The results suggest over the past 27 years, the world’s oceans have retained 60 per cent more heat each year than research teams had previously thought.
This represents an enormous amount of additional energy stored on Earth, indicating the planet is far more sensitive to fossil-fuel emissions than past studies have shown.
It also raises further serious doubts over whether current temperature goals – to limit the global temperature increase to 1.5C above pre-industrial levels – are attainable.
If accurate, the study is likely to have a major impact on climate modelling, which has largely used lower estimates for the amount of energy stored in the oceans.
Most previous models for ocean heat uptake have used data taken from the Argo Float programme, in which about 4,000 buoys drifting at around 1,000m below the surface of the oceans take temperature and other readings. It has been fully operational since 2007. Issues with the system are that it only measures the upper part of the ocean, and there remain huge gaps in coverage.
But the new study provides a separate estimate with data going back to 1991, and uses “dependable” measurements of atmospheric oxygen (O2) and carbon dioxide (CO2) – levels of which increase as the ocean warms and releases gases – as a “whole-ocean thermometer”, the authors wrote.
The study, published in the journal Nature, says 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.
This amounts to the oceans absorbing an amount of heat energy 150 times greater than the energy humans produce as electricity annually, over each year of the past quarter of a century.
“We thought that we got away with not a lot of warming in both the ocean and the atmosphere for the amount of CO2 that we emitted,” lead author Dr Laure Resplandy of Princeton University, told The Washington Post.
“But we were wrong. The planet warmed more than we thought. It was hidden from us just because we didn’t sample it right. But it was there. It was in the ocean already.”
The study comes three weeks after scientists from the IPCC (Intergovernmental Panel on Climate Change) commissioned by the United Nations issued the most urgent and far-reaching call yet for world governments to cut their greenhouse gas emissions and stop global warming.
“It is a big concern,” Dr Resplandy said.
“If you look at the IPCC 1.5C, there are big challenges ahead to keep those targets, and our study suggests it’s even harder because we close the window for those lower pathways,” she told the BBC.
Scientists already know the ocean takes up roughly 90 per cent of all excess energy produced as the Earth warms, so calculating the actual amount of energy makes it possible to estimate the level of surface warming we can expect, said the study’s co-author Ralph Keeling, a Scripps Oceanography geophysicist.
“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,” Professor Keeling said.
The findings suggest if governments are to prevent temperatures from rising above 2C, emissions of carbon dioxide, the chief greenhouse gas produced by human activities, must be reduced by 25 per cent compared to what was previously estimated, Dr Resplandy said.
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