OSLO—A U.N. panel on climate change is set to give its strongest warning yet that human use of fossil fuels is stoking global warming, informed sources said on Friday.
A draft of the report by 2,500 scientists says it is "very likely" that human activities were the main cause of warming in the past 50 years, strengthening a conclusion in their last study in 2001 that it was "likely", they said.
But the Intergovernmental Panel on Climate Change will also project less extreme bands than it did in 2001 for temperature and sea level rises in the 21st century. That means toning down both the most catastrophic and least damaging scenarios.
The report is due to be unveiled on Feb. 2 in Paris, after final review and approval by governments.
"It is very likely that... greenhouse gas increases (from human activities) caused most of the globally average temperature increases since the mid-20th century," a source who had seen the draft quoted it as saying.
The 2001 report defined "very likely" as meaning a 90-99 percent probability and "likely" as a 66-90 percent chance.
The draft projects that world temperatures will rise by 2.0-4.5 Celsius (3.6-8.1 Fahrenheit) by 2100 unless the world manages drastic cuts in greenhouse gas emissions from factories, cars and power plants.
Such a temperature rise could cause huge disruption to agriculture, trigger more floods, heatwaves and desertification and melt glaciers. The 2001 report had projected a wider possible range, of between 1.4-5.8 (2.5-10.4) Celsius.
The sources said the IPCC would also narrow both ends of the band of projected sea level rises, projected in the 2001 report at between 9 and 88 centimetres (3.5-34.7 inches) by 2100. Details were not available.
"There's good news that the top extremes for temperature and sea level rises have been cut," one source said. "But anyone hoping that the rise will be at the bottom end of the range will be disappointed."
"This is still a draft and its a draft until it's been approved by governments," another scientific source said.
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