Rising heat content of Earth’s oceans

Earth’s Sun-warmed surface radiates heat into the atmosphere. As the concentration of carbon dioxide and other greenhouse gases increases, more of that extra heat fails to escape into space. Although the trapped heat warms Earth’s lower atmosphere and land surface, more than 90% of it ends up in the ocean. How the ocean heat content (OHC) has changed over the past half century is the subject of a new study led by Lijing Cheng of the Institute of Atmospheric Physics in Beijing. Resolving OHC across time and space is challenging. Since 2005, a network of autonomous floats called Argo has continuously profiled the ocean’s temperature at depths down to 2000 m with steadily increasing spatial coverage (see the article by Karim Sabra, Bruce Cornuelle, and Bill Kuperman, Physics Today, February 2016, page 32 ). Before then, and even in Argo’s early years, measurements were less frequent, more sparse, and unevenly distributed. Cheng and his collaborators tackled the challenge through an interpolation scheme that took advantage of oceanographers’ understanding of how the ocean’s behavior is correlated through space and time. To validate the scheme, they demonstrated that they could reproduce data from recent, densely sampled observations by interpolating from a sparser sampling of those same observations. Among the findings: OHC remained fairly steady from 1960 through 1980; thereafter, it increased significantly. What’s more, the study found that OHC is 13% higher than the value cited in the most recent (2014) report of the Intergovernmental Panel on Climate Change. As the figure shows, increases in OHC extended to lower depths mainly after 1990. The study by Cheng and colleagues also revealed that Earth’s five oceans are sequestering heat at different rates. The Atlantic Ocean had the largest OHC increase from 1960 to 2015; it was 3.5 times as high as that of the Pacific Ocean, which covers twice the area. Circulation within and between oceans is the likely cause of the differences, which may themselves change if global warming alters circulation patterns. (L. Cheng et al., Sci. Adv. 3, e1601545, 2017, doi:10.1126/sciadv.1601545 .)