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.

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".

It's not just bad for polar bears.

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.