OSLO, Norway (Reuters) — Climate change could quickly get worse if huge amounts of extra heat absorbed by the oceans are released back into the air.
Scientists announced the finding after unveiling new research that shows oceans have helped mitigate the effects of warming since 2000.
Heat-trapping gases are being emitted into the atmosphere faster than ever, and the 10 hottest years since records began have all taken place since 1998.
However, the rate at which the Earth’s surface is heating up has slowed since 2000, causing scientists to search for an explanation for the pause.
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Experts in France and Spain said the oceans took up more warmth from the air around 2000. That would help explain the slowdown in surface warming but would also suggest the pause may be only temporary and brief.
“Most of this excess energy was absorbed in the top 700 metres of the ocean at the onset of the warming pause, 65 percent of it in the tropical Pacific and Atlantic oceans,” they wrote in the journal Nature Climate Change.
Lead author Virginie Guemas of the Catalan Institute of Climate Sciences in Barcelona, Spain, said the hidden heat may return to the atmosphere in the next decade, stoking warming again.
“If it is only related to natural variability, then the rate of warming will increase soon,” she said.
Caroline Katsman of the Royal Netherlands Meteorological Institute, an expert who was not involved in the latest study, said heat absorbed by the ocean will come back into the atmosphere if it is part of an ocean cycle such as the El Nino warming and La Nina cooling events in the Pacific Ocean.
She said the study broadly confirmed earlier research by her institute, but it was unlikely to be the full explanation of the warming pause at the surface because it applied only to the onset of the slowdown around 2000.
The pace of climate change has big economic implications because almost 200 governments agreed in 2010 to limit surface warming to less than 2 C above pre-industrial levels, mainly by shifting from fossil fuels.
Surface temperatures have already risen by 0.8 C. Two degrees is widely seen as a threshold for dangerous changes such as more droughts, mudslides, floods and rising sea levels.
Some governments and climate change skeptics argue that the slowdown in the rising trend shows less urgency to act. Governments have agreed to work out a global deal to combat climate change by the end of 2015.
Last year was the ninth warmest since records began in the 1850s, according to the United Nations’ World Meteorological Organization, and 2010 was the warmest, just ahead of 1998.
Apart from 1998, the 10 hottest years have all been since 2000.
Guemas’s study showed that natural La Nina weather events in the Pacific around 2000 brought cool waters to the surface that absorbed more heat from the air. In another set of natural variations, the Atlantic also soaked up more heat.
“Global warming is continuing, but it’s being manifested in somewhat different ways,” said Kevin Trenberth of the U.S. National Center for Atmospheric Research.
Warming can go to the air, water, land or to melting ice and snow.
He said warmth is spreading to ever deeper ocean levels, and pauses in surface warming could last 15 to 20 years.
“Recent warming rates of the waters below 700 metres appear to be unprecedented,” he and colleagues wrote in a study last month in the journal Geophysical Research Letters.
