Impacts, risks, and adaptation in the United States: 4th U.S. national climate assessment, volume II

Key message 3: Warming and acidifying oceans

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. 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.

Oceans occupy over 70% of the planet’s surface and host unique ecosystems and species, including those important for global commercial and subsistence fishing. For this reason, it is essential to highlight the fact that observed changes in the global average temperature of the atmosphere represent only a small fraction of total warming. Since the 1950s, the oceans have absorbed 93% of the excess heat in the earth system that has built up as a result of increasing concentrations of greenhouse gases in the atmosphere.31 ,32 Significant increases in heat content have been observed over the upper 6,560 feet (2,000 m) of the ocean since the 1960s, with surface oceans warming by about 1.3° ± 0.1°F (0.7° ± 0.1°C) globally from 1900 to 2016.

Oceans’ net uptake of CO2 each year is approximately equal to a quarter of that emitted to the atmosphere annually from human activities. It is primarily controlled by the difference between CO2 concentrations in the atmosphere and ocean, with small variations from year to year due to changes in ocean circulation and biology. This carbon uptake is making near-surface ocean waters more acidic, which in turn can harm vulnerable marine ecosystems (see Ch. 9: Oceans; Ch. 26: Alaska; Ch. 27: Hawai‘i & Pacific Islands). Although tropical coral reefs are the most frequently cited casualties of ocean warming and acidification, ecosystems at higher latitudes can be more vulnerable than those at lower latitudes as they typically have a lower buffering capacity against changing acidity. Regionally, acidification is greater along the U.S. coast than the global average, as a result of upwelling (for example, in the Pacific Northwest), changes in freshwater inputs (such as in the Gulf of Maine), and nutrient input (as in urbanized estuaries).

Easterling D. R., Fahey D. W., Hayhoe K., Doherty S., Kossin J. P., Sweet W., Vose R. S., Wehner M. F. & Wuebbles D. J., 2018. Our Changing Climate. In:
Reidmiller D. R., Avery C. W., Easterling D. R., Kunkel K. E., Lewis K. L. M., Maycock T. K. & Stewart B. C. (Eds.), Impacts, Risks, and Adaptation in the United States: Fourth National Climate Assessment, Volume II. U.S. Global Change Research Program, Washington, DC. Chapter.

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