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Oceans warming faster than previously thought, challenging climate targets

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The ocean has been absorbing much more heat than previously thought, according to a new study, which suggests that significant global warming could occur much faster than previously thought.

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, said the 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.

The researchers reported 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.

First author Laure Resplandy said their estimate is more than 60 per cent 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,” Resplandy said, “Our data show that it would have warmed by 6.5° Celsius every decade since 1991. In comparison, the estimate of the last IPCC assessment report would correspond to a warming of only 4° Celsius every decade.”

Scientists know that the ocean takes up roughly 90 per cent 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.

“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.

The researchers’ findings suggest that if society is to prevent temperatures from rising above the 2° Celsius mark as agreed upon in the Paris Agreement, emissions of carbon dioxide, the chief greenhouse gas produced by human activities, must be reduced by an additional 25 per cent beyond what was previously estimated.

The results are the first to come from a measuring technique independent from the dominant method behind existing research, she said.

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.

The team used 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 with the heat-energy content of the ocean.

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