Question:
You are given the full text of a scientific research paper.

    Your task is to generate a structured **policy brief** that includes the following components:

    - A concise, descriptive **title** that captures the central theme or policy issue addressed in the paper (do not label it with “Title:”).
    - **Policy Problem** - What societal or strategic challenge is the paper addressing?
    - **Scientific Research Findings** - What are the key results relevant to this issue?
    - **Scientific Research Study Methods** - What methodology or data supports these findings?
    - **Policy Implications** - How might the findings guide or influence policy?

    Write each section in a clear, accessible style suitable for policymakers. Avoid overly technical language, but use essential terms when necessary. Structure the output as a labeled five-part policy brief document.

    Scientific Research Paper:
    Nature Climate Change | Volume 12 | December 2022 | 1129–1135
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nature climate change
https://doi.org/10.1038/s41558-022-01508-0
Article
Ratcheting of climate pledges needed to limit
peak global warming
Gokul Iyer
  1,4, Yang Ou
  1,4, James Edmonds
  1, Allen A. Fawcett
  2,
Nathan Hultman
  3, James McFarland2, Jay Fuhrman1, Stephanie Waldhoff
  1
& Haewon McJeon
  1
The new and updated emission reduction pledges submitted by countries
ahead of the Twenty-Sixth Conference of Parties represent a meaningful
strengthening of global ambition compared to the 2015 Paris pledges. Yet,
limiting global warming below 1.5 °C this century will require countries to
ratchet ambition for 2030 and beyond. Here, we explore a suite of emissions
pathways to show that ratcheting near-term ambition through 2030 will be
crucial to limiting peak temperature changes. Delaying ratcheting ambition
to beyond 2030 could still deliver end-of-century temperature change
of less than 1.5 °C but would result in higher temperature overshoot over
many decades with the potential for adverse consequences. Ratcheting
near-term ambition would also deliver benefits from enhanced non-CO2
mitigation and facilitate faster transitions to net-zero emissions systems in
major economies.
Many countries and other non-national actors announced new cli-
mate ambition, actions and targets ahead of the Twenty-Sixth Confer-
ence of Parties (COP26), held in Glasgow in November 2021. COP26
provided the first real demonstration of the 2015 Paris Agreement’s
mechanism to regularly revisit and enhance national climate strate-
gies1. By the end of COP26, 151 countries submitted updated and new
nationally determined contributions (NDCs) outlining plans to cut
GHG emissions by 20302. Many countries also communicated official or
unofficial long-term strategies (LTSs) that outline emission reduction
strategies through the mid-century3 and net-zero emissions targets4.
Although the updated and new 2030 pledges suggest higher ambition
compared to the 2015 Paris pledges5,6, limiting global warming below
1.5 °C this century—the aspirational goal of the Paris Agreement—will
require countries to further ratchet or increase ambition in 2030 and
beyond6–12. Importantly, recognizing the need for countries to ratchet
their ambition beyond their current pledges, Article IV of the Glasgow
Climate Pact accelerates the previously expected timeline for revising
these NDCs and calls for countries “to revisit and strengthen the 2030
targets in their nationally determined contributions … to align with the
Paris Agreement temperature goal by the end of 2022”13. In addition,
the Pact calls for countries that have “not yet done so to communicate
new or updated nationally determined contributions and long-term low
greenhouse gas emission development strategies to net-zero emissions
by or around mid-century”. The Pact also “emphasizes the urgent need
for Parties to increase their efforts to collectively reduce emissions
through accelerated action”.
As the international community responds to these calls for ratchet-
ing ambition, there is a strong need to understand both the long-term
temperature outcomes of ratcheting ambition in 2030 and beyond
and what this ratcheting implies for sectoral and regional emissions.
To address this need, we explore a suite of high ambition emissions
pathways—developed using the Global Change Analysis Model (GCAM;
Methods)14—in which countries are assumed to use various combina-
tions of three strategies to ratchet ambition: (1) increasing near-term
actions through 2030, (2) accelerating post-2030 emissions reductions
and (3) moving forward the timing of dates that countries pledge to hit
net-zero emissions. We then use a reduced-form climate model (Hec-
tor)15 to compute the end-of-century and peak temperature change
implications of the emissions pathways. Our study builds off and
extends previous modelling studies that have explored high ambition
emissions pathways6,8,16–20. In doing so, our study makes a timely con-
tribution by exploring pathways that take the 2021 pledges made until
Received: 3 May 2022
Accepted: 21 September 2022
Published online: 10 November 2022
 Check for updates
1Joint Global Change Research Institute, Pacific Northwest National Laboratory and University of Maryland, College Park, MD, USA. 2US Environmental
Protection Agency, Washington, DC, USA. 3Center for Global Sustainability, School of Public Policy, University of Maryland, College Park, MD, USA.
4These authors contributed equally: Gokul Iyer, Yang Ou.
 e-mail: haewon.mcjeon@pnnl.gov

