For each of the past 25 years, 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 Scripps Institution of Oceanography at the University of California San Diego and Princeton University.
The strong ocean warming the researchers found suggests that Earth is more sensitive to fossil-fuel emissions than previously thought. Study lead author Laure Resplandy, a Princeton assistant professor of geosciences, said that this estimate is more than 60 percent higher than the figure in the most recent assessment report from the United Nations Intergovernmental Panel on Climate Change (IPCC).
The study appears in the journal Nature Nov. 1
“Imagine if the ocean was only 30 feet deep,” said Resplandy, a former postdoctoral researcher at Scripps Oceanography. “Our data shows that it would have warmed by 6.5℃ (11.7℉) every decade since 1991. In comparison, the estimate of the last IPCC assessment report would correspond to a warming of only 4℃ (7.2℉) 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 that can be expected, said co-author Ralph Keeling, a Scripps Oceanography geophysicist and Resplandy's former postdoctoral advisor.
“The result significantly increases the confidence we can place in estimates of ocean warming and therefore helps reduce uncertainty about 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 more than 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.
Measuring ocean heat
To calculate total heat content, previous estimates relied on millions of measurements of ocean temperature. Many came from a network of robotic sensors developed by Scripps researchers known as Argo. Gaps in coverage, however, made this approach uncertain. Argo makes comprehensive measurements of ocean temperature and salinity across the globe, but complete network data only goes 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 made at stations around the world. With it, they determined 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 are 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 with the heat-energy content of the ocean.
The researchers estimate that the world's oceans took up more than 13 zettajoules of heat energy each year between 1991 and 2016. A joule is the standard unit of energy. One zettajoule equals one sextillion (or the number 1 followed followed by 21 zeroes) joules.
Resplandy and Keeling worked with co-authors Yassir Eddebbar and Mariela Brooks from Scripps, Rong Wang from Fudan University in China, Laurent Bopp from Ecole 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 funded by the Climate Program Office of the National Oceanic and Atmospheric Administration (grant NA13OAR4310219) and the Princeton Environmental Institute.