SCIENCE science.orggreater political acceptance of mitigation ac-
tions, albeit within a cobenefits framing. In
Brazil, the success of a forest policy, enabled
in part by institutional strengthening of the
forest bureaucracy, led to creation of climate-
focused bodies. Deliberative institutions in
South Africa are engaged in creation of a just
transition narrative around climate change,
which may provide more favorable ground
for strategic institution building than would
a narrow mitigation framing.
Over time, as climate politics in countries
shift, in part through the ratcheting effect
that deeper institutionalization of climate
change can provide, conditions for mitiga-
tion-centric institutions may emerge. The
emergence of the German Climate Change
Act reflects greater political convergence
around climate change, spurred also by pub-
lic mobilization. Even after these conditions
are reached, climate institutions are needed
to preserve hard-won political gains. The UK
Climate Change Committee’s analyses have
also been credited with supporting formula-
tion of the country’s net-zero target.
These examples suggest that countries
develop climate institutions in context-de-
pendent ways. Universal best-practice for-
mulations are of less use than an approach
based on understanding how climate politics
and climate institutions shape each other.
Although choices may therefore be con-
strained, there is scope for building a virtu-
ous cycle from seemingly modest initial in-
stitutions. Both the analytical and normative
agenda of climate policy requires further at-
tention to the role of climate institutions. j
REFERENCES AND NOTES
- A. Averchenkova, S. Fankhauser, M. Nachmany, Tr e n d s
in Climate Change Legislation (Edward Elgar Publishing,
2017). - G. Iacobuta, N. K. Dubash, P. Upadhyaya, M. Deribe, N.
Höhne, Clim. Policy 18 , 1114 (2018). - K. Harrison, L. M. Sundstrom, Global Commons,
Domestic Decisions: The Comparative Politics of Climate
Change (MIT Press, 2010). - J. Meckling, J. Nahm, Governance (Oxford) 31 , 74 1
(2018). - M. Mildenberger, Carbon Captured: How Business and
Labor Control Climate Politics (MIT Press, 2020). - M. Aklin, M. Mildenberger, Glob. Environ. Polit. 20 , 4
(2020). - P. A. Hall, R. C. R. Taylor, Polit. Stud. 44 , 936 (1996).
- J. Mahoney, K. A. Thelen, Eds., Explaining Institutional
Change: Ambiguity, Agency, and Power (Cambridge
University Press, 2010). - N. K. Dubash, Env. Polit. 30 (sup1), 1 (2021).
- Varieties of Climate Governance, Env. Polit. 30 (suppl. 1),
1 (2021). - J. J. L. Candel, R. Biesbroek, Policy Sci. 49 , 211 (2016).
- J. J. Finnegan, APSA Preprints 10.33774/apsa-2019-
6ft75 (2019). - M. Lockwood, C. Kuzemko, C. Mitchell, R. Hoggett,
Environ. Plan. C Polit. Space 35 , 312 (2017).
ACKNOWLEDGMENTS
The authors acknowledge financial support from the John
D. and Catherine T. MacArthur Foundation that enabled two
project workshops (grant 19-1905-153965-CLS).
SUPLEMENTARY MATERIALS
science.org/doi/10.1126/science.abm1157
10.1126/science.abm1157
CLIMATE POLICYCan updated climate pledges
limit warming well below 2°C?
Increased ambition and implementation are essential
A
s part of the 2015 Paris Agreement,
countries agreed to regularly revisit
and enhance their national climate
strategies and, every 5 years, to offer
new emissions targets in the form of
nationally determined contributions
(NDCs) ( 1 ). This year’s 26th Conference of
Parties provides a waypoint in this updat-
ing process as countries have been offering
enhanced or completely new NDCs ( 2 , 3 )
(henceforth, updated pledges) ( 4 ). We find
that compared with the 2015 pledges, the
updated pledges suggest a strengthening of
ambition through 2030. By calculating prob-
abilistic temperature outcomes over the 21st
century for five emissions scenarios (see the
figure and table S1), we find that the updated
pledges provide a stronger near-term founda-
tion to deliver on the long-term goals of the
Paris Agreement of reducing the probability
of the worst levels of temperature change this
century and increasing the likelihood of lim-
iting temperature change to well below 2°C.
