Evidence for climate change has grown stronger and it is now ''virtually certain'' that human greenhouse gas emissions trap energy that warms the planet, according to a leaked draft of the next major Intergovernmental Panel on Climate Change report.
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.
The draft expresses even more confidence than the 2007 report that changes being observed across the planet are historically ''significant, unusual or unprecedented''.
It says that 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 of the University of NSW, said the theory had been rejected later in the report after a review of peer-reviewed research.
John Church, a lead author of the report and a senior CSIRO scientist, said: ''The report is still a working draft and these people have had access under conditions of not releasing the draft and not passing it on, and they have broken that agreement.''
Dr Church said the weight of evidence linking human activity and climate change had strengthened since the previous report.
The draft builds on the findings of the previous IPCC report that warming of the climate system is ''unequivocal''.
It says: ''There is consistent evidence from observations of a net energy uptake of the Earth System due to an imbalance in the energy budget. It is virtually certain that this is caused by human activities, primarily by the increase in CO2 concentrations.''
The IPCC was formed in 1988 on the request of UN member countries, and has released four major assessments of the climate to help guide the negotiations on a global treaty to cut emissions. Each assessment report is approved by all governments.
In 2007 it shared the Nobel Peace Prize with former US president Al Gore for its work on climate change, but it has also been criticised for being too conservative and for errors found in its last assessment report.
The IPCC draft report says global average temperatures had risen about 0.8 degrees between 1901 and 2010, mostly since the 1970s.
Other findings include that:
There is a high level of confidence average global temperatures will rise between one and 3.7 degrees by 2081-2100. The IPCC assumes some climate policies will be introduced.
It is extremely likely humans have caused more than half the temperature increase since the 1950s. There is a high level of confidence this has caused large-scale changes in the ocean, ice and sea levels.
Global sea levels are likely to rise between 0.29 and 0.82 metres by 2081-2100. Higher rises cannot by ruled out, but scientific understanding is insufficient to evaluate their probability.