...
... Pg C in this study vs. 42-141 Pg C in literature; by 2300, 222.2-247.6 Pg C in this study vs. 157-313 Pg C in literature)(Yokohata et al., 2020;Schneider von Deimling et al., 2015;Gasser et al., 2018) ( ...
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... As the land carbon cycle described in the previous section does not account for permafrost carbon, we implemented this feedback using the emulator developed by Gasser et al. (2018) but aggregated into a unique global region. Figure 4 gives a representation of the permafrost module as described in the following. ...
... Thawed carbon is not directly emitted to the atmosphere: it is split into three thawed carbon sub-pools (C th,j ) that have Figure 4. The permafrost carbon model in Pathfinder is taken from Gasser et al. (2018). The frozen pool dynamic lags behind a theoretical value that is determined by the temperature anomaly. ...
... The permafrost module's parameters are recalibrated using the same algorithm as used by Gasser et al. (2018) but adapted to the global formulation of Pathfinder. First, the algorithm is run once to obtain a set of parameters reproducing the behaviour of the global average of five permafrost models (with data from UVic (MacDougall, 2021) added to the four original models). ...
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... Part of the permafrost carbon will be released into the atmosphere as CH4, but the radiative forcing from this additional process will probably be smaller by an order of magnitude than permafrost CO2 emissions ( Figure 5.29c in Canadell et al., 2021, WGI AR6 Chapter 5). In low-emission scenarios, however, the permafrost region may release a significant fraction of the remaining carbon budget (Gasser et al., 2018;Kleinen and Brovkin, 2018). ...
... Due to a slow response to ongoing warming, changes in high-latitude ecosystems, including increased greenhouse gas emissions from thawing soils, could continue for decades even after anthropogenic emissions have ceased and global temperature has stabilized (Eliseev et al., 2014;de Vrese and Brovkin, 2021), as is illustrated in a conceptual diagram in Figure 9. By 2100, the permafrost region may release a substantial fraction of the remaining carbon budget (Gasser et al., 2018;Kleinen and Brovkin, 2018). ...
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... While the RCP 8.5 is a high-end scenario, there is still a great deal of uncertainty regarding permafrost carbon stocks, thawing processes, and subsequent microbial decomposition of CO 2 and CH 4 , as well as other potential feedbacks [8]. Another study found that the permafrost feedback effect reduces the carbon budget, when set at avoiding 2 • C, by 100 GtC [9], accounting for roughly 24% of our remaining carbon budget [6]. Therefore, permafrost feedback is the primary positive climate feedback we examine in this paper. ...
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While the Intergovernmental Panel on Climate Change (IPCC) physical science reports usually assess a handful of future scenarios, the Working Group III contribution on climate mitigation to the IPCC's Sixth Assessment Report (AR6 WGIII) assesses hundreds to thousands of future emissions scenarios. A key task in WGIII is to assess the global mean temperature outcomes of these scenarios in a consistent manner, given the challenge that the emissions scenarios from different integrated assessment models (IAMs) come with different sectoral and gas-to-gas coverage and cannot all be assessed consistently by complex Earth system models. In this work, we describe the “climate-assessment” workflow and its methods, including infilling of missing emissions and emissions harmonisation as applied to 1202 mitigation scenarios in AR6 WGIII. We evaluate the global mean temperature projections and effective radiative forcing (ERF) characteristics of climate emulators FaIRv1.6.2 and MAGICCv7.5.3 and use the CICERO simple climate model (CICERO-SCM) for sensitivity analysis. We discuss the implied overshoot severity of the mitigation pathways using overshoot degree years and look at emissions and temperature characteristics of scenarios compatible with one possible interpretation of the Paris Agreement. We find that the lowest class of emissions scenarios that limit global warming to “1.5 ∘C (with a probability of greater than 50 %) with no or limited overshoot” includes 97 scenarios for MAGICCv7.5.3 and 203 for FaIRv1.6.2. For the MAGICCv7.5.3 results, “limited overshoot” typically implies exceedance of median temperature projections of up to about 0.1 ∘C for up to a few decades before returning to below 1.5 ∘C by or before the year 2100. For more than half of the scenarios in this category that comply with three criteria for being “Paris-compatible”, including net-zero or net-negative greenhouse gas (GHG) emissions, median temperatures decline by about 0.3–0.4 ∘C after peaking at 1.5–1.6 ∘C in 2035–2055. We compare the methods applied in AR6 with the methods used for SR1.5 and discuss their implications. This article also introduces a “climate-assessment” Python package which allows for fully reproducing the IPCC AR6 WGIII temperature assessment. This work provides a community tool for assessing the temperature outcomes of emissions pathways and provides a basis for further work such as extending the workflow to include downscaling of climate characteristics to a regional level and calculating impacts.
