Can updated climate pledges limit warming well below 2°C?

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, including rapidly declining costs of photovoltaic cells, the expansion of electric vehicles, and the availability of CO₂ removal measures such as bioenergy in conjunction with carbon capture 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 updated pledges. The Reference–No policy scenario 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 reductions in renewable energy. The Current policy and Current policy–Continued ambition scenarios assume that countries implement current sectoral policies and national climate 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 decarbonize their economies at the same annual decarbonization rate—defined as the annual rate of improvement in CO₂ emissions per unit of gross domestic product (GDP)—that was required to implement the policies between 2015 and 2030. By contrast, the Current policy–Continued ambition scenario assumes a minimum decarbonization rate of 2% per year beyond 2030. In other words, this scenario assumes that countries continue to decarbonize 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 ambition and Updated pledges–Increased ambition scenarios assume that countries achieve the updated pledges through 2030 as stated. Collectively, the updated pledges result in lower CO₂ emissions from energy and industry in 2030 by 15% relative to the 2015 pledges (which corresponds to a 15% increase relative to 2005), suggesting a strengthening of ambition 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 between 2015 and their LTS target year.

Countries with net-zero pledges are assumed to achieve net-zero emissions in the target year and then continue to keep their emissions 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 assumptions 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 literature (see the figure). However, many scenarios in the literature result in net-negative emissions in the second half of the century. By contrast, our scenarios assume that countries reduce emissions at most to net-zero because no country has committed to netnegative emissions thus far.

We estimate probabilistic temperature outcomes for the emission scenarios using a simple climate model [Model for the Assessment of Greenhouse Gas Induced Climate Change (MAGICC)] in a probabilistic setup (see materials and methods, section S3) that accounts for the latest understanding of climate 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 policy scenarios, they result in lower probabilities of temperature change exceeding 4°C in 2100 and higher probabilities of limiting temperature change below 2°C. Under the Updated pledges–Continued ambition scenario, the probability of global mean surface temperature 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%, respectively, 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 virtually eliminated but the probabilities of limiting temperature change below 2°C and 1.5°C also increase to 60 and 11% respectively (compared with 31 and 5% with the 2015 pledges).

To understand the role of improved climate science since the original Fawcett et al. study (11), we performed additional simulations 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 accounts for the improved understanding of climate science (see materials and methods, 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 uncertainties versus updates to emissions trajectories between the two studies.

Results from the additional simulations suggest that most of the differences in probabilistic 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 technological 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 limiting 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).

Our analysis is based on a single integrated assessment model driven by a set of assumptions about future population, GDP, technological change, and resources. Other models and assumptions could result in different quantitative results. However, the core qualitative insight of this analysis would remain: if implemented as stated, updated pledges offered 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 generated 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 cooperation 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 opportunities 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 could entail substantial political, financial, institutional, social, and behavioral challenges (14). Surmounting these challenges with policies and measures and successfully achieving the targets would substantially reduce global climate risks, essentially avoiding the worst outcomes from climate change and substantially improving the probability of limiting global warming to well below 2°C.

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.