By Nina Chestney and Alister Doyle
LONDON (Reuters) - International climate scientists are more certain than ever that humans are responsible for global warming, rising sea levels and extreme weather events, according to a leaked draft report by an influential panel of experts.
The early draft, which is still subject to change before a final version is released in late 2013, showed that a rise in global average temperatures since pre-industrial times was set to exceed 2 degrees Celsius by 2100, and may reach 4.8 Celsius.
"It is extremely likely that human activities have caused more than half of the observed increase in global average surface temperatures since the 1950s," the Intergovernmental Panel on Climate Change (IPCC) draft report said.
"Extremely likely" in the IPCC's language means a level of certainty of at least 95 percent. The next level is "virtually certain", or 99 percent, the greatest possible certainty for the scientists.
The IPCC's previous report, in 2007, said it was at least 90 percent certain that human activities, led by burning fossil fuels, were the cause of rising temperatures.
The draft was shown on a climate change skeptic blog.
The IPCC said the unauthorized, premature posting of the draft may lead to confusion because the report was still work in progress and was likely to change before it is released.
A United Nations conference last week aimed at curbing emissions of greenhouse gases blamed for global warning yielded no progress and three countries - Canada, Russia and Japan - have abandoned the Kyoto Protocol limiting the emissions.
The United States never ratified it in the first place, and it excludes developing countries where emissions are growing most quickly.
Countries agreed to extend Kyoto to 2020, but only those covering less than 15 percent of world greenhouse gas emissions signed up. Developing nations said they would push next year for a radical U.N. mechanism to compensate them for the impact of climate change.
The IPCC said it had "high confidence" that human activity had caused large-scale changes in oceans, in ice sheets or mountain glaciers, and in sea levels in the second half of the twentieth century, according to the draft.
It said some extreme weather events had also changed due to human influences.
THREAT TO CITIES
The draft's scenarios forecast a rise in temperatures of between 0.2 and 4.8 Celsius this century - a narrower band than in 2007. But in almost all of the scenarios, the rise would exceed 2 degrees Celsius.
Governments pledged in 2010 to try to stop global temperatures rising by more than 2 degrees, a threshold seen by scientists as the maximum to avoid more extreme weather, droughts, floods, and other climate change impacts.
Carbon dioxide concentrations in the atmosphere were the highest in 800,000 years, according to the draft report.
The draft also said that sea levels were likely to rise by between 29 and 82 centimeters by the end of the century - compared to 18-59 centimeters projected in the 2007 report.
Rising sea levels are a threat to people living in low-lying areas, from Bangladesh to the cities of New York, London and Buenos Aires. They open up the risk of storm surges, coastal erosion and, in the worst case scenario, the complete swamping of large areas of land.
The IPCC carries weight because it brings together all scientific research on climate change and informs policymakers.
Many countries want to study the final IPCC report before signing up to a new global pact to cut greenhouse gas emissions.
The draft included a possible future acceleration of ice loss from Antarctica and Greenland, which was omitted in 2007. It stopped short of including some research carried out since 2007 that suggested seas may rise by up to 2 meters by 2100.
(Editing by Tom Pfeiffer)