Many climate change impacts and associated economic damages in the United States can be substantially reduced over the course of the 21st century through global-scale reductions in greenhouse gas emissions, though the magnitude and timing of avoided risks vary by sector and region (very high confidence). The effect of near-term emissions mitigation on reducing risks is expected to become apparent by mid-century and grow substantially thereafter (very high confidence).

This finding is from chapter 29 of Impacts, Risks, and Adaptation in the United States: The Fourth National Climate Assessment, Volume II.

Process for developing key messages:

The scope for this chapter was determined by the federal Fourth National Climate Assessment (NCA4) Steering Committee, which is made up of representatives from the U.S. Global Change Research Program (USGCRP) member agencies (see App. 1: Process for more information regarding the Steering Committee). The scope was also informed by research needs identified in the Third National Climate Assessment (NCA3) and in subsequent gap analyses.d6eb34ef-1bfb-4b90-a397-f6bb363086a0 Prospective authors were nominated by their respective agency, university, organization, or peers. All prospective authors were interviewed with respect to their qualifications and expertise. Authors were selected to represent the diverse perspectives relevant to mitigation, with the final team providing perspectives from federal and state agencies, nonfederal climate research organizations, and the private sector. The author team sought public input on the chapter scope and outline through a webinar and during presentations at conferences and workshops.

The chapter was developed through technical discussions of relevant evidence and expert deliberation by the report authors during extensive teleconferences, workshops, and email exchanges. These discussions were informed by the results of a comprehensive literature review, including the research focused on estimating the avoided or reduced risks of climate change. The authors considered inputs submitted by the public, stakeholders, and federal agencies and improved the chapter based on rounds of review by the public, National Academies of Sciences, Engineering, and Medicine, and federal agencies. The author team also engaged in targeted consultations during multiple exchanges with contributing authors from other chapters of this assessment, as well as authors of the Climate Science Special Report (CSSR). For additional information on the overall report process, see Appendix 1: Process.

Description of evidence base:

There are multiple lines of research and literature available to characterize the effect of large-scale GHG mitigation in avoiding or reducing the long-term risks of climate change in the United States. Recent multisector impacts modeling projects, all of which feature consistent sets of scenarios and assumptions across analyses, provide improved capabilities to compare impacts across sectors and regions, including the effect of global GHG mitigation in avoiding or reducing risks.0b30f1ab-e4c4-4837-aa8b-0e19faccdb94^,fad9e8ec-8951-4daa-9a9c-e093ef86af16^,0006123e-10a3-4501-a89c-95a7921a9c3d^,9f559c9b-c78e-4593-bcbe-f07661d29e16 The results of these coordinated modeling projects consistently show reductions in impacts across sectors due to large-scale mitigation. For most sectors, this effect of mitigation typically becomes clear by mid-century and increases substantially in magnitude thereafter. In some sectors, mitigation can provide large benefits. For example, by the end of the century, reduced climate change under a lower scenario (RCP4.5) compared to a higher one (RCP8.5) avoids (on net, and absent additional risk reduction through adaptation) thousands to tens of thousands of deaths per year from extreme temperatures,0b30f1ab-e4c4-4837-aa8b-0e19faccdb94^,9f559c9b-c78e-4593-bcbe-f07661d29e16 hundreds to thousands of deaths per year from poor air quality,^{0b30f1ab-e4c4-4837-aa8b-0e19faccdb94,81f96860-7931-48b6-9d57-32682728636f} and the loss of hundreds of millions of labor hours.0b30f1ab-e4c4-4837-aa8b-0e19faccdb94^,fad9e8ec-8951-4daa-9a9c-e093ef86af16^,9f559c9b-c78e-4593-bcbe-f07661d29e16

Beyond these multisector modeling projects, an extensive literature of sector-specific studies compares impacts in the United States under alternative scenarios. A careful review of these studies, especially those published since the Third National Climate Assessment, finds strong and consistent support for the conclusion that global GHG mitigation can avoid or reduce the long-term risks of climate change in the United States. For example, mitigation is projected to reduce the risk of adverse impacts associated with extreme weather events,79a24453-16b4-4acf-9a83-cc902de94033^,09f73f2d-8a26-469f-9607-25e2fd05539f temperature-related health effects,ea2ea20a-5d62-49ac-a89b-9a7951711a1b^,1ad1d794-bc57-4e48-ab28-0e2b65767cb9^,ec9926c5-6257-49b3-8bfd-c9a02c0bf75b agricultural yields,^{2cc7d464-2d35-46c7-abd7-2ca6da85496b,bc6c6b92-e049-4b86-b772-8d35032d3cb0,fba7132f-b922-4228-8ad5-6336d0c3de76} and wildfires.3592f4d3-26dc-401c-8d9e-03791923637b^,b7e764c8-8912-4d18-8dd3-1555ab8da1c2^,4d1a8689-4d86-4a2f-996a-1702f2de6ddd

