In the absence of more significant global mitigation efforts, climate change is projected to impose substantial damages on the U.S. economy, human health, and the environment (very high confidence). Under scenarios with high emissions and limited or no adaptation, annual losses in some sectors are estimated to grow to hundreds of billions of dollars by the end of the century (high confidence). It is very likely that some physical and ecological impacts will be irreversible for thousands of years, while others will be permanent (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:

Recent scientific and economic advances are improving the ability to understand and quantify the physical and economic impacts of climate change in the United States, including how those risks can be avoided or reduced through large-scale GHG mitigation. While the projected impacts of climate change across sectors and regions are well documented throughout this assessment, several multisector modeling projects are enabling the comparison of effects through the use of consistent scenarios and assumptions.0b30f1ab-e4c4-4837-aa8b-0e19faccdb94^,fad9e8ec-8951-4daa-9a9c-e093ef86af16^,0006123e-10a3-4501-a89c-95a7921a9c3d^,9f559c9b-c78e-4593-bcbe-f07661d29e16 A well-recognized conclusion from the literature produced by these projects is that climate change is projected to adversely affect the U.S. economy, human health, and the environment, each of which is further detailed below. These estimated damages increase over time, especially under a higher scenario (RCP8.5). For sectors where positive effects are observed in some regions or for specific time periods (for example, reduced mortality from extreme cold temperatures or beneficial effects on crop yields), the effects are typically dwarfed by changes happening overall within the sector or at broader scales (for example, comparatively larger increases in mortality from extreme heat or many more crops experiencing adverse effects).0b30f1ab-e4c4-4837-aa8b-0e19faccdb94^,fad9e8ec-8951-4daa-9a9c-e093ef86af16^,0006123e-10a3-4501-a89c-95a7921a9c3d^,9f559c9b-c78e-4593-bcbe-f07661d29e16 In Figure 29.2, wildfire is the only sector showing positive effects, a result driven in this particular study by projected shifts to vegetation with longer fire return intervals.0b30f1ab-e4c4-4837-aa8b-0e19faccdb94 However, it is important to note that the analysis underlying this result did not quantify the broader economic effects associated with these vegetative shifts, including ecosystem disruption and changes to ecosystem services. See Chapter 6: Forests for a discussion on the weight of evidence regarding projections of future wildfire activity, which generally show increases in annual area burned over time. See Chapter 25: Southwest for a discussion on aridification toward the end of this century under high emissions.

There is robust and consistent evidence that climate change is projected to adversely affect many components of the U.S. economy. Increasing temperatures, sea level rise, and changes in extreme events are projected to affect the built environment, including roads, bridges, railways, and coastal development. For example, coastal high tide flooding is projected to significantly increase the hours of delay for vehicles.b4808700-a94a-44da-b2bb-d360a83146f1 Annual damages to coastal property from sea level rise and storm surge, assuming no adaptation, are projected to range in the tens to hundreds of billions of dollars by the end of the century under RCP8.5 (Ch. 8: Coastal).0b30f1ab-e4c4-4837-aa8b-0e19faccdb94^,9f559c9b-c78e-4593-bcbe-f07661d29e16 Projected annual repair costs in order for roads, bridges, and railways to maintain levels of service in light of climate change range in the billions to tens of billions of dollars under RCP8.5.0b30f1ab-e4c4-4837-aa8b-0e19faccdb94^,64f34eaa-2b79-4940-b45b-1fdc2c9e5b86 Numerous studies suggest that regional economies can also be at risk, especially when they are tied to environmental resources or ecosystem services that are particularly vulnerable to climate change. For example, projected declines in coral reef-based recreationc3eee222-c3b5-4e90-a034-5e90f96c2687^,b07595f7-51db-4005-801f-29225fa042f7^,3d9112b9-6aa1-4614-9599-6966c9591ef9 would lead to decreases in tourism revenue; shorter seasons for winter recreation would likely lead to the closure of ski areas and resorts;80dd6dfe-4dea-4253-a65b-53f620805f9a^,1f240a8b-51f2-4841-aa71-bdb97c735fcf^,69deec00-efdd-44f2-b2fd-2b92252d1bec^,b1729fa8-3fbf-4311-a2d0-e0b36ccb9fb6 and increased risks of harmful algal blooms can limit reservoir recreation (Ch. 3: Water).28077cd1-c29f-48ae-a068-2cdcef880807^,0b8fa6b3-3b73-469f-9288-5332d530ac92

