Natural changes may offset global warming briefly

LONDON Wed Apr 30, 2008 6:40pm BST

1 of 2. Giant tabular icebergs surrounded by ice floe drift in Vincennes Bay in the Australian Antarctic Territory January 11, 2008. Natural climate changes may offset human-caused global warming over the next decade, keeping ocean temperatures the same or even temporarily cooling them slightly, German researchers said on Wednesday.

Credit: Reuters/Torsten Blackwood/Pool

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LONDON (Reuters) - Natural climate changes may offset human-caused global warming over the next decade, keeping ocean temperatures the same or even temporarily cooling them slightly, German researchers said on Wednesday.

However, this short-term situation might create a problem if policymakers regarded it as a sign they could ease efforts to limit greenhouse gases or play down global warming.

"The natural variations change climate on this timescale and policymakers may either think mitigation is working or that there is no global warming at all," said Noel Keenlyside, a climate researcher at the Leibniz Institute of Marine Sciences in Germany who led the study.

Climate researchers have long predicted more greenhouse gases in the atmosphere would spur a general warming trend over the next 100 years. The study in the journal Nature is one of the first to take a shorter-term view.

This is useful because natural changes as opposed to human causes may play a bigger role in the short term, Keenlyside said.

His team made a computer model that takes into account natural phenomena such as sea surface temperatures and ocean circulation patterns.

They checked their work by producing a set of forecasts using data recorded over the past 50 years and found the retrospective forecasts were accurate, Keenlyside said.

"This is important because policies are made in the short term," Keenlyside said. "Our results show we might not have as much change in climate over the next 10 years."

A United Nations climate panel report this year predicted temperatures would rise between 1.8 and 4 degrees Celsius this century, in part because of fossil fuels that produce carbon dioxide, the main greenhouse gas.

Scientists say rising temperatures could cause seas to rise sharply, glaciers to melt and storms and droughts to become more intense. These in turn may force mass migrations of climate refugees.

One possible reason for the relative cooling effect in the next decade is the predicted weakening of a system that brings warm water northward into the North Atlantic and offsets an expected rise in greenhouse gases, Keenlyside said.

"The first attempts at decadal prediction suggest that reasonably accurate forecasts of the combined effects of increasing greenhouse-gas concentrations and natural climate variations can be made," Richard Wood of Britain's Met Office, who was not involved in the study, wrote in a Nature commentary.

(Reporting by Michael Kahn, Editing by Maggie Fox and Robert Woodward)

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