An attempt by climate sceptics to hijack the latest UN report on global warming by selectively leaking claims that it is caused by sunspots rather than man-made emissions of carbon dioxide has backfired.

Sceptics described the forthcoming report of the Intergovernmental Panel on Climate Change (IPCC) as a “game changer” because of its apparent support for the controversial theory that solar activity, interacting with cosmic rays from deep space, can explain global warming.

Alec Rawls, a Republican blogger in the United States who signed himself up as an expert IPCC reviewer, decided to leak the panel’s Fifth Assessment Report (AR5) on the grounds that it is a taxpayer-funded document.

Mr Rawls claimed the report suggests that the IPCC has finally come round to the idea that solar activity – sunspots – is partly responsible for the observed global temperatures rise seen over the past half century.

“The admission of strong evidence for enhanced solar forcings changes everything. The climate alarmists can’t continue to claim that warming was almost entirely due to human activity over a period when solar warming effects, now acknowledged to be important, were at a maximum,” Mr Rawls said.

“The final draft of [the IPCC report] is not scheduled to be released for another year, but the public needs to know now how the main premise and conclusions of the IPCC story line have been undercut by the IPCC itself,” he said.

However, climate scientists pointed out that Mr Rawls has selectively quoted from the draft report and has ignored other parts of the document stating that solar activity and cosmic rays cannot explain the increase in global temperatures seen over the past half century, as sceptics have repeatedly claimed.

The AR5 draft report states that although there is “some evidence” that solar activity combined with cosmic rays may influence the formation of clouds, and therefore temperatures, but the phenomenon is “too weak” to influence the climate in any significant way.

The other major problem with the sunspots idea is that solar activity has largely flat-lined over the past 50 years, whereas average global temperatures have continued to rise, the IPCC report says.

“The lack of trend in the cosmic ray intensity over the last 50 years provides another strong argument against the hypothesis of a major contribution of cosmic rays to on-going climate change,” the draft report says.

Professor Bill McGuire of University College London said that the IPCC report reiterates the widely accepted view among scientists that climate change is not a natural process but the consequence of human activities.

“Alec Rawls’ interpretation of what IPCC5 says is quite simply wrong. In fact, while temperatures have been ramping up in recent decades, solar activity has been pretty subdued,” Professor McGuire said

Professor Corinne Le Quéré of the University of East Anglia, a lead author of previous IPCC reports as well as AR5, said that nothing has changed in terms of the overall view that global warming is exacerbated by greenhouse gas emissions and that the influence of solar activity was small.

“The evidence for man-made climate change is overwhelming, and there is nothing in the draft IPCC report leaked today that suggests the contrary,” Professor Le Quere said.

Bob Ward of the Grantham Research Institute on Climate Change at the London School of Economics, another IPCC reviewer, said that Rawls has tried to misrepresent the IPCC by cherry picking quotes out of context to support the idea that solar activity and cosmic rays have influenced the climate.

“In fact, the draft report concludes that there are strong arguments against the cosmic ray theory, while there is compelling evidence that greenhouse gas emissions are driving the unequivocal rise in global average temperature,” Mr Ward said.

The IPCC said that it regrets the unauthorised release of the draft report by one of many hundreds of reviewers who have all undertaken to keep the details confidential.

“The unauthorized and premature posting of the [draft reports], which are works in progress, may lead to confusion because the text will necessarily change in some respects once all the review comments have been addressed,” it said.