"key-message-29-4" . "29.4"^^ . "

Interactions between mitigation and adaptation are complex and can lead to benefits, but they also have the potential for adverse consequences (very high confidence). Adaptation can complement mitigation to substantially reduce exposure and vulnerability to climate change in some sectors (very high confidence). This complementarity is especially important given that a certain degree of climate change due to past and present emissions is unavoidable (very high confidence).

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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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Global-scale reductions in GHG emissions are projected to reduce many of the risks posed by climate change. However, Americans are already experiencing, and will continue to experience, impacts that have already been committed to because of past and present emissions.{{< tbib '5' '9f559c9b-c78e-4593-bcbe-f07661d29e16' >}}^,{{}} In addition, multisector modeling frameworks demonstrate that mitigation is unlikely to completely avoid the adverse impacts of climate change.^{{{< tbib '2' '0b30f1ab-e4c4-4837-aa8b-0e19faccdb94' >}}^,{{}}^,{{}}^,{{}}^,{{}}} These factors will likely necessitate widespread adaptation to climate change (Ch. 28: Adaptation); an expanding literature consistently indicates potential for the reduction of long-term risks and economic damages of climate change.{{< tbib '2' '0b30f1ab-e4c4-4837-aa8b-0e19faccdb94' >}}^,{{}}^,{{}}^,{{}} However, it is important to note that adaptation can require large up-front costs and long-term commitments for maintenance (Ch. 28: Adaptation), and uncertainty exists in some sectors regarding the applicability and effectiveness of adaptation in reducing risk.{{< tbib '101' '6f504af2-a3a0-46c3-a8bd-9f5f266bd5bf' >}}

Because of adaptation’s ability to reduce risk in ways that mitigation cannot, and vice versa, the weight of the evidence shows that the two strategies can act as complements. Several recent studies jointly model the effects of mitigation and adaptation in reducing overall risk to the impacts of climate change in the United States, focusing on infrastructure (e.g., Larsen et al. 2017, Melvin et al. 2016, Neumann et al. 2014)^{{{< tbib '71' '5b27123a-8c6d-4e85-bd48-841436fdf9eb' >}}^,{{}}^,{{}}} and agriculture (e.g., Kaye and Quemada 2017, Challinor et al. 2014, Lobell et al. 2013).{{< tbib '108' '94c2d912-8ac9-4c32-958c-6918f5cc079a' >}}^,{{}}^,{{}} Exploration of this mitigation and adaptation nexus is also advancing in the health sector, with both mitigation and adaptation (such as behavioral changes or physiological acclimatization) being projected to reduce deaths from extreme temperatures{{< tbib '100' '1ad1d794-bc57-4e48-ab28-0e2b65767cb9' >}} in both the higher and lower emissions scenarios that are the focus of this chapter. Similarly, energy efficiency investments are reducing GHG emissions and operating costs and improving resilience to future power interruptions from extreme weather events (Ch. 14: Human Health). While more studies exploring the joint effects of mitigation and adaptation are needed, recent literature finds that combined mitigation and adaptation actions can substantially reduce the risks posed by climate change in several sectors.{{< tbib '2' '0b30f1ab-e4c4-4837-aa8b-0e19faccdb94' >}}^,{{}}^,{{}} However, several studies highlight that mitigation and adaptation can also interact negatively. While these studies are more limited in the literature, sectors exhibiting potential negative co-effects from mitigation and adaptation include the bioenergy–water resource nexus{{< tbib '114' '9bfd3c12-a45e-4317-b640-6deadff2a790' >}} and changes in electricity demand and supply in response to increased use of air conditioning.{{< tbib '2' '0b30f1ab-e4c4-4837-aa8b-0e19faccdb94' >}}^,{{}}

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There is very high confidence that the dual strategies of mitigation and adaptation being taken at national, regional, and local levels provide complementary opportunities to reduce the risks posed by climate change. Studies consistently find that adaptation would be particularly important for impacts occurring over the next several decades, a time period in which the effects of large-scale mitigation would not yet be easily recognizable. However, further analysis is needed to help resolve uncertainties regarding the timing and magnitude of adaptation, including the potential positive and negative co-effects with mitigation.

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It is well understood that adaptation will likely reduce climate risks and that adaptation and mitigation interact. However, there are uncertainties regarding the magnitude, timing, and regional/sectoral distribution of these effects. Developing a full understanding of the interaction between mitigation and adaptation, with detailed accounting of potential positive and negative co-effects, is an important research objective that is only beginning to be explored in the detail necessary to inform effective implementation of these policies. Quantifying the effectiveness of adaptation requires detailed analyses regarding the timing and magnitude of how climate is projected to affect people living in the United States and their natural and built environments. As such, the uncertainties described under Key Messages 1 and 2 are also relevant here. Further, uncertainty exists regarding the effectiveness of adaptation measures in improving resilience to climate impacts. For some sectors, such as coastal development, protection measures (for example, elevating structures) have been well studied and implemented to reduce risk. However, the effectiveness of adaptation in other sectors, such as the physiological response to more intense heat waves, is only beginning to be understood.

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