National Topics

The national topic chapters (Chs. 4–20) summarize current and future risks related to climate change and what can be done to reduce those risks for a variety of societal and economic sectors of the United States. This Assessment builds on the range of topics covered in NCA4 by adding two new chapters: Economics (Ch. 19) and Social Systems and Justice (Ch. 20).

Likelihood

Virtually Certain	Very Likely	Likely	As Likely as Not	Unlikely	Very Unikely	Exceptionally Unlikely
99%–100%	90%–100%	66%–100%	33%–66%	0%–33%	0%–10%	0%–1%

Confidence Level

Very High	High	Medium	Low
Strong evidence (established theory, multiple sources, well-documented and accepted methods, etc.) High consensus	Moderate evidence (several sources, some consistency, methods vary and/or documentation limited, etc.) Medium consensus	Suggestive evidence (a few sources, limited consistency, methods emerging, etc.) Competing schools of thought	Inconclusive evidence (limited sources, extrapolations, inconsistent findings, poor documentation and/or methods not tested, etc.) Disagreement or lack of opinions among experts

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