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Cumulative carbon emissions budgets consistent with 1.5 °C global warming
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The Paris Agreement ¹ commits ratifying parties to pursue efforts to limit the global temperature increase to 1.5 °C relative to pre-industrial levels. Carbon budgets ²⁻⁵ consistent with remaining below 1.5 °C warming, reported in the IPCC Fifth Assessment Report (AR5) 2,6,8, are directly based on Earth system model (Coupled Model Intercomparison Project Phase 5) ⁷ responses, which, on average, warm more than observations in response to historical CO2 emissions and other forcings 8,9 . These models indicate a median remaining budget of 55 PgC (ref. ¹⁰, base period: year 1870) left to emit from January 2016, the equivalent to approximately five years of emissions at the 2015 rate 11,12 . Here we calculate warming and carbon budgets relative to the decade 2006-2015, which eliminates model-observation differences in the climate-carbon response over the historical period ⁹, and increases the median remaining carbon budget to 208 PgC (33-66% range of 130-255 PgC) from January 2016 (with mean warming of 0.89 °C for 2006-2015 relative to 1861-1880 ¹³⁻¹⁸ ). There is little sensitivity to the observational data set used to infer warming that has occurred, and no significant dependence on the choice of emissions scenario. Thus, although limiting median projected global warming to below 1.5 °C is undoubtedly challenging ¹⁹⁻²¹, our results indicate it is not impossible, as might be inferred from the IPCC AR5 carbon budgets 2,8 .
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Global Carbon Budget 2017
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  • Mar 2018
Accurate assessment of anthropogenic carbon dioxide (CO2) emissions and their redistribution among the atmosphere, ocean, and terrestrial biosphere – the “global carbon budget” – is important to better understand the global carbon cycle, support the development of climate policies, and project future climate change. Here we describe data sets and methodology to quantify the five major components of the global carbon budget and their uncertainties. CO2 emissions from fossil fuels and industry (EFF) are based on energy statistics and cement production data, respectively, while emissions from land-use change (ELUC), mainly deforestation, are based on land-cover change data and bookkeeping models. The global atmospheric CO2 concentration is measured directly and its rate of growth (GATM) is computed from the annual changes in concentration
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Uncertainty in projected climate change arising from uncertain fossil-fuel emission factors
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Emission inventories are widely used by the climate community, but their uncertainties are rarely accounted for. In this study, we evaluate the uncertainty in projected climate change induced by uncertainties in fossil-fuel emissions, accounting for non-CO2 species co-emitted with the combustion of fossil-fuels and their use in industrial processes. Using consistent historical reconstructions and three contrasted future projections of fossil-fuel extraction from Mohr et al we calculate CO2 emissions and their uncertainties stemming from estimates of fuel carbon content, net calorific value and oxidation fraction. Our historical reconstructions of fossil-fuel CO2 emissions are consistent with other inventories in terms of average and range. The uncertainties sum up to a ±15% relative uncertainty in cumulative CO2 emissions by 2300. Uncertainties in the emissions of non-CO2 species associated with the use of fossil fuels are estimated using co-emission ratios varying with time. Using these inputs, we use the compact Earth system model OSCAR v2.2 and a Monte Carlo setup, in order to attribute the uncertainty in projected global surface temperature change (∆T) to three sources of uncertainty, namely on the Earth system's response, on fossil-fuel CO2 emission and on non-CO2 co-emissions. Under the three future fuel extraction scenarios, we simulate the median ∆T to be 1.9, 2.7 or 4.0 °C in 2300, with an associated 90% confidence interval of about 65%, 52% and 42%. We show that virtually all of the total uncertainty is attributable to the uncertainty in the future Earth system's response to the anthropogenic perturbation. We conclude that the uncertainty in emission estimates can be neglected for global temperature projections in the face of the large uncertainty in the Earth system response to the forcing of emissions. We show that this result does not hold for all variables of the climate system, such as the atmospheric partial pressure of CO2 and the radiative forcing of tropospheric ozone, that have an emissions-induced uncertainty representing more than 40% of the uncertainty in the Earth system's response.
