Change in global ocean heat content (∆OHC). a, ∆OHC derived from hydrographic and atmospheric observations (normalized to zero in 2007, ±1σ uncertainty). b, Linear least-squares trends for 1991–2016, 1993–2016 and 2007–2016 (±1σ uncertainty). Hydrography-based ∆OHC estimates combine warming rates at ocean depths of 0 to 2,000m (from Cheng and co-authors (CHEN), Pacific Marine Environmental Laboratory (PMEL), Meteorological Research Institute (MRI), and National Centers for Environmental Information (NCEI) estimates) with the revised deep ocean warming (at depths of more than 2,000 m). The atmospheric-based estimate (this study), which uses observed atmospheric potential oxygen trends (∆APO Climate ) and model-based ∆APO Climate-to-∆OHC ratios, does not resolve interannual variations. Graphic: Resplandy, et al., 2018 / Nature

By Morgan Kelly and Robert Monroe
1 November 2018
(Princeton University) – For each year during the past quarter century, the world’s oceans have absorbed an amount of heat energy that is 150 times the energy humans produce as electricity annually, according to a study led by researchers at Princeton and the Scripps Institution of Oceanography at the University of California-San Diego. The strong ocean warming the researchers found suggests that Earth is more sensitive to fossil-fuel emissions than previously thought. [But see Scientists acknowledge key errors in study of how fast the oceans are warming. –Des]The researchers reported [pdf] in the journal Nature on 1 November 2018 that the world’s oceans took up more than 13 zettajoules — which is a joule, the standard unit of energy, followed by 21 zeroes — of heat energy each year between 1991 and 2016. The study was funded by the National Oceanic and Atmospheric Administration and the Princeton Environmental Institute.First author Laure Resplandy, an assistant professor of geosciences and the Princeton Environmental Institute, said that her and her co-authors’ estimate is more than 60 percent higher per year than the figure in the 2014 Fifth Assessment Report on climate change from the United Nations Intergovernmental Panel on Climate Change (IPCC).“Imagine if the ocean was only 30 feet deep,” said Resplandy, who was a postdoctoral researcher at Scripps. “Our data show that it would have warmed by 6.5 degrees Celsius [11.7 degrees Fahrenheit] every decade since 1991. In comparison, the estimate of the last IPCC assessment report would correspond to a warming of only 4 degrees Celsius [7.2 degrees Fahrenheit] every decade.”Scientists know that the ocean takes up roughly 90 percent of all the excess energy produced as the Earth warms, so knowing the actual amount of energy makes it possible to estimate the surface warming we can expect, said co-author Ralph Keeling, a Scripps Oceanography geophysicist and Resplandy’s former postdoctoral adviser.“The result significantly increases the confidence we can place in estimates of ocean warming and therefore helps reduce uncertainty in the climate sensitivity, particularly closing off the possibility of very low climate sensitivity,” Keeling said.Climate sensitivity is used to evaluate allowable emissions for mitigation strategies. Most climate scientists have agreed in the past decade that if global average temperatures exceed pre-industrial levels by 2℃ (3.6℉), it is all but certain that society will face widespread and dangerous consequences of climate change.The researchers’ findings suggest that if society is to prevent temperatures from rising above that mark, emissions of carbon dioxide, the chief greenhouse gas produced by human activities, must be reduced by 25 percent compared to what was previously estimated, Resplandy said.The researchers’ results are the first to come from a measuring technique independent from the dominant method behind existing research, she said.

Databased estimates of global ∆APO Climate. a, ∆APO Climate estimated from observed APO (∆APO OBS) from the Scripps Institution of Oceanography network (1991–2016), and corrected by taking into account fossil-fuel burning, ocean anthropogenic carbon uptake and anthropogenic aerosol deposition. b, The increase in global ∆APO Climate (±1σ interval) exceeds the range of 26-year trends expected from the natural variations in four Earth system models (CESM, GFDL, IPSL and UVic, shown in grey). Graphic: Resplandy, et al., 2018 / Nature

Previous estimates relied on millions of spot measurements of ocean temperature, which were interpolated to calculate total heat content. Gaps in coverage, however, make this approach uncertain. A network of robotic sensors known as Argo now makes comprehensive measurements of ocean temperature and salinity across the globe, but the network only has complete data going back to 2007 and only measures the upper half of the ocean. Several reassessments of heat content have been made in recent years using the ocean-temperature data – including the recent Argo data — which has led to upward revisions of the IPCC estimate. Resplandy and her co-authors used Scripps’ high-precision measurements of oxygen and carbon dioxide in the air to determine how much heat the oceans have stored during the time span they studied. They measured ocean heat by looking at the combined amount of O2 and CO2 in air, a quantity they call “atmospheric potential oxygen” or APO. The method depends on the fact that oxygen and carbon dioxide are both less soluble in warmer water.As the ocean warms, these gases tend to be released into the air, which increases APO levels. APO also is influenced by burning fossil fuels and by an ocean process involving the uptake of excess fossil-fuel CO2. By comparing the changes in APO they observed with the changes expected due to fossil-fuel use and carbon dioxide uptake, the researchers were able to calculate how much APO emanated from the ocean becoming warmer. That amount coincides the heat-energy content of the ocean.Resplandy and Keeling worked with co-authors Yassir Eddebbar and Mariela Brooks from Scripps, Rong Wang from Fudan University in China, Laurent Bopp from École Normale Supérieure in France, Matthew Long from the National Center for Atmospheric Research, John Dunne from the NOAA Geophysical Fluid Dynamics Laboratory, and Wolfgang Koeve and Andreas Oschlies from the GEOMAR Helmholtz Centre for Ocean Research in Germany.The study, “Quantification of ocean heat uptake from changes in atmospheric O2 and CO2 composition,” was published in the journal Nature on 1 November 2018. The work was funded by the Climate Program Office of the National Oceanic and Atmospheric Administration (grant NA13OAR4310219) and the Princeton Environmental Institute.

Earth’s oceans have absorbed 60 percent more heat per year than previously thoughtObserved link between potential oxygen and ocean heat. OPO concentrations in situ (OPO, yellow) and at saturation based on O2 and CO2 solubility (OPO sat, grey) as a function of ocean temperature in the GLODAPv2 database. Graphic: Resplandy, et al., 2018 / Nature

ABSTRACT: The ocean is the main source of thermal inertia in the climate system1. During recent decades, ocean heat uptake has been quantified by using hydrographic temperature measurements and data from the Argo float program, which expanded its coverage after 20072,3. However, these estimates all use the same imperfect ocean dataset and share additional uncertainties resulting from sparse coverage, especially before 20074,5. Here we provide an independent estimate by using measurements of atmospheric oxygen (O2) and carbon dioxide (CO2)—levels of which increase as the ocean warms and releases gases—as a whole-ocean thermometer. We show that the ocean gained 1.33 ± 0.20  × 1022 joules of heat per year between 1991 and 2016, equivalent to a planetary energy imbalance of 0.83 ± 0.11 watts per square metre of Earth’s surface. We also find that the ocean-warming effect that led to the outgassing of O2 and CO2 can be isolated from the direct effects of anthropogenic emissions and CO2 sinks. Our result—which relies on high-precision O2 measurements dating back to 19916—suggests that ocean warming is at the high end of previous estimates, with implications for policy-relevant measurements of the Earth response to climate change, such as climate sensitivity to greenhouse gases7 and the thermal component of sea-level rise8.

Quantification of ocean heat uptake from changes in atmospheric O2 and CO2 composition