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Global warming may 'stop', scientists predict
Telegraph.co.uk
01 Maggio 2008
Global warming will stop until at least 2015 because of natural variations in the climate, scientists have said
Researchers studying long-term changes in sea temperatures said they now expect a "lull" for up to a decade while natural variations in climate cancel out the increases caused by man-made greenhouse gas emissions.
The average temperature of the sea around Europe and North America is expected to cool slightly over the decade while the tropical Pacific remains unchanged.
This would mean that the 0.3°C global average temperature rise which has been predicted for the next decade by the UN's Intergovernmental Panel on Climate Change may not happen, according to the paper published in the scientific journal Nature.
However, the effect of rising fossil fuel emissions will mean that warming will accelerate again after 2015 when natural trends in the oceans veer back towards warming, according to the computer model.
Noel Keenlyside of the Leibniz Institute of Marine Sciences, Kiel, Germany, said: "The IPCC would predict a 0.3°C warming over the next decade. Our prediction is that there will be no warming until 2015 but it will pick up after that."
He stressed that the results were just the initial findings from a new computer model of how the oceans behave over decades and it would be wholly misleading to infer that global warming, in the sense of the enhanced greenhouse effect from increased carbon emissions, had gone away.
The IPCC currently does not include in its models actual records of such events as the strength of the Gulf Stream and the El Nino cyclical warming event in the Pacific, which are known to have been behind the warmest year ever recorded in 1998.
Today's paper in Nature tries to simulate the variability of these events and longer cycles, such as the giant ocean "conveyor belt" known as the meridional overturning circulation (MOC), which brings warm water north into the North East Atlantic.
This has a 70 to 80-year cycle and when the circulation is strong, it creates warmer temperatures in Europe. When it is weak, as it will be over the next decade, temperatures fall. Scientists think that variations of this kind could partly explain the cooling of global average temperatures between the 1940s and 1970s after which temperatures rose again.
Writing in Nature, the scientists said: "Our results suggest that global surface temperature may not increase over the next decade, as natural climate variations in the North Atlantic and tropical Pacific temporarily offset the projected anthropogenic [manmade] warming."
The study shows a more pronounced weakening effect than the Met Office's Hadley Centre, which last year predicted that global warming would slow until 2009 and pick up after that, with half the years after 2009 being warmer than the warmest year on record, 1998.
Commenting on the new study, Richard Wood of the Hadley Centre said the model suggested the weakening of the MOC would have a cooling effect around the North Atlantic.
"Such a cooling could temporarily offset the longer-term warming trend from increasing levels of greenhouse gases in the atmosphere.
"That emphasises once again the need to consider climate variability and climate change together when making predictions over timescales of decades."
But he said the use of just sea surface temperatures might not accurately reflect the state of the MOC, which was several miles deep and dependent on factors besides temperatures, such as salt content, which were included in the Met Office Hadley Centre model.
If the model could accurately forecast other variables besides temperature, such as rainfall, it would be increasingly useful, but climate predictions for a decade ahead would always be to some extent uncertain, he added.
By Charles Clover
Source > Telegraph.co.uk
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