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CO2 loss by permafrost thawing implies additional emissions reductions to limit warming to 1.5 or 2 ◦C
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Large amounts of carbon are stored in the permafrost of the northern high latitude land. As permafrost degrades under a warming climate, some of this carbon will decompose and be released to the atmosphere. This positive climate-carbon feedback will reduce the natural carbon sinks and thus lower anthropogenic CO 2 emissions compatible with the goals of the Paris Agreement. Simulations using an ensemble of the JULES-IMOGEN intermediate complexity climate model (including climate response and process uncertainty) and a stabilization target of 2 • C, show that including the permafrost carbon pool in the model increases the land carbon emissions at stabilization by between 0.09 and 0.19 Gt C year −1 (10th to 90th percentile). These emissions are only slightly reduced to between 0.08 and 0.16 Gt C year −1 (10th to 90th percentile) when considering 1.5 • C stabilization targets. This suggests that uncertainties caused by the differences in stabilization target are small compared with those associated with model parameterisation uncertainty. Inertia means that permafrost carbon loss may continue for many years after anthropogenic emissions have stabilized. Simulations suggest that between 225 and 345 Gt C (10th to 90th percentile) are in thawed permafrost and may eventually be released to the atmosphere for stabilization target of 2 • C. This value is 60-100 Gt C less for a 1.5 • C target. The inclusion of permafrost carbon will add to the demands on negative emission technologies which are already present in most low emissions scenarios.
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Pathways to 1.5 °C and 2 °C warming based on observational and geological constraints
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Restricting global warming to remain below agreed targets requires limiting carbon emissions, the principal driver of anthropogenic warming. However, there is significant uncertainty in projecting the amount of carbon that can be emitted, in part due to the limited number of Earth system model simulations and their discrepancies with present-day observations. Here, we demonstrate a novel approach to reduce the uncertainty of climate projections; using theory and geological evidence we generate a very large ensemble (3×10<sup>4</sup>) of projections that closely match records for nine key climate metrics, including warming and ocean heat content. Our analysis narrows the uncertainty in surface warming projections and reduces the range in equilibrium climate sensitivity. We find that a warming target of 1.5°C above the preindustrial requires the total emitted carbon from the start of year 2017 to be less than 195 to 205 PgC (in over 66% of simulations), while a warming target of 2 °C is only likely if the emitted carbon remains less than 395 to 455 PgC. At current emission rates, these warming targets are reached in 17 to 18 years and 35 to 41 years, respectively, so that there is a limited window to develop a more carbon-efficient future.
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Emission budgets and pathways consistent with limiting warming to 1.5°C
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The Paris Agreement has opened debate on whether limiting warming to 1.5 °C is compatible with current emission pledges and warming of about 0.9 °C from the mid-nineteenth century to the present decade. We show that limiting cumulative post-2015 CO2 emissions to about 200 GtC would limit post-2015 warming to less than 0.6 °C in 66% of Earth system model members of the CMIP5 ensemble with no mitigation of other climate drivers, increasing to 240 GtC with ambitious non-CO2 mitigation. We combine a simple climate–carbon-cycle model with estimated ranges for key climate system properties from the IPCC Fifth Assessment Report. Assuming emissions peak and decline to below current levels by 2030, and continue thereafter on a much steeper decline, which would be historically unprecedented but consistent with a standard ambitious mitigation scenario (RCP2.6), results in a likely range of peak warming of 1.2–2.0 °C above the mid-nineteenth century. If CO2 emissions are continuously adjusted over time to limit 2100 warming to 1.5 °C, with ambitious non-CO2 mitigation, net future cumulative CO2 emissions are unlikely to prove less than 250 GtC and unlikely greater than 540 GtC. Hence, limiting warming to 1.5 °C is not yet a geophysical impossibility, but is likely to require delivery on strengthened pledges for 2030 followed by challengingly deep and rapid mitigation. Strengthening near-term emissions reductions would hedge against a high climate response or subsequent reduction rates proving economically, technically or politically unfeasible.
