By Alister Doyle
OSLO, Jan 30 (Reuters) - Predicting how clouds will form in a warmer world remains a haze in a UN climate report due on Friday, affecting projected rises in temperatures and sea levels, scientists say.
The UN climate panel, an authoritative group on global warming numbering 2,500 experts, is set to give its strongest warning yet that human activities are heating the planet and that warming may cause huge damage to nature by 2100.
A draft report has plugged many gaps since a last report in 2001, such as anomalies between temperatures measured by satellites or at the earth's surface or how far tiny, glinting particles of air pollution reflect sunlight back into space.
But cloud formation in the 21st century – hard enough for weather forecasters to predict for tomorrow – is among the remaining puzzles.
"Large uncertainties remain about how clouds might respond to global climate change," according to a draft of the report under review by the Intergovernmental Panel on Climate Change (IPCC), meeting in Paris.
Warmer air can absorb more moisture, meaning more clouds and so more rain and snow in many regions. But much more cloud cover might also brake warming because more sunlight will bounce off the white tops back into space.
The report, the first of four on global climate this year by the IPCC, is due to be issued on Friday and will guide governments trying to work out policies to brake warming.
The report says more water vapour will bring more rains and snow to many regions towards the poles, such as northern Europe, Canada, the northeast United States and the Arctic.
Tibetan Rains
In winter, precipitation would also increase in northern Asia and the Tibetan plateau, it says.
By contrast, rains are likely to decrease in many subtropical regions. And parts of Africa and Europe around the Mediterranean are likely to get drier, and winter rains would decrease in southwestern Australia, it says.
In many regions, downpours will be more intense.
More snows could also offset any thaw of the vast Antarctic ice cap and the smaller cap on Greenland. If both melted over thousands of years world sea levels would be about 65 metres (around 215 ft) higher than today.
"In a warmer climate, models suggest that the ice sheets could accumulate more snowfall, tending to lower sea level," the draft says. But it adds that rapid thawing at the fringes has probably outweighed any such trend in recent years.
"In the interior of Greenland, the ice has been thickening," said Catherine Myrmehl, of the Nansen Environmental and Remote Sensing Center in Norway, based upon satellite readings. Many scientists reckon Greenland is losing ice overall.
The IPCC is likely to predict a "best estimate" of a temperature rise of 3 Celsius (5.4 Fahrenheit) by 2100 over pre-industrial times. And it is set to predict sea level rises this century of between 28 and 43 cms (11-17 inches), a lower band than forecast in the 2001 report.
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