finding 13.4 : key-finding-13-4

Increasing sea surface temperatures, rising sea levels, and changing patterns of precipitation, winds, nutrients, and ocean circulation are contributing to overall declining oxygen concentrations at intermediate depths in various ocean locations and in many coastal areas. Over the last half century, major oxygen losses have occurred in inland seas, estuaries, and in the coastal and open ocean (high confidence). Ocean oxygen levels are projected to decrease by as much as 3.5% under the higher scenario (RCP8.5) by 2100 relative to preindustrial values (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: 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.

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 Bopp et al. 2013,fa10bfab-8f7c-4d8c-8435-8284a05d78e5 and Schmidtko et al. 2017.b2a0160d-032f-4a96-8cb1-321e09950172 Evidence arises from extensive global measurements of the 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 (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.

This finding was derived from figure -.2: Confidence / Likelihood

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