Nature Climate Change | Volume 12 | December 2022 | 1129–1135
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section included in the Supplementary Information, we explore more
ambitious emission cuts in 2030 that lie within the literature range
(Supplementary Fig. 1). We also examine the sensitivity of our results to
the methodology used to develop the NDC+ and NDC++ assumptions
(Supplementary Section 2).
Beyond 2030, countries are assumed to achieve the same level of
decarbonization rate—defined as the annual rate of improvement in
GHG emissions per unit of gross domestic product (GDP)—as the rate
between 2015 and 2030 or a minimum rate if their decarbonization
rate is below this minimum rate. Our central assumption about the
post-2030 minimum decarbonization rate is 2% and our sensitivity
assumptions are 5% and 8%. These assumptions are consistent with
previous studies and are consistent with the average and high rates
observed historically (Methods).
In all our pathways, countries are assumed to achieve their official
LTSs and net-zero pledges if any. Countries with LTSs are assumed to
achieve their LTSs in the target year following a linear path (Methods).
Beyond the target year, countries are assumed to follow a path defined
by the minimum decarbonization rate. Our pathways vary in their
assumptions about the timing of net-zero pledges if any. Our central
assumption is that countries with net-zero pledges achieve net-zero
emissions in the target year following a linear path and then continue
to keep their emissions constant beyond that year. We consider two
alternative sensitivity assumptions in which countries with net-zero
pledges advance the timing of achieving net-zero emissions by 5
or 10 years.
The full combination of the above assumptions results in 27
emissions pathways. We note that our high ambition pathways are
meant to be illustrative and they do not imply feasibility, which would
require accounting for a variety of considerations including ethical
and political24–26.
Temperature outcomes
Using a simple reduced-form climate model (Hector; Methods)15 we
study the implications of the emissions pathways for end-of-century
and peak global mean surface temperature changes (Fig. 2). Our results
show—consistent with other studies—that if countries achieve their
2021 NDCs, official LTSs and net-zero pledges as stated, global surface
temperature change can be limited to <2 °C warming this century5,6,9–12.
In addition, many of our high ambition pathways with the NDC+ and
NDC++ emission levels in 2030 result in <1.5 °C temperature change
in 2100. Even if ratcheting of ambition is delayed to beyond 2030,
end-of-century temperature change can be returned to <1.5 °C. How-
ever, that would require substantial ratcheting of post-2030 ambition
beyond historically observed decarbonization rates (Methods)—as in
the pathways in which countries achieve the NDC emission level in 2030
followed by an 8% minimum decarbonization rate.
Ratcheting ambition in the near-term—as in our NDC++ pathways—
has marked implications for peak temperature changes. Ratcheting
ambition in the near-term results in lower levels of peak warming. For
instance, the peak temperature change in the pathway with NDC emis-
sion level in 2030 followed by a 8% minimum decarbonization rate and
net-zero pledges in the specified target years is 1.77 °C compared to
1.82 °C in the pathway with NDC emission level in 2030 followed by a
2% minimum decarbonization rate. By contrast, the peak temperature
change in the pathway with NDC++ emissions in 2030 followed by a 2%
minimum decarbonization rate and net-zero pledges in the specified
target years is 1.68 °C. Ratcheting ambition in the near- and long-term—
as in the pathways with NDC++ emission level in 2030 followed by an
8% minimum decarbonization rate—reduces peak temperature change
further (peak temperature changes of 1.67 °C if net-zero pledges are
assumed to be achieved in the specified target years). This is an impor-
tant finding since higher peak temperature changes and therefore
higher temperature overshoots—that is, an exceedance of global mean
temperature change above the intended threshold before returning to
the end of COP26 as a starting point and providing important insights
on the long-term temperature change and sectoral and regional emis-
sions implications of ratcheting ambition beyond those pledges (see
Supplementary Section 1 for a detailed literature review).
Emissions pathways
Our emissions pathways (Fig. 1) explore a combination of three
strategies that countries might use to ratchet and achieve ambition:
(1) increasing ambition in the near-term through 2030, (2) increasing
post-2030 decarbonization rates or (3) achieving net-zero pledges
sooner.
In 2030, countries are assumed to achieve one of three ambition
levels, namely, NDC, NDC+ and NDC++. Our central assumption is that
countries achieve the updated or new pledges submitted until the end
of COP26 (Methods). To construct the NDC+ and NDC++ sensitivities,
we begin with the ambition level implied in the updated or new pledges
as assessed by Climate Action Tracker (CAT)21,22. CAT provides one
of five rating categories for the country pledges (‘1.5 °C Paris Agree-
ment compatible’, ‘almost sufficient’, ‘insufficient’, ‘highly insufficient’
and ‘critically insufficient’). In the NDC+ sensitivity, we assume that
countries rated ‘critically insufficient’ and ‘highly insufficient’ by CAT
reduce their emissions by 30% below their current NDC. In the NDC++
sensitivity, we assume that countries rated ‘critically insufficient’,
‘highly insufficient’, ‘insufficient’ and ‘not assessed’ by CAT also reduce
their emissions by 30% below their current NDC. Although arbitrary,
the 30% assumption enables us to explore the implications of greater
ambition and also helps ensure that global 2030 emissions are consist-
ent with existing high ambition scenarios. The GHG emissions in 2030
in our NDC++ pathways lie at the higher end of pathways explored in
the recent modelling literature6,23. In an additional sensitivity analysis
Global GHG emissions (GtCO2e yr–1)
–20
–10
0
10
20
30
40
50
60
1990
2000
2010
2020
2030
2040
2050
Year
2060
2070
2080
2090
2100
Historical emissions
UNEP “Below 1.5 °C”
range in 2030
Central:  2% decarbonization
Net-zero pledges achieved in target year
Most ambitious: 8% decarbonization
Timing of net-zero pledges advanced by 10 years
NDC NDC+ NDC++
Fig. 1 | Global GHG emissions in the pathways modelled using the GCAM.
The emissions pathways vary across assumptions about ambition level in 2030,
post-2030 minimum decarbonization rate and timing of net-zero for countries
with net-zero pledges. See text for detailed description of assumptions. The black
colour corresponds to the ‘NDC’ cases, orange to the ‘NDC+’ cases and blue to the
‘NDC++’ cases. Each colour group comprises nine pathways. The thick bold lines
in each colour group correspond to the central assumptions about post-2030
minimum decarbonization (2%) and year of net-zero (target year as specified).
The thick dashed lines correspond to the most ambitious pathway within each
colour group. The lighter lines within each colour group correspond to different
assumptions about the post-2030 minimum decarbonization rate and timing
of net-zero pledges. The shaded green area represents 15–85 percentile range
of 1.5 °C pathways with no or limited overshoot from the IPCC SR1.5 report18.
See Supplementary Section 2 and Supplementary Table 1 for a mapping of our
pathways with the recently published IPCC Sixth Assessment Report20,52.

