The oceans absorb every year a quantity of thermal energy that is 150 times more than the energy that humans produce as electricity, 60 percent more than what had been estimated.
The strong oceanic warming that the Princeton researchers and the Scripps Institution of Oceanography found suggests that Earth is more sensitive to fossil fuel emissions than previously thought.
The researchers reported in the journal Nature that the world's oceans consumed more than 13 zetajoules, which is a joule, the standard unit of energy, followed by 21 zeros, of thermal energy each year between 1991 and 2016.
The first author, Laure Resplandy, assistant professor of Geosciences and the Princeton Environmental InstituteHe said the estimate is more than 60 percent higher than the figure in the 2014 Fifth Assessment Report on climate change of the United Nations Intergovernmental Panel on Climate Change (IPCC).
"Imagine if the ocean was only 10 meters deep," said Resplandy, who was a postdoctoral researcher at Scripps. "Our data shows that it would have warmed by 6.5 degrees Celsius every decade since 1991. By comparison, the estimation of the last IPCC evaluation report would correspond to a heating of only 4 degrees Celsius every decade."
Scientists know that the ocean occupies approximately 90 percent of all the excess energy produced as the Earth heats up, so knowing the actual amount of energy makes it possible to estimate the surface warming we can expect, co-author Ralph Keeling, a geophysicist at Scripps Oceanography and a former Resplandy postdoctoral advisor, said in a statement.
"The result significantly increases the confidence that we can place in the estimates of the warming of the ocean and, therefore, helps reduce uncertainty in climate sensitivity, in particular, eliminating the possibility of a sensitivity to very low climate, "said Keeling.
Climate sensitivity is used to assess the emissions allowed for mitigation strategies. Most climate scientists have agreed in the last decade that if average global temperatures exceed pre-industrial levels by 2 degrees Celsius, it is almost 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 main greenhouse gas produced by human activities, must be reduced by 25 percent compared to what had been previously estimated, said Resplandy.
Previous estimates were based on millions of point measurements of the ocean temperature, which were interpolated to calculate the total heat content. However, gaps in coverage make this approach uncertain. A robotic sensor network known as Argo now performs exhaustive measurements of ocean temperature and salinity around the world, but the network only has complete data dating back to 2007 and it only measures the upper half of the ocean. Several reassessments of heat content have been made in recent years using ocean temperature data, including recent Argo data, which have led to upward revisions of the IPCC estimate.
Increase in APO levels
Resplandy and his coauthors 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 they studied. They measured the heat of the ocean by observing the combined amount of O2 and CO2 in the air, an amount they call "atmospheric oxygen potential" or APO. The method depends on the fact that oxygen and carbon dioxide are less soluble in warmer water.
As the ocean heats up, these gases tend to be released into the air, what increases the levels of APO. The APO is also influenced by the burning of fossil fuels and by an oceanic process that involves the absorption of excess CO2 from fossil fuel. By comparing the changes in the APO that they observed with the expected changes due to the use of fossil fuels and the consumption of carbon dioxide, the researchers were able to calculate the amount of APO that emanated from the ocean to warm up. That amount matches the heat-energy content of the ocean.