science.org SCIENCEGRAPHIC: K. FRANKLIN/SCIENCEing rapidly declining costs of photovoltaic
cells, the expansion of electric vehicles, and
the availability of CO 2 removal measures such
as bioenergy in conjunction with carbon cap-
ture and storage, afforestation, and direct air
capture (see materials and methods, sections
S2 and S4; and table S2).
The Reference–No policy, Current policy,
and Current policy–Continued ambition
scenarios serve as counterfactuals against
which to compare scenarios with the up-
dated pledges. The Reference–No policy sce-
nario assumes no new emission mitigation
policy through 2100. Compared with Fawcett
et al. ( 11 ), the Reference–No policy scenario
in this study results in lower emissions
throughout the century (see fig. S1). This is
primarily driven by policy and technological
developments since 2015, such as reduced
coal power investment and rapid cost reduc-
tions in renewable energy. The Current pol-
icy and Current policy–Continued ambition
scenarios assume that countries implement
current sectoral policies and national cli-
mate mitigation measures through 2030 ( 5 ).
Examples of these policies include increased
deployment of renewable energy, improved
energy efficiency, and additional investment
in public transportation.
Beyond 2030, the Current policy scenario
assumes that countries continue to decarbon-
ize their economies at the same annual decar-
bonization rate—defined as the annual rate
of improvement in CO 2 emissions per unit
of gross domestic product (GDP)—that was
required to implement the policies between
2015 and 2030. By contrast, the Current pol-
icy–Continued ambition scenario assumes
a minimum decarbonization rate of 2% per
year beyond 2030. In other words, this sce-
nario assumes that countries continue to de-
carbonize their economies at the same rate
as that required to implement the current
policies between 2015 and 2030 or increase
to 2% per year if the decarbonization rate is
below this level.The Updated pledges–Continued ambi-
tion and Updated pledges–Increased ambi-
tion scenarios assume that countries achieve
the updated pledges through 2030 as stated.
Collectively, the updated pledges result in
lower CO 2 emissions from energy and indus-
try in 2030 by 15% relative to the 2015 pledges
(which corresponds to a 15% increase relative
to 2005), suggesting a strengthening of ambi-
tion since the original 2015 submissions (see
fig. S1). Beyond 2030, both scenarios assume
that countries without official UNFCCC LTSs
achieve the same level of decarbonization
rate as the rate between 2015 and 2030 or a
minimum rate if their decarbonization rate
is below this minimum rate. Countries with
LTSs are assumed to follow their LTSs until
the target year, followed by the path defined
by the minimum decarbonization rate be-
tween 2015 and their LTS target year.
Countries with net-zero pledges are as-
sumed to achieve net-zero emissions in the
target year and then continue to keep theirINSIGHTS | POLICY FORUM
(^1) Joint Global Change Research Institute, Pacific Northwest National Laboratory and University of Maryland, College Park, MD, USA. (^2) Center for Global Sustainability, School of Public Policy,
University of Maryland, College Park, MD, USA.^3 US Department of State, Washington, DC 20520, USA.^4 US Environmental Protection Agency, Washington, DC, USA.^5 Climate Analytics, Berlin,
Germany.^6 NewClimate Institute, Berlin, Germany.^7 International Institute for Applied Systems Analysis, Laxenburg, Austria.^8 NewClimate Institute, Cologne, Germany.^9 Environmental Systems
Analysis Group, Wageningen University and Research, Wageningen, Netherlands.^10 Copernicus Institute of Sustainable Development, Utrecht University, Utrecht, Netherlands.^11 Australian-
German Climate and Energy College, The University of Melbourne, Parkville, Victoria, Australia.^12 School of Geography, Earth and Atmospheric Sciences, The University of Melbourne, Parkville,
Victoria, Australia.^13 Climate Resource, Northcote, Victoria, Australia.^14 Grantham Institute, Imperial College London, London, UK. Email: [email protected]
–10
0
10
20
30
40
50
60
70
80
1990 2000 2010 2020 2030 2040 2050 2060 2070 2080 2090 2100
Years
NDCs
Global CO2 emissions from energy and industry (GtCO2/year)
IPCC AR5 pathways: Baseline range
IPCC SR1.5 pathways (15th to 85th percentile range): >66% chance below 2°C
IPCC SR1.5 pathways (15th to 85th percentile range): 1.5°C with no or limited overshoot
Emissions pathways
Illustrative 50%
1.5°C scenario
Updated pledges–
Increased ambition
Updated pledges–
Continued ambition
Current policy–
Continued ambition
Current policy
Reference–
No policy
0 10 20 30 40 50 60 70 80 90 100
Likelihood of projected warming until 2100 (%)
1 to 1.5 ̊C 1.5 to 2 ̊C 2 to 3 ̊C 3 to 4 ̊C >4 ̊C
Projected warming until 2100 relative to preindustrial levels
GtCO 2 , gigatonnes of carbon dioxide; IPCC AR5, IPCC Fifth Assessment Report scenario; IPCC SR1.5, IPCC Special Report on 1.5°C scenario; NDCs, nationally determined contributions.
694 5 NOVEMBER 2021 • VOL 374 ISSUE 6568
Emissions pathways and temperature probabilities
Five scenarios modeled using the Global Change Analysis Model (GCAM; materials and methods, section S2) are shown (left). Thick gray lines reflect global CO 2 emissions
from fossil fuel production and utilization and from industrial processes. The scenarios represent deterministic pathways that share all assumptions other than those about
the stringency of emissions mitigation policies through 2030 and beyond (materials and methods, section S4). The open circle shows emissions in 2030 with new and
updated pledges ( 4 ). Shaded areas show ranges for baseline, 2 ̊C, and 1.5 ̊C pathways per the Intergovernmental Panel on Climate Change (IPCC) ( 15 ). Faint lines behind the
IPCC ranges represent emissions trajectories that determine the range. Likelihoods of different levels of increase in global mean surface temperature change through 2100
relative to preindustrial levels are shown (right). Temperature outcomes are based on the full suite of greenhouse gas, aerosol, and short-lived species emissions generated by
GCAM. The illustrative 50% scenario corresponds to an emission pathway (fig. S1) that achieves a 50% chance of limiting 2100 warming below 1.5°C.