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below the intended level—could expose natural and human systems
to substantial risks potentially leading to irreversible and adverse
consequences such as the loss of some ecosystems27–29.
Furthermore, ratcheting near-term ambition could markedly
reduce the number of years of overshoot before returning to 1.5 °C
this century (Supplementary Table 1 and Supplementary Fig. 2). For
example, the pathways with NDC emission level in 2030 followed by
a 2% or 5% minimum decarbonization rate and net-zero pledges in
the specified target years do not return to 1.5 °C this century and the
number of years of overshoot in the pathway with an 8% minimum
decarbonization rate is 67. By contrast, in the pathways with NDC++
emission level in 2030, the number of years of overshoot reduces to
58, 56 and 47 years respectively.
Advancing the timing of net-zero pledges can be an important
ratcheting strategy as it could provide the extra push required in the
long-term to return 2100 warming to <1.5 °C and further reduce temper-
ature overshooting. For example, in the pathway with NDC+ emissions
in 2030 followed by a 5% minimum decarbonization rate and net-zero
pledges in the specified target years, the 2100 and peak temperature
changes are, respectively, 1.51 °C and 1.72 °C. Advancing the timing of
net-zero pledges by 10 years brings the 2100 and peak temperature
changes down to 1.46 °C and 1.68 °C.
Sectoral implications of ratcheting ambition
Ratcheting ambition would entail rapid reductions in CO2 emissions
from all sectors of the energy system (Fig. 3 and Supplementary Fig. 4),
especially through the mid-century. While some sectors (for exam-
ple, electricity, buildings and industry) decarbonize faster due to the
availability of many low-carbon technology alternatives, others (for
example, transportation) decarbonize slower due to fewer options.
Ratcheting ambition in the near-term results in quicker transitions
to net-zero emissions energy systems30–32. For example, in the pathway
with NDC emissions in 2030 followed by a 2% minimum decarboniza-
tion rate, global CO2 emissions do not get to net-zero this century.
Ratcheting near-term ambition—as in the pathway with NDC++ emis-
sions in 2030 followed by 2% minimum decarbonization rate—advances
the year of global net-zero CO2 emissions to 2053. Ratcheting both
near-term and long-term ambition—as in the pathway with NDC++
emissions in 2030 followed by an 8% minimum decarbonization rate—
advances the year of global net-zero CO2 emissions further to 2052
(Supplementary Table 1). Such accelerated declines in CO2 emissions
are accompanied by rapid transformations throughout the global
energy system to phase out fossil fuel-based infrastructures and scale
up low-carbon technologies such as renewables, nuclear and carbon
capture and storage (Supplementary Figs. 5–8).
Ratcheting near-term ambition also implies greater reductions
in non-CO2 emissions, some of which have higher global warming
potentials and shorter atmospheric lifetimes and therefore play an
important role in both stabilizing long-term temperature change and
limiting peak near-term warming (Fig. 3). Non-CO2s respond to climate
policy in two ways33. First, fuel switching and associated phasing out of
carbon-intensive fuels due to climate policy reduce associated non-CO2
emissions (for example, fugitive methane emissions from resource pro-
duction). Thus, higher CO2 ambition implies higher non-CO2 ambition.
Second, non-CO2 emissions that are largely unaffected by fuel switching
such as hydrofluorocarbon (HFC) emissions from cooling energy use
and industrial process emissions (perfluorocarbons (PFCs) and sulfur
hexafluoride) respond to climate policy through the implementation
of additional control measures. In our analysis, the NDC+ and NDC++
pathways result, respectively, in 18% and 24% reduction in methane
emissions from the energy system in 2030 relative to 2020. In terms
of total methane emissions from energy and agricultural systems, the
reduction is, respectively, 4% and 8%. In comparison, over a hundred
countries made a commitment under the Global Methane Pledge—a
key outcome of COP26—to collectively reduce methane emissions
by at least 30% (ref. 34). Future work could explore higher ambition to
reduce methane emissions—especially from agriculture—than sug-
gested by our scenarios.
Nevertheless, some non-CO2 emissions such as methane emis-
sions from cattle due to enteric fermentation are hard to abate35–37.
Hence, achieving net-zero GHG emissions beyond 2030 and continued
decarbonization over the longer term will require the deployment of
carbon dioxide removal (CDR) technologies38,39 to offset these emis-
sions. Our pathways assume the availability of CDR technologies such
as bioenergy in combination with carbon capture and storage (BECCS)
and direct air capture (DAC) in addition to terrestrial sinks40 and assume
that the relative roles of CDR deployment versus mitigation in other
sectors largely depend on economics. Although the scale of CDR in our
pathways is consistent with the extant literature (Supplementary Figs. 9
and 10), an important caveat that could affect the feasibility and scale of
2% decarbonization
5% decarbonization
8% decarbonization
2% decarbonization
5% decarbonization
8% decarbonization
2% decarbonization
5% decarbonization
8% decarbonization
Global surface temperature change (°C)
NDC
NDC+
NDC++
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
2
Net-zero pledges
Central: Net-zero pledges achieved in target year
Ratchet: Timing of net-zero pledges advanced by
               5 and 10 years
Post-2030 decarbonization rate
Central: 2% minimum decarbonization rate
Ratchet: 5% and 8% minimum decarbonization rate
2030 ambition
Central: Countries achieve NDCs submitted until end
               of COP26 (NDC)
Ratchet: Select countries increase 2030 ambition by
               30% (NDC+)
Almost all countries increase 2030 ambition
by 30% (NDC++)
2100
Peak
Fig. 2 | Temperature change in 2100 (inner bars) and peak temperature
change (outer bars) outcomes of the emissions pathways explored in
this study. The grey colour corresponds to the ‘NDC’ cases, orange to the
‘NDC+’ cases and blue to the ‘NDC++’ cases. See text for description about
the construction of the NDC+ and NDC++ cases. The lighter shades within
each colour group correspond to different assumptions about the post-2030
minimum decarbonization rate. While the first bar within each colour and
shading group corresponds to the central case in which countries with net-zero
pledges are assumed to achieve their pledges in the target year as stated, the
second and third bars assume that countries advance the accomplishment of
their pledges by 5 and 10 years respectively.

