LONDON - A leaked draft of the UN's most comprehensive study ever on climate change shows increasing evidence that links human activity to global warming.

It is "extremely likely" mankind is responsible for more than half of the observed temperature rises since the 1950s, a United Nations agency said in a draft report. In the UN's last study, in 2007, human influence on the temperature rise was deemed "very likely."

The document was posted by a blogger, Alec Rawls, on the website www.stopgreensuicide.com. In that post, Rawls, who is also an official reviewer of the study, said he regards his confidentiality agreement with the UN agency responsible for the research as "vitiated by the systematic dishonesty of the report."

The report by the UN Intergovernmental Panel on Climate Change is due for publication starting in September. It will come in three parts and then a final summary, culminating in October 2014. It's intended to guide envoys from 194 countries working on adopting a new treaty by 2015 that would fight climate change.

The IPCC said in a statement Friday that the leak "interferes" with the drafting process, and that the report is a "work in progress."

"This report further confirms that there really is no doubt about the fact that the Earth is warming, and there is no reasonable doubt that greenhouse-gas emissions from human activities are the primary driver of that warming," Bob Ward, policy director at the Grantham Research Institute on Climate Change and the Environment at the London School of Economics, said in a phone interview. "The evidence is now more robust."

More than 830 authors and editors from 85 countries are compiling the IPCC's so-called fifth assessment report. The terms used are assigned probabilities, with "virtually certain" representing 99 percent to 100 percent; extremely likely being at least 95 percent; "very likely" is 90 percent and up; and "likely" is at least 66 percent.

In 2007, the IPCC said global warming is an unequivocal fact and that "most of the observed increases in globally-averaged temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations."

"Most of the main aspects of the science of climate change have been well established and were well established in the last report in 2007," Ward said.