UN: 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 and on the earth's surface or how far 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. But increased cloud cover might also slow warming because more sunlight will bounce off the tops of clouds and be reflected 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 determine policies to slow global warming.
The report says more water vapour will bring more rain and snow to regions towards the poles, such as northern Europe, Canada, the northeast US and the Arctic. In winter, precipitation would also increase in northern Asia and the Tibetan plateau.
By contrast, rains are likely to decrease in many subtropical regions. 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 65m (215ft) 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.
The IPCC is likely to predict a "best estimate" of a temperature rise of 3 degrees by 2100 over pre-industrial times. It is set to predict sea-level rises of 28cm to 43cm (11 to 17 inches) this century, a lower band than forecast in the 2001 report.
Climate change: four reports due this year
The UN scientific group advising on climate change, the Intergovernmental Panel on Climate Change, is due to make several reports this year.
The first, to be published in Paris on Friday, will give evidence linking human activities, led by use of fossil fuels, to a warming in the past 50 years. It will project likely climate changes to 2100.
A draft of the report, Physical Science Basis of Climate Change, says there is at least a 90 per cent chance that human activities are the main cause of global warming, scientific sources say.
The previous report in 2001 said the link was "likely", or at least a 66 per cent chance. It will also project a "best estimate" of a temperature rise of three degrees above pre-industrial levels.
The second report, Impacts, Adaptation and Vulnerability, to be published in Brussels in April, will detail the likely impacts of climate change. Australian newspaper the Age said a draft projects that 200 to 700 million more people could face food shortages by 2080 and that 1.1-3.2 billion more could suffer water shortages.
The third report , Mitigation of Climate Change, will be published in Bangkok in May and will analyse ways to fight global warming. The fourth, Synthesis Report, to be published in Valencia in November, will sum up.