The finding that the magnitude and timing of avoided risks vary by sector and region, as well as due to changes in socioeconomics and adaptive capacity, is consistently supported by the broad literature base of multisector analyses (e.g., Hsiang et al. 2017, O’Neill et al. 2017, EPA 2017, Houser et al. 20150b30f1ab-e4c4-4837-aa8b-0e19faccdb94^,fad9e8ec-8951-4daa-9a9c-e093ef86af16^,0006123e-10a3-4501-a89c-95a7921a9c3d^,9f559c9b-c78e-4593-bcbe-f07661d29e16) and focused sector studies (e.g., Melvin et al. 2016, Neumann et al. 20145b27123a-8c6d-4e85-bd48-841436fdf9eb^,df6fcad4-f0ea-4c60-97e1-ae2a40455f51). Complex spatial patterns of avoided risks are commonly observed across sectors, including for human health effects (e.g., Fann et al. 2015, Sarofim et al. 20161ad1d794-bc57-4e48-ab28-0e2b65767cb9^,54a66159-1675-43bb-b5d3-a9b7f283e4de), agriculture (e.g., Beach et al. 201549d89afb-f314-4386-8d38-213e66de8cad), and water resources (e.g., Chapra et al. 2017, Wobus et al. 2017, EPA 201380dd6dfe-4dea-4253-a65b-53f620805f9a^,28077cd1-c29f-48ae-a068-2cdcef880807^,fc630495-cb5f-496c-a759-87166b8569a6).

The weight of evidence among studies in the literature indicates that the difference in climate impact outcomes between different scenarios is more modest through the first half of the century,0b30f1ab-e4c4-4837-aa8b-0e19faccdb94^,0006123e-10a3-4501-a89c-95a7921a9c3d^,9f559c9b-c78e-4593-bcbe-f07661d29e16^,b87babf4-a67d-4e2c-8a8d-a660b34aec3a as the human-forced response may not yet have emerged from the noise of natural climate variability.9c909a77-a1d9-477d-82fc-468a6b1af771 In evaluating and quantifying multisector impacts across alternative scenarios, the literature generally shows that the effect of near-term mitigation in avoiding damages increases substantially in magnitude after 2050.0b30f1ab-e4c4-4837-aa8b-0e19faccdb94^,0006123e-10a3-4501-a89c-95a7921a9c3d^,9f559c9b-c78e-4593-bcbe-f07661d29e16 For example, mitigation under RCP4.5 is projected to reduce the number of premature deaths and lost labor hours from extreme temperatures by 24% and 21% (respectively) by 2050, and 58% and 48% by 2090.0b30f1ab-e4c4-4837-aa8b-0e19faccdb94 For coastal impacts, where inertia in the climate system leads to smaller differences in rates of sea level rise across scenarios, the effects of near-term mitigation only become evident toward the end of the century (Ch. 8: Coastal).0b30f1ab-e4c4-4837-aa8b-0e19faccdb94^,9f559c9b-c78e-4593-bcbe-f07661d29e16^,3bae2310-7572-47e2-99a4-9e4276764934

New information and remaining uncertainties:

Quantifying the multisector impacts of climate change involves a number of analytic steps, each of which has its own potential sources of uncertainty. The timing and magnitude of projected future climate change are uncertain due to the ambiguity introduced by human choices, natural variability, and scientific uncertainty, which includes uncertainty in both scientific modeling and climate sensitivity. One of the most prominent sources involves the projection of climate change at a regional level, which can vary based on assumptions about climate sensitivity, natural variability, and the use of any one particular climate model. Advancements in the ability of climate models to resolve key aspects of atmospheric circulation, improved statistical and dynamic downscaling procedures, and the use of multiple ensemble members in impact analyses have all increased the robustness of potential climate changes that drive impact estimates described in the recent literature. However, key uncertainties and challenges remain, including the structural differences between sectoral impact models, the ability to simulate future impacts at fine spatial and temporal resolutions, and insufficient approaches to quantify the economic value of changes in nonmarket goods and services.e311cbe3-cf61-445a-ae6f-130056df0558 In addition, the literature on economic damages of climate change in the United States is incomplete in coverage, and additional research is needed to better reflect future socioeconomic change, including the ability of adaptation to reduce risk.

Assessment of confidence based on evidence:

There is very high confidence that large-scale reductions in GHG emissions throughout the 21st century are projected to reduce the level of climate change projected to occur in the United States, along with the adverse impacts affecting human health and the environment. Across the literature, there are limited instances where mitigation, compared to a higher emissions scenario, does not provide a net beneficial outcome for the United States. While the content of this chapter is primarily focused on the 21st century, confidence in the ability of mitigation to avoid or reduce impacts improves when considering impacts beyond 2100.

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