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Quantifying uncertainties of permafrost carbon–climate feedbacks
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  • Jun 2017
  • BG
The land surface models JULES (Joint UK Land Environment Simulator, two versions) and ORCHIDEE-MICT (Organizing Carbon and Hydrology in Dynamic Ecosystems), each with a revised representation of permafrost carbon, were coupled to the Integrated Model Of Global Effects of climatic aNomalies (IMOGEN) intermediate-complexity climate and ocean carbon uptake model. IMOGEN calculates atmospheric carbon dioxide (CO2) and local monthly surface climate for a given emission scenario with the land–atmosphere CO2 flux exchange from either JULES or ORCHIDEE-MICT. These simulations include feedbacks associated with permafrost carbon changes in a warming world. Both IMOGEN–JULES and IMOGEN–ORCHIDEE-MICT were forced by historical and three alternative future-CO2-emission scenarios. Those simulations were performed for different climate sensitivities and regional climate change patterns based on 22 different Earth system models (ESMs) used for CMIP3 (phase 3 of the Coupled Model Intercomparison Project), allowing us to explore climate uncertainties in the context of permafrost carbon–climate feedbacks. Three future emission scenarios consistent with three representative concentration pathways were used: RCP2.6, RCP4.5 and RCP8.5. Paired simulations with and without frozen carbon processes were required to quantify the impact of the permafrost carbon feedback on climate change. The additional warming from the permafrost carbon feedback is between 0.2 and 12 % of the change in the global mean temperature (ΔT) by the year 2100 and 0.5 and 17 % of ΔT by 2300, with these ranges reflecting differences in land surface models, climate models and emissions pathway. As a percentage of ΔT, the permafrost carbon feedback has a greater impact on the low-emissions scenario (RCP2.6) than on the higher-emissions scenarios, suggesting that permafrost carbon should be taken into account when evaluating scenarios of heavy mitigation and stabilization. Structural differences between the land surface models (particularly the representation of the soil carbon decomposition) are found to be a larger source of uncertainties than differences in the climate response. Inertia in the permafrost carbon system means that the permafrost carbon response depends on the temporal trajectory of warming as well as the absolute amount of warming. We propose a new policy-relevant metric – the frozen carbon residence time (FCRt) in years – that can be derived from these complex land surface models and used to quantify the permafrost carbon response given any pathway of global temperature change.
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ORCHIDEE-MICT (v8.4.1), a land surface model for the high latitudes: model description and validation
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  • Jan 2018
The high-latitude regions of the Northern Hemisphere are a nexus for the interaction between land surface physical properties and their exchange of carbon and energy with the atmosphere. At these latitudes, two carbon pools of planetary significance – those of the permanently frozen soils (permafrost), and of the great expanse of boreal forest – are vulnerable to destabilization in the face of currently observed climatic warming, the speed and intensity of which are expected to increase with time. Improved projections of future Arctic and boreal ecosystem transformation require improved land surface models that integrate processes specific to these cold biomes. To this end, this study lays out relevant new parameterizations in the ORCHIDEE-MICT land surface model. These describe the interactions between soil carbon, soil temperature and hydrology, and their resulting feedbacks on water and CO2 fluxes, in addition to a recently developed fire module. Outputs from ORCHIDEE-MICT, when forced by two climate input datasets, are extensively evaluated against (i) temperature gradients between the atmosphere and deep soils, (ii) the hydrological components comprising the water balance of the largest high-latitude basins, and (iii) CO2 flux and carbon stock observations. The model performance is good with respect to empirical data, despite a simulated excessive plant water stress and a positive land surface temperature bias. In addition, acute model sensitivity to the choice of input forcing data suggests that the calibration of model parameters is strongly forcing-dependent. Overall, we suggest that this new model design is at the forefront of current efforts to reliably estimate future perturbations to the high-latitude terrestrial environment.
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Increased nitrous oxide emissions from Arctic peatlands after permafrost thaw
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  • May 2017
Significance The Arctic is warming rapidly, causing permafrost soils to thaw. Vast stocks of nitrogen (>67 billion tons) in the permafrost, accumulated thousands of years ago, could now become available for decomposition, leading to the release of nitrous oxide (N 2 O) to the atmosphere. N 2 O is a strong greenhouse gas, almost 300 times more powerful than CO 2 for warming the climate. Although carbon dynamics in the Arctic are well studied, the fact that Arctic soils store enormous amounts of nitrogen has received little attention so far. We report that the Arctic may become a substantial source of N 2 O when the permafrost thaws, and that N 2 O emissions could occur from surfaces covering almost one-fourth of the entire Arctic.
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