--- chapter: doi: 10.7930/NCA4.2018.CH29 identifier: mitigation-avoiding-and-reducing-long-term-risks number: 29 report_identifier: nca4 sort_key: 129 title: Reducing Risks through Emissions Mitigation url: https://nca2018.globalchange.gov/chapter/29 chapter_identifier: mitigation-avoiding-and-reducing-long-term-risks cited_by: [] confidence: '

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.

' contributors: [] evidence: "

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.{{< tbib '2' '0b30f1ab-e4c4-4837-aa8b-0e19faccdb94' >}}^,{{}}^,{{}}^,{{}} 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,{{< tbib '2' '0b30f1ab-e4c4-4837-aa8b-0e19faccdb94' >}}^,{{}} hundreds to thousands of deaths per year from poor air quality,^{{{< tbib '2' '0b30f1ab-e4c4-4837-aa8b-0e19faccdb94' >}}^,{{}}} and the loss of hundreds of millions of labor hours.{{< tbib '2' '0b30f1ab-e4c4-4837-aa8b-0e19faccdb94' >}}^,{{}}^,{{}}

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,{{< tbib '29' '79a24453-16b4-4acf-9a83-cc902de94033' >}}^,{{}} temperature-related health effects,{{< tbib '99' 'ea2ea20a-5d62-49ac-a89b-9a7951711a1b' >}}^,{{}}^,{{}} agricultural yields,^{{{< tbib '187' '2cc7d464-2d35-46c7-abd7-2ca6da85496b' >}}^,{{}}^,{{}}} and wildfires.{{< tbib '73' '3592f4d3-26dc-401c-8d9e-03791923637b' >}}^,{{}}^,{{}}

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. 2015{{< tbib '2' '0b30f1ab-e4c4-4837-aa8b-0e19faccdb94' >}}^,{{}}^,{{}}^,{{}}) and focused sector studies (e.g., Melvin et al. 2016, Neumann et al. 2014{{< tbib '71' '5b27123a-8c6d-4e85-bd48-841436fdf9eb' >}}^,{{}}). 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. 2016{{< tbib '100' '1ad1d794-bc57-4e48-ab28-0e2b65767cb9' >}}^,{{}}), agriculture (e.g., Beach et al. 2015{{< tbib '192' '49d89afb-f314-4386-8d38-213e66de8cad' >}}), and water resources (e.g., Chapra et al. 2017, Wobus et al. 2017, EPA 2013{{< tbib '167' '80dd6dfe-4dea-4253-a65b-53f620805f9a' >}}^,{{}}^,{{}}).

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,{{< tbib '2' '0b30f1ab-e4c4-4837-aa8b-0e19faccdb94' >}}^,{{}}^,{{}}^,{{}} as the human-forced response may not yet have emerged from the noise of natural climate variability.{{< tbib '6' '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.{{< tbib '2' '0b30f1ab-e4c4-4837-aa8b-0e19faccdb94' >}}^,{{}}^,{{}} 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.{{< tbib '2' '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).{{< tbib '2' '0b30f1ab-e4c4-4837-aa8b-0e19faccdb94' >}}^,{{}}^,{{}}

