Prepared on behalf of the United Nations every five or six years to summarise climate change research, the panel report draws on hundreds of peer-reviewed papers. 

Compared with the previous report in 2007, the draft expresses even more confidence that changes being observed across the planet are ''significant, unusual or unprecedented'' on historic time scales.

It says carbon dioxide is the biggest cause of climate change, far outweighing natural causes. The concentration of atmospheric carbon dioxide is the highest in 800,000 years.

The draft document - due to be released next year - was leaked by climate sceptic Alec Rawls, who runs a website called ''Stop Green Suicide'' and volunteered as a report reviewer.

Mr Rawls claimed the report contained an ''admission'' that galactic cosmic rays were having a strong influence on the Earth's climate.

But a lead author of the relevant section of the report, Professor Steve Sherwood from the University of New South Wales, told Fairfax Media the theory had been rejected later in the report after a review of peer-reviewed research.