By Environment Correspondent Alister Doyle
OSLO, Dec 8 (Reuters) - A thaw of Arctic ice and snow is linked to worsening summer heatwaves and downpours thousands of miles south in Europe, the United States and other areas, underlying the scale of the threat posed by global warming, scientists said on Sunday.
Their report, which was dismissed as inconclusive by some other experts, warned of increasingly extreme weather across "much of North America and Eurasia where billions of people will be affected".
The study is part of a drive to work out how climate change affects the frequency of extreme weather, from droughts to floods. Governments want to know the trends to plan everything from water supplies to what crops to plant.
But the science of a warming Arctic is far from settled.
Writing in the journal Nature Climate Change, experts in China and the United States said they could not conclusively say the Arctic thaw caused more extreme weather, or vice versa.
But they said they had found evidence of a relationship between the two. Rising temperatures over thawing snow on land and sea ice in the Arctic were changing atmospheric pressure and winds, the report said.
The changes slowed the eastward movement of vast meandering weather systems and meant more time for extreme weather to develop - such as a heatwave in Russia in 2010, droughts in the United States and China in 2011 and 2012, or heavy summer rains that caused floods in Britain in 2012, the paper added.
"The study contributes to a growing body of evidence that ... the melting Arctic has wide-ranging implications for people living in the middle latitudes," lead author Qiuhong Tang of the Chinese Academy of Sciences told Reuters.
Sea ice in the Arctic shrank to a record low in 2012 and the U.N.'s panel of climate scientists says it could almost vanish in summers by 2050 with rising greenhouse gas emissions.
But some scientists said other factors, including the usual vagaries of weather or changing sea temperatures, may explain some recent extremes rather than changes in the Arctic.
"The jury is still very much out," James Screen, an expert at Exeter University in England, said of efforts to see if there is a link between a melting Arctic and extremes further south in the northern hemisphere.
Some evidence in Sunday's study was "plausible ... but far from conclusive," he said, adding that some of the data were not statistically significant and might be random variations.
"For people on the streets, what really matters is whether the extremes are changing or not. But from the scientific perspective we want to understand why," he said. Better understanding is vital to make reliable predictions.
In September, the U.N.'s panel of climate scientists raised the probability that most global warming since 1950 has a human cause - mostly gases released by burning fossil fuels - to 95 percent from 90 in a previous assessment in 2007.
James Overland, of the U.S. National Oceanic and Atmospheric Administration, said many extremes studied were in the past decade, too short to know for sure if they were enhanced by Arctic ice and snow melt or not.
"Sceptics remain unconvinced that Arctic/mid-latitude linkages are proven, and this work will do little to change their viewpoint," he wrote in a comment in Nature Climate Change.
Still, he said there was a high potential for an Arctic influence, given the outlook for a further thaw. (Reporting By Alister Doyle)