" files: [] gcmd_keywords: - definition: |- Glaciers are masses of land ice, formed by the further recrystallization of firn, flowing continuously from higher to lower elevations. Ice sheets are a continuous sheet of land ice that covers a very large area and moves outward in many directions. This type of ice mass is so thick as to mask the land surface contours, in contrast to the smaller and thinner highland ice. The continental glacier of Greenland is sometimes called the Inland Ice. This term is often used to describe the great ice masses that characterized the ice ages. href: https://data.globalchange.gov/gcmd_keyword/8603db51-3484-4439-8b3b-a06f48e8c686.yaml identifier: 8603db51-3484-4439-8b3b-a06f48e8c686 label: GLACIERS/ICE SHEETS parent_identifier: fa0a36c3-2503-4662-98cd-7f3e74ce9f80 uri: /gcmd_keyword/8603db51-3484-4439-8b3b-a06f48e8c686 - definition: |- Scientific field of study of the land environment immediately affected by marine processes. Includes variables pertaining to both coastal features and the processes that affect them. href: https://data.globalchange.gov/gcmd_keyword/b6fd22ab-dca7-4dfa-8812-913453b5695b.yaml identifier: b6fd22ab-dca7-4dfa-8812-913453b5695b label: COASTAL PROCESSES parent_identifier: 91697b7d-8f2b-4954-850e-61d5f61c867d uri: /gcmd_keyword/b6fd22ab-dca7-4dfa-8812-913453b5695b - definition: 'The science and art of preventing disease, prolonging life and promoting health through the organized efforts and informed choices of society, organizations, public and private, communities and individuals.' href: https://data.globalchange.gov/gcmd_keyword/da2c70fd-d92b-45be-b159-b2c10cb387c6.yaml identifier: da2c70fd-d92b-45be-b159-b2c10cb387c6 label: PUBLIC HEALTH parent_identifier: fb93d937-c17c-45d0-a9e3-ca5c8a800ca8 uri: /gcmd_keyword/da2c70fd-d92b-45be-b159-b2c10cb387c6 - definition: 'Refers to the total number of inhabitants constituting a particular race, class, or group in a specified area.' href: https://data.globalchange.gov/gcmd_keyword/085edf65-1c8c-414a-b8e4-a1a08ff08f22.yaml identifier: 085edf65-1c8c-414a-b8e4-a1a08ff08f22 label: POPULATION parent_identifier: fb93d937-c17c-45d0-a9e3-ca5c8a800ca8 uri: /gcmd_keyword/085edf65-1c8c-414a-b8e4-a1a08ff08f22 - definition: |- Models that aid in decision support of a specific topic such as population, irrigation scheduling, coastal zone decisions, ecosystem distribution, crop yields, water use, energy generation, telecommunications, and space weather activity. href: https://data.globalchange.gov/gcmd_keyword/9a1dd3c3-a126-437e-ad04-9dc0a382d567.yaml identifier: 9a1dd3c3-a126-437e-ad04-9dc0a382d567 label: SOCIAL AND ECONOMIC MODELS parent_identifier: e1f20631-b5b9-438c-b5c2-b1fa0fce100a uri: /gcmd_keyword/9a1dd3c3-a126-437e-ad04-9dc0a382d567 - definition: |- Mitigation includes any activities that prevent a hazard emergency, reduce the chance of an emergency happening, or lessen the damaging effects of unavoidable emergencies. href: https://data.globalchange.gov/gcmd_keyword/a2e9c7b9-96fd-449f-91db-7ab5e2dd679e.yaml identifier: a2e9c7b9-96fd-449f-91db-7ab5e2dd679e label: HAZARDS MITIGATION parent_identifier: 464de0a5-2bb9-4172-9fd3-1634cbc4e739 uri: /gcmd_keyword/a2e9c7b9-96fd-449f-91db-7ab5e2dd679e - definition: 'Any measurement or statistic of, related, or based on the production, distribution, and consumption of goods and services.' href: https://data.globalchange.gov/gcmd_keyword/cdbe5ef5-408d-489d-b6ff-4482ce4a99c7.yaml identifier: cdbe5ef5-408d-489d-b6ff-4482ce4a99c7 label: ECONOMIC RESOURCES parent_identifier: fb93d937-c17c-45d0-a9e3-ca5c8a800ca8 uri: /gcmd_keyword/cdbe5ef5-408d-489d-b6ff-4482ce4a99c7 - definition: "The observed physical cover and natural use of the land including the vegetation and human construction, which covers the earth's surface." href: https://data.globalchange.gov/gcmd_keyword/e5815f58-8232-4c7f-b50d-ea71d73891a9.yaml identifier: e5815f58-8232-4c7f-b50d-ea71d73891a9 label: LAND USE/LAND COVER parent_identifier: 6a426480-c58f-4b6b-8e35-0975b7f6edb5 uri: /gcmd_keyword/e5815f58-8232-4c7f-b50d-ea71d73891a9 href: https://data.globalchange.gov/report/nca4/chapter/mitigation-avoiding-and-reducing-long-term-risks/finding/key-message-29-3.yaml identifier: key-message-29-3 ordinal: 3 parents: - activity_uri: ~ label: scenario rcp_4_5 note: '' publication_type_identifier: scenario relationship: prov:wasDerivedFrom url: /scenario/rcp_4_5 - activity_uri: ~ label: scenario rcp_8_5 note: '' publication_type_identifier: scenario relationship: prov:wasDerivedFrom url: /scenario/rcp_8_5 process: "

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.{{< tbib '155' '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.

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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).

' 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.{{< tbib '85' '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.

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