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CDR deployment that is not fully accounted for in our pathways is that
they could interact with societal priorities other than climate, creating
varying degrees of synergies and tradeoffs depending on the type and
scale of CDR measures used41–44. In addition, since not all countries are
equally endowed with CDR potential, achieving net-zero pledges and
ratcheting ambition might need to be supported by cooperative strat-
egies and/or trade38,45. A simple sensitivity analysis that explores the
implications of limited CDR availability suggests that the high ambition
pathways explored in this study are feasible under no availability of DAC
but that results in greater reductions in CO2 emissions from energy and
industrial sectors (Supplementary Figs. 11 and 12). Further research is
required to better understand the role of CDR and emissions trading
in high ambition emissions pathways16.
Regional implications of ratcheting ambition
The implications of ratcheting ambition for regional emissions
depend on whether countries currently have net-zero pledges. For
countries with net-zero pledges (for example, China, India and the
USA), cumulative emissions grow and then remain flat beyond the
year of net-zero under the pathway with central assumptions (Fig. 4).
For such countries, ratcheting ambition in the near-term—as in the
NDC++ pathways—results in slower growth of emissions and a plateau-
ing of emissions at a lower peak level. In addition, ratcheting ambition
in the near-term for such countries also facilitates an advancement of
the timing of net-zero CO2 emissions (Fig. 5). For example, ratcheting
near-term ambition from NDC to NDC++ emissions in 2030 in China,
Brazil and the USA results in an advancement of the year of net-zero
CO2 emissions from 2058, 2041 and 2046 to 2057, 2037 and 2044,
respectively. This advancement occurs despite the target years for
the official net-zero pledges—which are modelled in terms of net-zero
GHG emissions for the above countries—remaining unchanged (see
Methods and Supplementary Table 2 for details on how net-zero
pledges are modelled) because it facilitates a more rapid phase out
of fossil-fuel-based infrastructure. Advancing the target year for net-
zero pledges further advances the timing of net-zero CO2 emissions
(Supplementary Table 3).
By contrast, cumulative emissions for emerging economies with-
out net-zero pledges (for example, Middle East, Africa and Southeast
Asia) grow throughout the century under the NDC pathways—albeit at
a slower rate under the 8% minimum decarbonization rate assumption
(Fig. 4). For such countries, ratcheting ambition both in the near-term
and in the long-term—as in the pathway with NDC++ emissions in 2030
followed by an 8% minimum decarbonization rate—is critical to acceler-
ate the phase out of fossil-based infrastructure and consequently get
to net-zero CO2 emissions sooner (Fig. 5).
Discussion
This study provides an ex-ante scientific underpinning to help design
revised and more ambitious pledges in response to the calls made in the
2021 Glasgow Climate Pact and to understand their potential tempera-
ture implications during the century. Our results underscore the impor-
tance that countries ratchet their ambition in the near-term—through
2030—to reduce overshooting and thus maximize long-term climate
benefits. Our study also underscores the potential hazards of delaying
the timing of ratcheting ambition. Although limiting global warming to
<1.5 °C by the end of the century is possible even if ratcheting ambition
2020
2030
2040
2050
2060
2070
2080
2090
2100
2020
2030
2040
2050
2060
2070
2080
2090
2100
−20
0
20
40
60
−20
Year
Year
0
20
40
60
−20
0
20
40
60
–20
0
20
40
60
GHG emissions (GtCO2e)
F−gases
N2O Agriculture and land-use
N2O energy
CH4 Agriculture and land-use
CH4 energy
CO2 transportation
CO2 other energy
CO2 industry
CO2 electricity
CO2 buildings
CO2 direct air capture
CO2 Bioenergy
CO2 land-use change
a
b
c
d
Fig. 3 | GHG emissions by sector and species in a subset of the pathways
explored in the study. a, NDC/2% decarbonization rate/net-zero pledges in
target years. b, NDC/8% decarbonization rate/net-zero pledges in target years. c,
NDC++/2%decarbonization rate/net-zero pledges in target years. d, NDC++/8%
decarbonization rate/net-zero pledges in target years. See Supplementary Fig.
3 for GHG emissions by sector and species in all of the pathways explored in this
study and Supplementary Fig. 4 for a version of the figure with differences in
panels b, c and d relative to a.

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is delayed, it would result in higher overshooting during the century
potentially for a period spanning decades which could lead to irreversi-
ble and adverse consequences for human and natural systems. Delaying
ratcheting of ambition may also require accelerating post-2030 decar-
bonization to rates that are substantially higher than historical rates.
For analysts, these results emphasize the need for future research to
2020
2030
2040
2050
2060
2070
2080
2090
2100
2020
2030
2040
2050
2060
2070
2080
2090
2100
0
500
1,000
1,500
2,000
2,500
3,000
3,500
4,000
0
500
1,000
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3,000
3,500
4,000
Year
Year
0
500
1,000
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3,000
3,500
4,000
0
500
1,000
1,500
2,000
2,500
3,000
3,500
4,000
Australia and New Zealand
Other Europe
Russia
EU
South America and Caribbean
Brazil
Mexico
Canada
USA
Africa
Middle East
Other Asia
India
China
Cumulative GHG emissions from 2020 (GtCO2e)
a
b
c
d
Fig. 4 | Cumulative GHG emissions by region in a subset of the pathways
explored in the study. a, NDC/2% decarbonization rate/net-zero pledges in
target years. b, NDC/8% decarbonization rate/net-zero pledges in target years.
c, NDC++/2%decarbonization rate/net-zero pledges in target years. d, NDC++/8%
decarbonization rate/net-zero pledges in target years. See Supplementary Fig. 13
for GHG emissions by region in all of the pathways explored in this study.
2030
2040
2050
2060
2070
2080
2090
>2100
Year of
net-zero CO2
>2100
2052
2054
2047
2058
2058
2057
2057
2046
2046
2044
2044
>2100
2087
2087
2051
2070
2070
2070
2070
>2100
2068
>2100
2062
2041
2041
2037
2037
2045
2045
2042
2042
2046
2046
2044
2044
2044
2044
2042
2042
2050
2050
2050
2050
a
b
c
d
Fig. 5 | Year of net-zero CO2 emissions in a subset of the pathways explored in the
study. a, NDC/2% decarbonization rate/net-zero pledges in target years. b, NDC/8%
decarbonization rate/net-zero pledges in target years. c, NDC++/2%decarbonization
rate/net-zero pledges in target years. d, NDC++/8% decarbonization rate/net-zero
pledges in target years. See Supplementary Table 3 for year of net-zero CO2 emissions
in all of the pathways explored in this study. Global maps in this figure are created
using an open-source R package53 and documented in ref. 54. “>2100” indicates the
year of net-zero CO2 emission occurs after 2100.

