finding 28.4 : key-message-28-4

Proactive adaptation initiatives—including changes to policies, business operations, capital investments, and other steps—yield benefits in excess of their costs in the near term, as well as over the long term (medium confidence). Evaluating adaptation strategies involves consideration of equity, justice, cultural heritage, the environment, health, and national security (high confidence).

This finding is from chapter 28 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 Fourth National Climate Assessment (NCA4) Federal Steering Committee, which is made up of representatives from the U.S. Global Change Research Program member agencies. The scope was also informed by research needs identified in the Third National Climate Assessment (NCA3). Authors for this NCA4 chapter were selected to represent a range of public- and private-sector perspectives and experiences relevant to adaptation planning and implementation.

This chapter was developed through technical discussions of relevant evidence and expert deliberation by chapter authors during teleconferences, e-mail exchanges, and a day-long in-person meeting. These discussions were informed by a comprehensive literature review of the evidence base for the current state of adaptation in the United States. The author team obtained input from outside experts in several important areas to supplement its expertise.

Description of evidence base:

Both limited field applications and literature reviews highlight adaptation co-benefits, including those associated with equity considerations.971185c1-a31f-4c15-8454-57f273b4ed33 Near-term benefits are assessed from observations of adaptation results, as well as from comparisons to similar situations without such responses; longer-term benefits are generally assessed from projections.

New information and remaining uncertainties:

Benefits are based on understanding the relevant systems so that one can compare similar cases and construct counterfactuals. Such understanding is excellent for many engineered systems (for example, how a storm drain performs under various rainfall scenarios) but is less robust for many biological systems. Benefit–cost ratios can have large uncertainties associated with estimates of costs, the projection of benefits, and the economic valuation of benefits. In addition, because expected differences in benefit–cost ratios are sufficiently large and the number of current examples is sufficiently low, there are large uncertainties in applying results from one case to another.

Assessment of confidence based on evidence:

There is suggestive evidence that provides medium confidence that many proactive adaptation actions offer significant benefits that exceed their costs. However, because of a small sample size and insufficient evaluation, it is in general hard to know the extent to which this is true in any particular case. There is strong agreement that evaluating adaptation involves consideration of a wide range of measures of social well-being.

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