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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: firstname.lastname@example.org
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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