The chairman of the IPCC, Rajendra Pachauri, celebrates the announcement of the Nobel Peace Prize for the organisation in 2007

The Intergovernmental Panel on Climate Change (IPCC) has warned against drawing too many conclusions from the latest leaked version of its upcoming, and eagerly awaited, Assessment Report 5 (AR5).

This massive tome will be published in four stages over the next year - the first part, the physical science behind climate change, will be presented in Stockholm on 26 September.

The process of compiling this report - with several hundred scientists, 195 governments and over 100 non-governmental organisations involved - has been particularly leaky, with at least three confidential drafts being made public in the last year.

According to the latest scoop, the scientists are set to say they are more convinced than ever that global warming is caused by humans. They will say they are 95% certain that our use of fossil fuels is the main reason behind the global rise in temperatures since the 1950s.

The panel will also outline why global temperatures have been rising more slowly since 1998, a controversial slowdown that scientists have been struggling to explain.

Sensitive questions

According to the leak, they will put it down to natural meteorological variations and other factors that could include greater absorption of heat into the deep oceans - and the possibility that the climate is less sensitive to carbon dioxide than had previously been believed.

Many climate sceptics have argued that this is a key factor behind the temperature slowdown, and a good reason not to believe the more extreme predictions of those they dismiss as warmist conspirators.

But those involved with the IPCC say that even now, just a month away from publication, you would be "foolish in the extreme" to take this latest leak as conclusive.

"It is guaranteed it will change," said Jonathan Lynn, spokesman for the IPCC. "In September, the scientists will go through the 15-page summary for policymakers, line by line."

"We've already given it to governments for their thoughts, and we've had 1,800 comments on that 15-page document," he said.

When the previous IPCC report came out in 2007, it ran into two major problems.

The first issue was the discovery of basic errors - including the embarrassing claim that all the glaciers in the Himalayas would have melted by 2035.

The second was the so called Climategate affair, in which leaked emails purported to show leading scientists trying to manipulate their data to make the report more damning.

Ultimately, several investigations showed the accusations of manipulation to be false.

Last chance saloon?

To try to ensure there was no repeat in this year's report, the IPCC determined to be as open as possible.

Almost anyone who claimed any expertise in the field could register to be a reviewer. There have already been several leaks as a result of this open approach.

"We are not trying to keep it secret," Jonathan Lynn told BBC News. "After the report is finished, we are going to publish all the comments and responses so that people can track the process.

"We just think it's misleading to get hold of these drafts and put them out and draw conclusions from them."

The ongoing problems with leaks is one of the reasons behind the mutterings that this large-scale, multi-faceted report from the IPCC could be its last.

There are some who argue that having a mega-event every seven years is misguided: science is moving much faster than that, and the process itself is too arduous for the hundreds of scientists involved.

"There are people who say: why not put all the drafts out there, let everyone look at them and that will be the end of it," said Jonathan Lynn. "I think there will certainly be an IPCC in the future but there may not be these big blockbuster events."

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