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explore emissions pathways that focus not only end-of-century tem-
perature targets but also alternative pathways that limit the degree
of temperature overshoot during the century—especially those with
higher near-term ambition than implied by the current set of NDCs. A
few studies have begun to explore such alternative pathways23,29 but
more community-wide studies—including intermodel comparison
efforts—could help collect robust insights about the costs and benefits
of ratcheting ambition in the near-term and the technological options
that could facilitate the implementation of higher near-term ambition.
Our study suggests a strong potential for non-CO2 mitigation in
facilitating the higher ambition needed. Previous analyses have shown
that to limit temperature change to 1.5 °C, mitigation strategies focused
only on CO2 reduction could require getting to net-zero two decades
sooner than comprehensive strategies that include non-CO2s as well33.
While the Global Methane Pledge34 is a step in the right direction to
motivate higher non-CO2 ambition, comprehensive strategies that
account for a wider suite of GHGs would ultimately be required to
enable cost-effective emission reductions33,46.
Our results also suggest that ratcheting near-term ambition could
enable faster transitions required to accomplish net-zero pledges,
especially in major emitting economies. These transitions could very
well be accomplished with limited availability of nascent technologies
such as CDR. However, the economic implications of these transitions
would hinge on the availability of CDR and other nascent technologies
such as CCS and the ability of grid infrastructures to expand rapidly
as technologies such as renewables scale up. The speed and scale at
which these technologies can be deployed depend on a variety of
factors, such as costs, access to financial capital, supply-chain issues,
land-use and geophysical constraints and other institutional, social and
behavioural factors26,47–50. Such factors could imply severe economic
consequences47,48. Future work should consider these factors and other
real-world political and ethical ramifications of ratcheting ambition
to better understand the feasibility of the high ambition pathways
explored in this study51. Nonetheless, ratcheting near-term ambition
and concurrently testing out and establishing policies and institutional
infrastructures that phase out fossil fuels and incentivize research
and development and deployment of more nascent technologies in
the near-term will be crucial to facilitate deeper emissions reductions
in the long-term needed to cost-effectively accomplish the long-term
goals of the Paris Agreement39.
Online content
Any methods, additional references, Nature Research reporting sum-
maries, source data, extended data, supplementary information,
acknowledgements, peer review information; details of author contri-
butions and competing interests; and statements of data and code avail-
ability are available at https://doi.org/10.1038/s41558-022-01508-0.
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the author(s) or other rightsholder(s); author self-archiving of the
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Methods
The Global Change Analysis Model
GCAM is an open-source model developed and maintained at the Pacific
Northwest National Laboratory’s Joint Global Change Research Insti-
tute. In this study, we use the version of the GCAM (v.5.3) used in ref. 5
and available in a public repository55. The full documentation of the
model is available at the GCAM documentation page (http://jgcri.
github.io/gcam-doc/) and the description here is a summary of the
online documentation and based on refs. 5,56–59.
GCAM includes representations of five systems: economy, energy,
agriculture and land-use, water and climate in 32 geopolitical regions
across the globe and the associated land allocation, water use and agri-
culture production across 384 land subregions and 235 water basins.
GCAM operates in 5-year time-steps from 2015 (calibration year) to
2100 by solving for the equilibrium prices and quantities of various
energy, agricultural, water, land-use and GHG markets in each time
period and in each region. GCAM is a dynamic recursive model. Hence,
solutions for each modelling period only depend on conditions in the
last modelling period. Outcomes of GCAM are driven by exogenous
assumptions about population growth, labour participation rates and
labour productivity in the 32 geopolitical regions, along with represen-
tations of resources, technologies and policy. GCAM tracks emissions
of 24 gases, including GHGs, short-lived species and ozone precursors,
endogenously based on the resulting energy, agriculture and land-use
systems as discussed in the following subsections.
The GCAM energy system contains representations of fossil
resources (coal, oil and gas), uranium and renewable sources (wind,
solar, geothermal, hydro and biomass and traditional biomass) along
with processes that transform these resources to final energy carriers
(electricity generation, refining, hydrogen production, gas processing
and district heat), which are ultimately used to deliver goods and ser-
vices demanded by end-use sectors (residential buildings, commercial
buildings, transportation and industry). Each of the sectors in GCAM
include technological detail. For example, the electricity generation
sector includes several different technology options to convert coal to
electricity such as pulverized coal with and without carbon capture and
storage (CCS) and coal integrated gasification combined cycle (IGCC)
with and without CCS. In every sector within GCAM, individual tech-
nologies compete for market share on the basis of the levelized cost of
a technology. The cost of a technology in any period depends on (1) its
exogenously specified non-energy cost, (2) its endogenously calculated
fuel cost and (3) any cost of emissions, as determined by the climate
policy. The first term, non-energy cost, represents capital, fixed and
variable operation and maintenance costs incurred over the lifetime
of the equipment (except for fuel or electricity costs), expressed per
unit of output. For example, the non-energy cost of coal-fired power
plant is calculated as the sum of overnight capital cost (amortized using
a capital recovery factor and converted to dollars per unit of energy
output by applying a capacity factor), fixed and variable operations and
maintenance costs. The second term, fuel or electricity cost, depends
on the specified efficiency of the technology, which determines the
amount of fuel or electricity required to produce each unit of output,
as well as the cost of the fuel or electricity. The various data sources
and assumptions are documented in the GCAM documentation page
(http://jgcri.github.io/gcam-doc/).
The prices of fossil fuels and uranium are calculated endogenously.
Fossil fuel resource supply in GCAM is modelled using graded resource
supply curves that represent increasing cost of extraction as cumula-
tive extraction increases. Wind and rooftop PV technologies include
resource costs that are also calculated from exogenous supply curves
that represent marginal costs that increase with deployment, such
as long-distance transmission line costs that would be required to
produce power from remote wind resources. Utility-scale solar pho-
tovoltaic and concentrated solar power technologies are assumed to
have constant marginal resource costs regardless of deployment levels.
In GCAM, technology choice is determined by market competition.
The market share captured by a technology increases as its costs decline
but GCAM uses a logit model of market competition. This approach
is designed to represent decision-making among competing options
when only some characteristics of the options can be observed60,61 and
avoids a ‘winner takes all’ response.
The agriculture and land-use component of GCAM represents
competition for land among alternative uses in 283 agro-economic
zones within the 32 regions. Land is allocated between alternative
uses such as food crops (including wheat, corn, rice, root and tuber
and other grain), commercial biomass, forests, pasture, grassland
and shrubs based on expected profitability according to a logit-share
mechanism similar to the energy system. The profitability in turn
depends on the productivity of the land-based product (for example,
mass of harvestable product per hectare), product price and non-land
costs of production (labour, fertilizer and so on). The productivity of
land-based products is subject to change over time based on future
estimates of crop productivity change. GCAM also tracks land from
desert, tundra and urban land. However, these are excluded from
economic competition and assumed to be fixed over time. Yields for all
crops are assumed to improve over time. These improvement rates vary
by region, with higher improvement rates in developing regions. The
energy system and the agriculture and land-use systems are hard linked
(coupled in code). Commercial biomass is demanded in the energy
system while its supply is modelled in the agriculture and land-use
component. Fertilizer supply is represented in the energy system while
fertilizer demand is modelled in the agriculture and land-use system.
Traditional biomass is not modelled in the agriculture and land-use
system but is instead represented through exogenous supply curves
that account for the opportunity cost associated with collecting tradi-
tional biomass—collecting traditional biomass requires labour which
becomes increasingly expensive as incomes rise.
GCAM tracks emissions of a variety of GHG species: CO2, CH4, N2O,
HFCs (HFC23, HFC32, HFC125, HFC134a, HFC143a, HFC152a, HFC227ea,
HFC43, HFC236fa, HFC365mfc and HFC245fa), PFCs (CF4 and C2F6)
and SF6. The CO2 emissions result from direct combustion of fossil
fuels and conversion to other forms. Once a fossil fuel is extracted,
the carbon in the fuel is either emitted or sequestered. The total CO2
emissions in the base year of GCAM (currently 2015) is calibrated to the
Carbon Dioxide Information Analysis Center database62 at the global
level and fossil fuel consumption in the base year is calibrated to the
International Energy Agency’s Energy Balances Database63. Global aver-
age emissions coefficients (for example, CO2 per GJ) are derived from
the ratio of the total emissions and the total fuel consumption for each
fossil fuel (coal, oil and gas). In each model period, CO2 emissions from
a technology are calculated as the product of global average emission
coefficients obtained above and fuel consumption by that technology
in that period. Agriculture and land-use change emissions depend on
the amount of land-use change, the equilibrium carbon density of the
ecosystem and region-specific growth profiles64.
GCAM also tracks non-CO2 emissions from the energy and agri-
cultural and land-use systems. Historical emissions of CH4, N2O and
F-gases are harmonized with the 2019 US Environmental Protection
Agency (EPA) Global Non-CO2 Greenhouse Gas Emission Projections
and Mitigation Potential report65. Historical emissions of short-lived
forcing agents (BC and OC) and air pollutants (SO2, NOx and PM2.5) are
calibrated to the Community Emissions Data System66. These histori-
cal emissions are then used to develop emission factors (emission per
energy input or service output of a specific technology). Emissions
factors are assumed to change over time if air pollution controls are
tightened (local air pollutants only) or a carbon price is applied (GHGs
only; not all sectors). Future emissions are estimated as the product of
the projected economic activity, the corresponding emission factor
for a given technology and emissions reductions estimated through
marginal abatement cost (MAC) curves. MAC curves are based on ref. 65.

