Every year during the past 25 years, the world's oceans have absorbed an amount of heat energy 150 times the energy humans produce as electricity annually.
This unexpectedly strong ocean warming suggests the Earth is more sensitive to fossil-fuel emissions than previously thought, said a published study led by researchers from Princeton, and the Scripps Institution of Oceanography at the University of California-San Diego.
Researchers reported that the world's oceans absorbed more than 13 zettajoules (13,000,000,000,000,000,000,000 joules) of heat energy each year between 1991 and 2016.
They estimate this is more than 60 percent higher 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 first author Laure Resplandy, an assistant professor of geosciences and the Princeton Environmental Institute. She said their data show it would have warmed by 6.5 degrees Celsius every decade since 1991. By comparison, the estimate of the last IPCC assessment report would correspond to a warming of only 4 degrees Celsius every decade.
Oceans absorb over 90 percent of all the excess energy produced as the Earth warms. Knowing this makes it possible to estimate the surface warming we can expect, said co-author Ralph Keeling, a Scripps Oceanography geophysicist.
The result significantly increases the confidence we can place in estimates of ocean warming. It also helps reduce uncertainty in the 'climate sensitivity,' and reduces the possibility of very low climate sensitivity, according to Keeling. Climate sensitivity is used to evaluate allowable emissions for mitigation strategies.
Most climate scientists have agreed that if global average temperatures exceed pre-industrial levels by 2 degrees Celsius, it is all but certain society will face widespread and dangerous consequences of climate change.
The study's findings suggest that if society is to prevent temperatures from rising above that mark, emissions of carbon dioxide must be reduced by 25 percent compared to what was previously estimated. CO2 is the chief greenhouse gas produced by human activities.
The study results are the first to come from a measuring technique independent from the dominant method behind existing research.
Researchers used Scripps' high-precision measurements of oxygen and CO2 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 the air. This quantity is called the Atmospheric Potential Oxygen or APO.
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, researchers were able to calculate how much APO emanated from the ocean becoming warmer. That amount coincides the heat-energy content of the ocean.
