The world’s oceans have absorbed 93% of the excess heat from human-induced warming since the mid-20th century and are currently absorbing more than a quarter of the carbon dioxide emitted to the atmosphere annually from human activities, making the oceans warmer and more acidic (very high confidence). Increasing sea surface temperatures, rising sea levels, and changing patterns of precipitation, winds, nutrients, and ocean circulation are contributing to overall declining oxygen concentrations in many locations (high confidence).

This finding is from chapter 2 of Impacts, Risks, and Adaptation in the United States: The Fourth National Climate Assessment, Volume II.

Process for developing key messages:

This chapter is based on the collective effort of 32 authors, 3 review editors, and 18 contributing authors comprising the writing team for the Climate Science Special Report (CSSR),75cf1c0b-cc62-4ca4-96a7-082afdfe2ab1 a featured U.S. Global Change Research Project (USGCRP) deliverable and Volume I of the Fourth National Climate Assessment (NCA4). An open call for technical contributors took place in March 2016, and a federal science steering committee appointed the CSSR team. CSSR underwent three rounds of technical federal review, external peer review by the National Academies of Sciences, Engineering, and Medicine, and a review that was open to public comment. Three in-person Lead Authors Meetings were conducted at various stages of the development cycle to evaluate comments received, assign drafting responsibilities, and ensure cross-chapter coordination and consistency in capturing the state of climate science in the United States. In October 2016, an 11-member core writing team was tasked with capturing the most important CSSR key findings and generating an Executive Summary. The final draft of this summary and the underlying chapters was compiled in June 2017.

The NCA4 Chapter 2 author team was pulled exclusively from CSSR experts tasked with leading chapters and/or serving on the Executive Summary core writing team, thus representing a comprehensive cross-section of climate science disciplines and supplying the breadth necessary to synthesize CSSR content. NCA4 Chapter 2 authors are leading experts in climate science trends and projections, detection and attribution, temperature and precipitation change, severe weather and extreme events, sea level rise and ocean processes, mitigation, and risk analysis. The chapter was developed through technical discussions first promulgated by the literature assessments, prior efforts of USGCRP,75cf1c0b-cc62-4ca4-96a7-082afdfe2ab1 e-mail exchanges, and phone consultations conducted to craft this chapter and subsequent deliberations via phone and e-mail exchanges to hone content for the current application. The team placed particular emphasis on the state of science, what was covered in USGCRP,75cf1c0b-cc62-4ca4-96a7-082afdfe2ab1 and what is new since the release of the Third NCA in 2014.dd5b893d-4462-4bb3-9205-67b532919566

Description of evidence base:

The Key Message and supporting text summarize the evidence documented in climate science literature as summarized in Rhein et al. (2013).bc140b4c-c2d9-4d99-a684-5c054dc5134f Oceanic warming has been documented in a variety of data sources, most notably by the World Ocean Circulation Experiment (WOCE),4ef3eb98-3ce7-4c94-8b1b-9a09ee951bfd Argo,295cc0c4-536f-49c5-abdc-3a3b4916fdba and the Extended Reconstructed Sea Surface Temperature v4 (ERSSTv4).865e132e-dd4a-4195-9ea0-c3c7d32d447e 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 sea surface temperature (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 Sweet et al. 2017c66bf5a9-a6d7-4043-ad99-db0ae6ae562c) and reduced Arctic/Antarctic ice sheets (see Taylor et al. 201761d6757d-3f7a-4e90-add7-b03de796c6c4) 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 ERSSTv4865e132e-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 (2016),594bee23-c085-4cdd-8480-9d6fd1658c4e suggesting confidence in these findings.

Evidence for oxygen trends arises from extensive global measurements of WOCE after 1989 and individual profiles before that.2dbd3f8b-a4f8-421f-b75f-8cb165b1a867 The first basin-wide dissolved oxygen surveys were performed in the 1920s.b2a0160d-032f-4a96-8cb1-321e09950172 The confidence level is based on globally integrated O₂ distributions in a variety of ocean models. Although the global mean exhibits low interannual variability, regional contrasts are large.

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 high confidence 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.

Uncertainties in ocean oxygen content (as estimated from the intermodel spread) in the global mean are moderate mainly because ocean oxygen content exhibits low interannual variability when globally averaged. Uncertainties in long-term decreases of the global averaged oxygen concentration amount to 25% in the upper 1,000 m for the 1970–1992 period and 28% for the 1993–2003 period. Remaining uncertainties relate to regional variability driven by mesoscale eddies and intrinsic climate variability such as ENSO.

Assessment of confidence based on evidence:

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.

Major ocean deoxygenation is taking place in bodies of water inland, at estuaries, and in the coastal and the open ocean (high confidence). Regionally, the phenomenon is exacerbated by local changes in weather, ocean circulation, and continental inputs to the oceans.

Provenance

This finding was derived from scenario rcp_8_5