SCIENCE science.orgemissions constant beyond that year. The
two scenarios differ in their assumptions
about the minimum decarbonization rate:
Whereas the Updated pledges–Continued
ambition scenario assumes a 2% minimum
decarbonization rate, the Updated pledges–
Increased ambition scenario assumes a 5%
rate. The 2 and 5% decarbonization rate as-
sumptions have been used previously and
are consistent with the average and high
rates observed historically ( 11 ).
We note that our emissions scenarios are
consistent with most scenarios in the lit-
erature (see the figure). However, many sce-
narios in the literature result in net-negative
emissions in the second half of the century.
By contrast, our scenarios assume that coun-
tries reduce emissions at most to net-zero
because no country has committed to net-
negative emissions thus far.
TEMPERATURE OUTCOMES
We estimate probabilistic temperature out-
comes for the emission scenarios using a
simple climate model [Model for the Assess-
ment of Greenhouse Gas Induced Climate
Change (MAGICC)] in a probabilistic setup
(see materials and methods, section S3) that
accounts for the latest understanding of cli-
mate uncertainties ( 12 ) (see the figure, right).
Because emission trajectories in the Updated
pledges scenarios are lower than those in
the Reference–No Policy and Current pol-
icy scenarios, they result in lower probabili-
ties of temperature change exceeding 4°C in
2100 and higher probabilities of limiting tem-
perature change below 2°C. Under the Up-
dated pledges–Continued ambition scenario,
the probability of global mean surface tem-
perature changes exceeding 4°C is virtually
eliminated (compared with a ~10% chance
with the 2015 NDCs) and the probabilities
of limiting temperature change to below 2°C
and 1.5°C increase to 34 and 1.5%, respec-
tively, compared with about 8 and 0% with
the 2015 pledges. By contrast, if countries
scale up mitigation efforts as in the Updated
pledges–Increased ambition scenario, not
only is the probability of exceeding 4°C virtu-
ally eliminated but the probabilities of limit-
ing temperature change below 2°C and 1.5°C
also increase to 60 and 11% respectively (com-
pared with 31 and 5% with the 2015 pledges).
THE ROLE OF IMPROVED CLIMATE SCIENCE
To understand the role of improved climate
science since the original Fawcett et al.
study ( 11 ), we performed additional simula-
tions by combining the emissions scenarios
in this study and those from Fawcett et al.
with the versions of MAGICC used in the
two studies (see fig. S4 and table S1). The
version of MAGICC used in this study ac-
counts for the improved understanding of
climate science (see materials and meth-
ods, section S3). Therefore, our additional
simulations help us understand how much
of the probabilistic temperature outcomes
obtained in this study can be attributed to
the improved understanding of climate un-
certainties versus updates to emissions tra-
jectories between the two studies.
Results from the additional simulations
suggest that most of the differences in proba-
bilistic temperature outcomes below 2°C
between Fawcett et al. ( 11 ) and this study
can be attributed to the updated emissions
trajectories, which are in turn driven by the
updated pledges, LTSs, net-zero pledges, and
other recent societal, policy, and technologi-
cal trends. For example, under the Increased
ambition scenarios, the updated emission
trajectories contribute to 27 percentage
points out of a total improvement of about 29
percentage points in the probabilities of lim-
iting temperature change to below 2°C, with
two percentage points being contributed by
an improved climate science understanding
of the narrowed climate sensitivity ranges
(see table S3).AMBITION NEEDS IMPLEMENTATION
Our analysis is based on a single integrated
assessment model driven by a set of assump-
tions about future population, GDP, techno-
logical change, and resources. Other models
and assumptions could result in different
quantitative results. However, the core quali-
tative insight of this analysis would remain:
if implemented as stated, updated pledges of-
fered by nations in the past year would lay
a stronger near-term foundation for reaching
global climate goals compared with the 2015
pledges. This underscores that the process
for countries to revisit their national climate
strategies and offer new targets has gener-
ated a notable increase in ambition over the
past 6 years. This increase in ambition could
spawn even higher ambition in the future: As
countries implement updated pledges, the
widespread development and deployment of
low-carbon technologies, expansion of policy
instruments across all sectors and levels of
government, and enhanced international co-
operation could drive down costs and enable
more cost-effective and stronger mitigation
in the long term ( 13 ). Nevertheless, to the
extent that countries continue to have oppor-
tunities to further enhance their ambition—
both before and after 2030— this would
provide additional and needed ambition to
substantially improve climate outcomes and
increase the likelihood to limit warming to
less than 1.5°C.
Ultimately, realizing the long-term climate
benefits described in this study will require
putting words into action by implementing
these newer and enhanced targets. This couldentail substantial political, financial, institu-
tional, social, and behavioral challenges ( 14 ).
Surmounting these challenges with policies
and measures and successfully achieving the
targets would substantially reduce global cli-
mate risks, essentially avoiding the worst out-
comes from climate change and substantially
improving the probability of limiting global
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ACKNOWLEDGMENTS
Y.O. and G.I. contributed equally to this work. The views and
opinions expressed in this paper are those of the authors
alone and do not necessarily state or reflect those of the
Environmental Protection Agency, the Department of State,
or the US government, and no official endorsement should
be inferred. Current policy projections and update pledges
used in this analysis were derived by Climate Action Tracker,
with thanks to all project team members who contributed.
Funding: This work was funded by US Environmental
Protection Agency IAA DW-089-92460001 (Y.O., G.I., M.B.,
J.E., P.P., S.Y., S.W., and H.M.) and German Federal Ministry for
the Environment, Nature Conservation and Nuclear Safety
grant 16_I_291_Global_A_CAT (C.F., A.G., S.G.-Z., M.J.G.,
N.H., L.J., and T.K.). Data and materials availability: GCAM
is an open -source community model available at https://
github.com/JGCRI/gcam-core/releases. Source code and
data associated with this analysis are available at https://doi.
org/10.5281/zenodo.5553171.SUPPLEMENTARY MATERIALS
science.org/doi/10.1126/science.abl897610.1126/science.abl89765 NOVEMBER 2021 • VOL 374 ISSUE 6568 695