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Article
https://doi.org/10.1038/s41558-022-01508-0
In our pathways, non-CO2 emissions can be controlled by two mech-
anisms. First, changes in activity (phasing out of carbon-intensive fuels
due to climate policy) will reduce non-CO2 emissions (for example,
fugitive CH4 from natural gas production). Second, for emission sources
without explicit representation of the underlying activity, emission
reductions are calculated off of MAC curves that are parametrized to
abatement technologies and abatement levels. MAC curves represent
the mitigation cost and corresponding emission reductions achievable
for each region, species and available source categories over time.
The version of GCAM used in this study includes important recent
technological and socioeconomic trends. First, the effect of COVID-19
on the global economy is reflected by incorporating the latest
country-specific International Monetary Fund GDP growth projec-
tions67. Second, electric power technology cost assumptions (capital
cost, operation and maintenance cost and efficiency) follow recent
trends and projections and are based on the 2019 National Renew-
able Energy Laboratory (NREL) Annual Technology Baseline68. These
assumptions entail substantial capital and operation and mainte-
nance cost reductions for most technologies, especially solar and wind
technologies. Third, the version of GCAM used in this study includes
electrification options in the transportation sector including electric
vehicles and electric trucks. Our transportation cost and energy inten-
sity assumptions are based on the NREL Electrification Futures Study69.
The version of GCAM used in this study assumes the availability
of three CDR options: afforestation, BECCS and DAC technologies.
The scale of each option is determined by economics. Our pathways
incentivize afforestation by assuming a gradual transition—by 2050—to
a regime in which CO2 emissions from land-use changes are valued at
the same price as emissions from the energy system59,70. As described
earlier, in GCAM, bioenergy competes for land with other land uses on
the basis of profitability. BECCS technologies are deployed in a variety
of sectors within the GCAM energy system including refining, electric-
ity generation and hydrogen production. Our assumptions for DAC
technologies are documented in Supplementary Table 4 and refs. 43,40.
Hector
Hector is the reduced-form carbon-cycle climate module that is avail-
able for use in GCAM15,71 and is an open-source model. This study is
based on Hector v.2.5. Hector has a three-part carbon cycle: one-pool
atmosphere, three-pool land and four-pool ocean. The model’s ter-
restrial carbon cycle includes primary production and respiration
fluxes while also accommodating arbitrary geographic divisions, such
as ecological biomes or political units. Hector’s ocean component
includes a detailed representation of the inorganic carbon cycle, cal-
culating air–sea fluxes and ocean pH (ref. 71). Hector reproduces the
global historical trends of atmospheric CO2, radiative forcing and
surface temperatures.
GCAM interacts with Hector through emissions. At every time step,
emissions from GCAM are passed to Hector. Hector then converts these
emissions to concentrations when necessary and calculates the associ-
ated radiative forcing, as well as the response of the climate system (for
example, surface temperature and carbon fluxes).
Emissions pathways
The representation of the NDCs in our central pathway is based on
ref. 5 and is explained in detail in the supplementary information to
that study. This study also includes 21 new and/or updated NDCs after
30 September 2021, including those from China, Pakistan and many
African and Middle Eastern countries that were not included in ref. 5
(Supplementary Table 5). We assume that the NDCs are achieved as
stipulated and focus on the climate outcomes of their successful imple-
mentation. Examining the likelihood of individual regions achieving
their submitted targets is beyond the scope of this study.
Our representation includes only ratified and quantifiable uncon-
ditional NDC commitments, including absolute emissions limit,
percentage emission reductions from a given reference level and emis-
sion intensity targets. Parties whose commitments included: (1) only
actions/policies, (2) non-GHG targets with no corresponding GHG
emissions target or (3) only sector-specific GHG emissions reduction
targets without attempting to quantify the impact on their overall GHG
footprints are assumed to have target year emissions equal to the GCAM
emissions in the default reference scenario without any climate policy
(‘Reference—No Policy’ in ref. 5). Likewise, in cases where a country’s
2025 and 2030 emissions based on its NDC are lower than the default
reference scenario in the same year, the NDC emissions are assumed to
be achieved as stipulated. In cases where a country’s NDC emissions are
higher, emissions are assumed to be equal to the reference scenario.
For countries that included multiple types of commitments in their
NDCs, such as economy-wide emissions reductions backed by sectoral
policies or targets, only the broadest commitment was considered.
For example, China’s NDC representation in GCAM is based on its
commitment to reduce its carbon intensity of GDP by 65% relative to
2005 and it does not explicitly model its targets for non-fossil energy
consumption or increased forest stock.
Similar to ref. 5, our pathways include LTSs and net-zero pledges.
For countries with LTSs that are different from a net-zero pledge (for
example, Mexico), emissions are assumed to meet their NDC commit-
ments in 2030 first. Beyond 2030, emissions linearly reduce to the LTS
in the specified target year and then continue to follow a path defined
by the decarbonization rate between 2015 and the LTS target year. For
countries with net-zero pledges, emissions are assumed to meet their
NDC commitments in 2030 first. Beyond 2030, emissions linearly reduce
to net-zero in the target year and then remain constant afterwards. In
the cases where countries have explicitly committed to net-zero CO2
emissions, such as South Korea, only CO2 emissions are constrained. This
study also includes additional net-zero pledges that were announced
after the completion of the ref. 5 study. These include pledges from India,
Brazil, Australia, New Zealand and Argentina (Supplementary Table 2).
Where the scope of net-zero targets is somewhat unclear (as in the case
of Japan) or in cases where countries use terms such as ‘carbon neutral’
and ‘net-zero GHG emissions’ interchangeably, we follow the CAT assess-
ment and assume a net-zero GHG target. For example, China announced
a ’carbon neutrality’ goal by 2060, which is assumed as a net-zero GHG
emission target in our main analysis. This assumption is consistent with
latest official interpretations of China’s net-zero pledge72.
Our post-2030 decarbonization rate assumptions are consistent
with refs. 57,5. However, our definition of decarbonization rate is based
on all GHGs while the definitions used by ref. 57,5 are based only on fossil
fuel and industrial CO2 emissions (note that the emissions scenarios
modelled in the studies of refs. 57,5 do include concurrent reductions
in non-CO2s in response to CO2 reductions that are facilitated by the
decarbonization rate assumptions). Our central assumption about
the post-2030 minimum decarbonization rate is 2% and our sensitiv-
ity assumptions are 5% and 8%. While the 2% rate has been achieved
routinely in history and represents a moderate level of post-2030
mitigation, the 5% and 8% decarbonization rate assumptions can be
considered as requiring more dedicated, stringent mitigation policies
(Supplementary Fig. 14). For additional context, the 2% decarboni-
zation rate falls under the higher end of the distribution of decar-
bonization rates implied in the ‘baseline’ scenarios assessed by the
Intergovernmental Panel on Climate Change (IPCC) Special Report on
1.5 °C (SR1.5) and the 5% and 8% assumptions lie at the peak of the distri-
bution of decarbonization rates in scenarios limiting global warming
to 1.5 °C (Supplementary Figs. 15 and 16)18. We note that 2% minimum
decarbonization rate assumption is not binding for any region since
the implied 2015–2030 decarbonization rate in the NDCs for all regions
is >2% (Supplementary Table 6).
Notably, there is some interaction and overlap among the three
strategies explored in this study that countries might use to ratchet
ambition. With higher 2030 ambition, the post-2030 minimum

