finding 13.1 : key-finding-13-1

The world’s oceans have absorbed about 93% of the excess heat caused by greenhouse gas warming since the mid-20th century, making them warmer and altering global and regional climate feedbacks. Ocean heat content has increased at all depths since the 1960s and surface waters have warmed by about 1.3° ± 0.1°F (0.7° ± 0.08°C) per century globally since 1900 to 2016. Under a higher scenario, a global increase in average sea surface temperature of 4.9° ± 1.3°F (2.7° ± 0.7°C) by 2100 is projected, with even higher changes in some U.S. coastal regions. (Very high confidence)

This finding is from chapter 13 of Climate Science Special Report: The Fourth National Climate Assessment: Volume I.

Process for developing key messages: There is very high confidence in measurements that show increases in the ocean heat content and warming of the ocean, based on the agreement of different methods. However, long-term data in total ocean heat uptake in the deep ocean are sparse leading to limited knowledge of the transport of heat between and within ocean basins.

Description of evidence base: The key finding and supporting text summarizes the evidence documented in climate science literature, including Rhein et al. 2013.bc140b4c-c2d9-4d99-a684-5c054dc5134f Oceanic warming has been documented in a variety of data sources, most notably the World Ocean Circulation Experiment (WOCE) ( and Argo databases ( and Extended Reconstructed Sea Surface Temperature (ERSST) v4 ( There is particular confidence in calculated warming for the time period since 1971 due to increased spatial and depth coverage and the level of agreement among independent SST observations from satellites, surface drifters and ships, and independent studies using differing analyses, bias corrections, and data sources.db777261-ee2e-4bf6-944e-a8831c595300 9f1fa4b6-2d4d-414a-8cee-4b6e589753bb 987c69a9-042a-486c-87c5-c21182ef9f10 Other observations such as the increase in mean sea level rise (see Ch. 12: Sea Level Rise) and reduced Arctic/Antarctic ice sheets (see Ch. 11: Arctic Changes) further confirm the increase in thermal expansion. For the purpose of extending the selected time periods back from 1900 to 2016 and analyzing U.S. regional SSTs, the ERSST version 4 (ERSSTv4)865e132e-dd4a-4195-9ea0-c3c7d32d447e is used. For the centennial time scale changes over 1900–2016, warming trends in all regions are statistically significant with the 95% confidence level. U.S. regional SST warming is similar between calculations using ERSSTv4 in this report and those published by Belkin,594bee23-c085-4cdd-8480-9d6fd1658c4e suggesting confidence in these findings. The projected increase in SST is based on evidence from the latest generation of Earth System Models (CMIP5)

New information and remaining uncertainties: Uncertainties in the magnitude of ocean warming stem from the disparate measurements of ocean temperature over the last century. There is low uncertainty in warming trends of the upper ocean temperature from 0–700 m depth, whereas there is more uncertainty for deeper ocean depths of 700–2,000 m due to the short record of measurements from those areas. Data on warming trends at depths greater than 2,000 m are even more sparse. There are also uncertainties in the timing and reasons for particular decadal and interannual variations in ocean heat content and the contributions that different ocean basins play in the overall ocean heat uptake.

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