As of 30 September of this year, 121 par-
ties covering 52% of global emissions have
submitted updated pledges ( 5 ). The updated
pledges, in most cases, target near-term re-
ductions—through 2025, 2030, or 2035 —and,
hence, they could play a critical role in
achieving the Paris Agreement’s longer-term
goals ( 1 ). Any assessment of the long-term
temperature implications of the updated
pledges will need to account for changes in
our scientific understanding of the climate
system, including uncertainties related
to the effect of global emissions on global
mean surface temperature change. Multiple
lines of evidence now suggest a narrower
range for climate sensitivity—defined as the
steady-state global temperature increase for
a doubling of carbon dioxide (CO 2 )—com-
pared with past assessments. Notably, new
evidence suggests higher likelihoods for cli-
mate sensitivity values near the middle part
of the range identified in previous literature
and lower likelihoods for values in the lower
and higher parts of the range ( 6 ).
Several recent policy, societal, and tech-nological developments could also affect fu-
ture emissions trajectories and, hence, tem-
perature outcomes. Foremost, 33 countries
representing 32% of global emissions have
communicated official long-term strategies
(LTSs) under the United Nations Framework
Convention on Climate Change (UNFCCC) or
unofficial long-term objectives that outline
emission reduction strategies through the
midcentury ( 7 ), and 52 parties representing
54% of global emissions have communicated
net-zero emissions targets ( 8 ). Other devel-
opments since 2015—such as the economic
slowdown driven by the COVID-19 pandemic
and subsequent recovery ( 9 ), the enactment
of new sectoral policies and regulations
across the globe ( 10 ), and technological ad-
vances driving rapidly declining costs of re-
newable energy, electric vehicles, and other
clean technologies (see table S2)—have all
affected near-term emission trajectories with
potential long-term implications.
Our analysis accounts for these recent de-
velopments. In doing so, this study provides
new and updated insights over previous stud-
ies that have examined the long-term tem-
perature outcomes of the original 2015 NDCs
(see materials and methods, section S1). We
also note that our analysis is intentionally
designed to facilitate easy comparison with
Fawcett et al. ( 11 )—a key study by a similar
team of authors—that evaluated the long-
term probabilistic temperature outcomes of
the original 2015 pledges using previous ver-
sions of the models used in the present study.
Comparing with Fawcett et al. allows us to
quantify how much of the new insights ob-
tained in this study can be attributed to an
improved understanding of climate science
versus updated emissions trajectories.EMISSIONS SCENARIOS
We developed our emissions scenarios (see
the figure, left) using the Global Change
Analysis Model (GCAM; see materials and
methods, section S2). Our scenarios include
the impacts of COVID-19 on the economy
and the latest technological trends, includ-By Ya ng Ou^1 , Gokul Iyer^1 , Leon Clarke^2 , Jae Edmonds^1 , Allen A. Fawcett^4 , Nathan Hultman2,3,
James R. McFarland^4 , Matthew Binsted^1 , Ryna Cui^2 , Claire Fyson^5 , Andreas Geiges^5 , Sofia
Gonzales-Zuñiga^6 , Matthew J. Gidden5,7, Niklas Höhne8,9, Louise Jeffery^6 , Takeshi Kuramochi8,10,
Jared Lewis11,12,13, Malte Meinshausen11,12,13, Zebedee Nicholls11,12,13, Pralit Patel^1 , Shaun
Ragnauth^4 , Joeri Rogelj7, 1 4, Stephanie Waldhoff^1 , S h a Yu^1 , Haewon McJeon^15 NOVEMBER 2021 • VOL 374 ISSUE 6568 693