Nature Climate Change
Article
https://doi.org/10.1038/s41558-022-01508-0
decarbonization rate assumption might no longer be binding in some
cases. For example, in the case of India, the 2015–2030 decarbonization
rates in the pathways with the NDC, NDC+ and NDC++ emission levels in
2030 are, respectively, 2.1%, 4.4% and 4.4% (see Supplementary Table
6 for 2015–2030 decarbonization rates under the NDC, NDC+ and
NDC++ emission levels in 2030). Hence, the 2% post-2030 minimum
decarbonization rate assumption would be binding only in the NDC
cases. In addition, advancing the timing of the net-zero pledges (for
countries with net-zero pledges) would result in higher post-2030
decarbonization rates. However, it is important to note that our mini-
mum decarbonization rate assumptions (2%, 5% and 8%) do not affect
the emission pathways of countries with net-zero pledges since these
countries are always assumed to achieve their pledges—in the specified
target years, 5 years in advance or 10 years in advance.
Data availability
Country ratings from the CAT are publicly available at https://climate-
actiontracker.org/countries/. The latest (v.2020) Human Develop-
ment Index is publicly available at https://hdr.undp.org/data-center/
human-development-index#/indicies/HDI. The datasets generated
during and analysed in the current study are available from a public
repository (https://doi.org/10.5281/zenodo.7069063). Source data
are provided with this paper.
Code availability
GCAM is an open-source community model available at https://github.
com/JGCRI/gcam-core/releases. The version of GCAM and additional
input files associated with this study are available at https://doi.
org/10.5281/zenodo.7069066.
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Acknowledgements
The research described in this paper was conducted with support
from the US EPA IAA DW-089-92460001 (G.I., Y.O., J.E., J.F., S.W. and
H.M.). 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 or the US Government and no
official endorsement should be inferred.
Author contributions
G.I. and Y.O. contributed equally to this study. G.I., Y.O., J.E.,
A.A.F., N.H. and H.M. designed the research. G.I. wrote the
first draft of the paper. Y.O. conducted the simulations. G.I., Y.O.,
J.E., J.F., S.W. and H.M. contributed to the modelling tools. J.M.
contributed to the writing of the paper. All authors contributed
to writing the paper.
Competing interests
The authors declare no competing interests.
Additional information
Supplementary information The online version contains
supplementary material available at
https://doi.org/10.1038/s41558-022-01508-0.
Correspondence and requests for materials should be addressed to
Haewon McJeon.
Peer review information Nature Climate Change thanks Matthias
Weitzel, Ioannis Dafnomilis and Kate Dooley for their contribution to
the peer review of this work.
Reprints and permissions information is available at
www.nature.com/reprints.

