By Alister Doyle, Environment Correspondent

OSLO, Oct 30 (Reuters) - Both Antarctica and the Arctic are getting less icy because of global warming, scientists said on Thursday in a study that extends evidence of man-made climate change to every continent.

Detection of a human cause of warming at both ends of the earth also strengthens a need to understand ice sheets on Antarctica and Greenland that would raise world sea levels by about 70 metres (230 ft) if they all melted, they said.

"We're able for the first time to directly attribute warming in both the Arctic and the Antarctic to human influences," said Nathan Gillett of England's University of East Anglia of a study he led with colleagues in the United States, Britain and Japan.

The Arctic has warmed sharply in recent years and sea ice shrank in 2007 to a record low. But Antarctic trends have been confusing -- some winter sea ice has expanded in recent decades, leaving doubts for some about whether warming was global.

The U.N. Climate Panel, which draws on work by 2,500 experts, said last year that the human fingerprint on climate "has been detected in every continent except Antarctica", which has insufficient observational coverage to make an assessment.

The scientists, writing in the journal Nature Geoscience, said the new findings filled that gap.

The study, comparing temperature records and four computer climate models, found a warming in both polar regions that could be best explained by a buildup of greenhouse gases, mainly from burning fossil fuels, rather than natural shifts.

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The link with human activities had been elusive in the polar regions because there are fewer than 100 temperature stations in the Arctic and just 20 in Antarctica, they said.

The scientists said temperatures had risen about 2 Celsius (3.6 Fahrenheit) in the past 40 years in the Arctic.

Temperatures in Antarctica, an icy deep freeze bigger than the United States, had gained by a few tenths of a degree. The Arctic is warming fast because darker water and ground soak up ever more heat than ice and snow that reflect the sun's rays.

The study also formally linked greenhouse gas emissions to rising temperatures in the Arctic, where big natural variations included a sharp temperature rise in the 1930s and 1940s.

The human cause had been hinted at by the U.N. Climate Panel last year, which said a human impact "has likely contributed to recent decreases in Arctic sea ice extent".

Scientists urged more study of ice and temperatures.

The U.N. Climate Panel projects that sea levels will rise by between 18 and 59 cm (7-23 inches) this century, part of shifts also likely to include more droughts, floods, heatwaves and more destructive storms.

"We really need to pay closer attention to what's going on with these ice sheets," Andrew Monaghan, of the U.S. National Center for Atmospheric Research, told a telephone news conference with Gillett.

Asked if the findings would affect his view of the likely pace of melting, he said: "I would say that it would lean towards a little bit bleaker side of the picture."

-- For Reuters latest environment blogs click on: http://blogs.reuters.com/environment/ (Editing by Giles Elgood)

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