Oceans absorb some 90 per cent of carbon dioxide emissions.
Climate change could get worse quickly if huge amounts of
extra heat absorbed by the oceans are released back into the air, scientists
said after unveiling new research showing that 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. But the rate at which the earth's surface is heating up has
slowed somewhat since 2000, causing scientists to search for an explanation for
the pause.
Experts in France and Spain said on Sunday that the oceans
took up more warmth from the air around 2000. That would help explain the
slowdown in surface warming but would also suggest that the pause may be only
temporary and brief.
"Most of this excess energy was absorbed in the top 700
metres (2,300 ft) of the ocean at the onset of the warming pause, 65 per cent
of it in the tropical Pacific and Atlantic oceans," they wrote in the
journal Nature Climate Change.
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Lead author Virginie Guemas of the Catalan Institute of
Climate Sciences in Barcelona 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 told Reuters.
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.
She said the study broadly confirmed earlier research by her
institute but that it was unlikely to be the full explanation of the warming
pause at the surface, since it only applied to the onset of the slowdown around
2000.
Threshold
The pace of climate change has big economic implications
since almost 200 governments agreed in 2010 to limit surface warming to less
than 2 degrees Celsius (3.6 F) 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 sceptics that man-made climate change
is a big problem, argue that the slowdown in the rising trend shows less
urgency to act. Governments have agreed to work out, by the end of 2015, a
global deal to combat climate change.
Last year was ninth warmest since records began in the
1850s, according to the U.N.'s World Meteorological Organisation, and 2010 was
the warmest, just ahead of 1998. Apart from 1998, the 10 hottest years have all
been since 2000.
Guemas's study, twinning observations and computer models,
showed that natural La Nina weather events in the Pacific around the year 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, for instance, to the air,
water, land or to melting ice and snow.
Warmth is spreading to ever deeper ocean levels, he said,
adding that pauses in surface warming could last 15-20 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.
The U.N. panel of climate scientists says it is at least 90
per cent certain that human activities - rather than natural variations in the
climate - are the main cause of warming in recent decade
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