    Policy Brief:

Answer:
** The path to 1.5 °C requires ratcheting of climate pledges**

**Policy Problem**A total of 151 countries submitted updated or new climate pledges to the 26th United Nations Conference of Parties (COP26) held in Glasgow in November 2021, outlining their plans to cut GHG emissions. Countries also communicated long-term strategies that outline emission reduction strategies to the mid-century and net-zero emissions targets. Although these updated and new pledges suggest higher ambition as compared with the 2015 Paris pledges, limiting global warming below 1.5 °C this century — the goal of the Paris Agreement — will require countries to further ratchet or increase ambition. Recognizing the need for more ambition, nations adopted the Glasgow Climate Pact, which calls for countries “to revisit and strengthen the…targets…to align with the Paris Agreement temperature goal by the end of 2022”. This study explores the long-term temperature outcomes of ratcheting or increasing ambition, as well as what this ratcheting implies for sectoral and regional emissions.

**Scientific Research Findings**We find that ratcheting near-term ambition to 2030 will be crucial to limiting peak temperature changes this century. If ratcheting is delayed, it would result in higher temperature overshooting — that is, an exceedance of global mean temperature change above the intended threshold before returning to below the intended level — over many decades, with the potential for adverse and irreversible consequences for human and natural systems. Our results also suggest that ratcheting near-term ambition could facilitate faster transitions to net-zero emissions systems — especially in major economies — resulting in faster reductions in emissions from all sectors of the economy for both CO2 and non-CO2 emissions. Although these transitions can be accomplished with limited availability of nascent technologies such as CO2 removal, further research is required to better understand the role of such technologies in high-ambition emissions pathways.

**Scientific Research Study Methods**We explored a suite of high-ambition emissions pathways in which countries ratchet and achieve ambition through a combination of increasing near-term ambition to 2030, accelerating post-2030 decarbonization, and advancing the dates for national net-zero pledges. We developed the pathways using the Global Change Analysis Model (GCAM), which includes representations of the economy, energy, agriculture and land use, water, and climate systems and their interactions in 32 geopolitical regions (including many individual countries) across the globe. GCAM tracks emissions of GHGs on the basis of the evolution of these systems over the century. These detailed representations allowed us to model countries’ emission reduction pledges and the sectoral and regional implications of ratcheting those pledges. We used a simple climate model (Hector) to compute the temperature change implications of the emissions pathways. Our study also includes sensitivity analyses on the availability of CO2removal technologies and the underlying assumptions and methodology used to construct our emissions pathways.

**Policy Implications**• Ratcheting near-term ambition to 2030 as called for by the Glasgow Climate Pact can help to reduce peak temperature change and bring us closer to the Paris Agreement’s 1.5 °C temperature goal.
• Delaying ratcheting of ambition to beyond 2030 could still limit end-of-century temperature change to below 1.5 °C but would result in higher temperature overshoot over many decades.
• Ratcheting near-term ambition could also facilitate faster transitions to net-zero emissions systems, especially in major economies.
• Ratcheting near-term ambition could lay strong institutional infrastructures required to phase out fossil fuels and incentivize research and development, and to deploy more nascent technologies.