finding 9.1 : key-finding-9-1

Human activities have contributed substantially to observed ocean–atmosphere variability in the Atlantic Ocean (medium confidence), and these changes have contributed to the observed upward trend in North Atlantic hurricane activity since the 1970s (medium confidence).

This finding is from chapter 9 of Climate Science Special Report: The Fourth National Climate Assessment: Volume I.

Process for developing key messages: The key message and supporting text summarizes extensive evidence documented in the climate science peer-reviewed literature. The uncertainties and points of consensus that were described in the NCA3 and IPCC assessments have continued.

Description of evidence base: The Key Finding and supporting text summarizes extensive evidence documented in the climate science literature and is similar to statements made in previous national (NCA3)dd5b893d-4462-4bb3-9205-67b532919566 and internationalf03117be-ccfe-4f88-b70a-ffd4351b8190 assessments. Data limitations are documented in Kossin et al. 2013f748a8e5-7925-4fb4-a64c-57dd77279670 and references therein. Contributions of natural and anthropogenic factors in observed multidecadal variability are quantified in Carslaw et al. 2013;97e50b82-dfba-40d7-9fab-3e9d3b75b1d5 Zhang et al. 2013;562bc859-63f4-4d69-8818-64a7f545912a Tung and Zhou 2013;025e3b4f-1901-4a21-9f07-8abc1984fd33 Mann et al. 2014;bb77b2a6-9db3-4f38-a5bc-bddb1a61867f Stevens 2015;4a432e17-c562-43fa-8f90-a6e087fcf4c3 Sobel et al. 2016;3e306a34-4ff7-4273-9012-1eea2fa13100 Walsh et al. 2015.e56b040c-4d53-400a-b13c-6bc3dbe3f2f4

New information and remaining uncertainties: Key remaining uncertainties are due to known and substantial heterogeneities in the historical tropical cyclone data and lack of robust consensus in determining the precise relative contributions of natural and anthropogenic factors in past variability of the tropical environment.

Assessment of confidence based on evidence: Confidence in this finding is rated as medium. Although the range of estimates of natural versus anthropogenic contributions in the literature is fairly broad, virtually all studies identify a measurable, and generally substantial, anthropogenic influence. This does constitute a consensus for human contribution to the increases in tropical cyclone activity since 1970.

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