Nov 1 2018
According to a research led by scientists at Princeton and the Scripps Institution of Oceanography at the University of California-San Diego, for each year during the last quarter century, the world’s oceans have absorbed an amount of heat energy that is 150 times greater than the energy humans generate as electricity each year.
Princeton and Scripps researchers report that the world's oceans absorbed 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. That’s 150 times more heat energy each year than the energy humans produce as electricity annually. The estimate is 60% higher than that used in the IPCC’s Fifth Assessment Report. (Photo by Abigale Wyatt, Princeton Department of Geosciences)
The intense ocean warming the scientists found indicates that Earth is more sensitive to fossil-fuel emissions than formerly believed.
The researchers explain in the journal Nature that the world’s oceans absorb more than 13 zettajoules—which is a joule, the standard unit of energy, followed by 21 zeroes—of heat energy per annum between 1991 and 2016. The research was sponsored 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 she and her co-authors’ estimate is over 60% 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.”
Researchers know that the ocean absorbs approximately 90% of all the surplus energy generated as the Earth warms, so being aware of the actual amount of energy makes it possible to estimate the surface warming that can be anticipated, 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 assess allowable emissions for mitigation approaches. Most climate scientists have agreed in the past decade that if global average temperatures surpass pre-industrial levels by 2 °C (3.6 °F), it is all but certain that society will face extensive and dangerous consequences of climate change.
The team’s findings indicate that if society is to stop temperatures from rising beyond that mark, emissions of carbon dioxide (CO2), the key greenhouse gas formed due to human activities, must be decreased by 25% compared to what was formerly estimated, Resplandy said.
The researchers’ results are the first to transpire from a measuring method autonomous from the dominant technique behind current research, she said.
Earlier estimates relied on millions of spot measurements of ocean temperature, which were interpolated to calculate total heat content. Gaps in coverage, however, make this method ambiguous. A network of robotic sensors called Argo currently makes wide-ranging measurements of ocean temperature and salinity across the globe, but the network only has comprehensive data going back to 2007 and only takes into the upper half of the ocean. Several revisions of heat content have been done in recent years using the ocean-temperature data—including the recent Argo data—which has resulted in upward revisions of the IPCC estimate.
Resplandy and her co-authors used Scripps’ high-precision measurements of oxygen (O2) and CO2 in the air to establish how much heat the oceans have absorbed during the time span they investigated. They measured ocean heat by studying the combined amount of O2 and CO2 in air, a quantity they term “atmospheric potential oxygen (APO).” The technique relies on the fact that CO2 and O2 are both less soluble in warmer water.
When the ocean starts to heat up, these gases tend to be discharged into the air, which raises APO levels. APO also is affected by burning fossil fuels and by an ocean process involving the uptake of surplus fossil-fuel CO2. By comparing the variations in APO they noticed with the changes expected because of fossil-fuel use and CO2 uptake, the scientists were able to calculate how much APO originated from the ocean becoming warmer. That amount concurs with the heat-energy content of the ocean.
Resplandy and Keeling partnered with co-authors Yassir Eddebbar and Mariela Brooks from Scripps, Rong Wang from Fudan University in China, Laurent Bopp from École Normale Supérieure in Paris, 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 November 1st. The research was sponsored by the Climate Program Office of the National Oceanic and Atmospheric Administration (grant NA13OAR4310219) and the Princeton Environmental Institute.