An increasing body of literature indicates that impacts to human health are likely to have some of the largest effects on the economy. Studies consistently indicate that climate-driven changes to morbidity and mortality can be substantial.81f96860-7931-48b6-9d57-32682728636f^,1ad1d794-bc57-4e48-ab28-0e2b65767cb9^,5ec155e5-8b77-438f-afa9-fbcac4d27690^,dbfb7cd9-7c82-43ea-a4e2-9e2eb0b851fd^,ec9926c5-6257-49b3-8bfd-c9a02c0bf75b^,9915b0f2-cf17-4aa3-a36f-32d18dfa11b1 In some sectors, the value of health damages is estimated to reach hundreds of billions of dollars per year under RCP8.5 by the end of the century. A large fraction of total health damages is due to mortality, quantified using the Value of a Statistical Life (VSL) approach based on standard VSL values used in federal government regulatory analysis.72d79359-0674-416f-ba9f-c0cd6e094fe4 For example, annual damages associated with extreme temperature-related deaths are estimated at $140 billion by the end of the century under RCP8.5, while lost wages from extreme temperatures, especially for outdoor industries, are projected at $160 billion per year by 2090.0b30f1ab-e4c4-4837-aa8b-0e19faccdb94 Adaptive actions, including physiological adaptation and increased availability of air conditioning, are projected to reduce extreme temperature mortality by approximately half; however, the implementation costs of those adaptations were not estimated. Although less studied compared to the research on the direct effects of temperature on health, climate-driven impacts to air quality81f96860-7931-48b6-9d57-32682728636f^,54a66159-1675-43bb-b5d3-a9b7f283e4de and aeroallergens5ec155e5-8b77-438f-afa9-fbcac4d27690^,971ee908-7da0-416e-8b6c-a72984d129ba are also projected to have large economic effects, due to increases in medical expenditures (such as emergency room visits) and premature mortality (Ch. 13: Air Quality).

Multiple lines of research have also shown that some climate change impacts will very likely be irreversible for thousands of years. For some species, the rate and magnitude of climate change projected for the 21st century is projected to increase the risk of extinction or extirpation (local-scale extinction) from the United States.25d5b793-3f5e-4c9f-9cb4-be71e84bf224^,05276a46-dc40-4839-9787-8f0b4defcf1c^,2116114a-1211-4afe-be2b-43a5e29f07f3^,819c7790-ed1c-4010-801f-9e513c72c5ce Coral reefs, coldwater fish, and high-elevation species are particularly vulnerable (Ch. 9: Oceans; Ch. 7: Ecosystems). The rapid and widespread climate changes occurring in the Arctic and Antarctic are leading to the loss of mountain glaciers and shrinking continental ice sheets.61d6757d-3f7a-4e90-add7-b03de796c6c4^,34b73570-133a-43d2-b326-521ecf7b09c6 The contribution of this land ice volume to the rate of global sea level rise is projected to affect U.S. coastlines for centuries (Ch. 8: Coastal).3bae2310-7572-47e2-99a4-9e4276764934^,ae82c8a3-3033-4103-91e9-926a27d1fa18^,924426db-fd9a-43d6-a9ea-3007a80e5795

New information and remaining uncertainties:

This Key Message reflects consideration of the findings of several recent multisector modeling projects (e.g., Hsiang et al. 2017, O’Neill et al. 2017, EPA 2017, Houser et al. 2015)0b30f1ab-e4c4-4837-aa8b-0e19faccdb94^,fad9e8ec-8951-4daa-9a9c-e093ef86af16^,0006123e-10a3-4501-a89c-95a7921a9c3d^,9f559c9b-c78e-4593-bcbe-f07661d29e16 released since NCA3. Despite these improvements to quantify the physical and economic impacts of climate change across sectors, uncertainty exists regarding the ultimate timing and magnitude of changes, particularly at local to regional scales. The sources of uncertainty vary by sector and the modeling approaches applied. Each approach also varies in its capacity to measure the ability of adaptation to reduce vulnerability, exposure, and risk. While the coverage of impacts has improved with recent advancements in the science, many important climate change effects remain unstudied, as do the interactions between sectors (Ch. 17: Complex Systems).e311cbe3-cf61-445a-ae6f-130056df0558 Finally, as climate conditions pass further outside the natural variability experienced over past several millennia, the odds of crossing thresholds or tipping points (such as the loss of Arctic summer sea ice) increase, though these thresholds are not well represented in current models.6b87bc9c-d8f5-438a-9693-7b33324f4c22^,08bc6610-586b-421c-a788-f5e18781ac52

Assessment of confidence based on evidence:

There is very high confidence that climate change is projected to substantially affect American livelihoods and well-being in the future compared to a future without climate change. The evidence supporting this conclusion is based on agreement across a large number of studies analyzing impacts across a multitude of sectors, scenarios, and regions. The literature clearly indicates that the adverse impacts of climate change are projected to substantially outweigh the positive effects. Although important uncertainties exist that affect our understanding of the timing and magnitude of some impacts, there is very high confidence that some effects will very likely lead to changes that are irreversible on human timescales.

Provenance

This finding was derived from scenario rcp_4_5
This finding was derived from scenario rcp_8_5