guardian.co.uk, Friday 20 November 2009 18.15 GMT

Climate sceptics claim leaked emails are evidence of collusion among scientists

Hundreds of emails and documents exchanged between world's leading climate scientists stolen by hackers and leaked online

Hundreds of private emails and documents allegedly exchanged between some of the world's leading climate scientists during the past 13 years have been stolen by hackers and leaked online, it emerged today.

The computer files were apparently accessed earlier this week from servers at the University of East Anglia's Climate Research Unit, a world-renowned centre focused on the study of natural and anthropogenic climate change.

Climate change sceptics who have studied the emails allege they provide "smoking gun" evidence that some of the climatologists colluded in manipulating data to support the widely held view that climate change is real, and is being largely caused by the actions of mankind.

The veracity of the emails has not been confirmed and the scientists involved have declined to comment on the story, which broke on a blog called The Air Vent.

The files, which in total amount to 160MB of data, were first uploaded on to a Russian server, before being widely mirrored across the internet. The emails were accompanied by the anonymous statement: "We feel that climate science is, in the current situation, too important to be kept under wraps. We hereby release a random selection of correspondence, code and documents. Hopefully it will give some insight into the science and the people behind it."

A spokesperson for the University of East Anglia said: "We are aware that information from a server used for research information in one area of the university has been made available on public websites. Because of the volume of this information we cannot currently confirm that all this material is genuine. This information has been obtained and published without our permission and we took immediate action to remove the server in question from operation. We are undertaking a thorough internal investigation and have involved the police in this inquiry."

In one email, dated November 1999, one scientist wrote: "I've just completed Mike's Nature [the science journal] trick of adding in the real temps to each series for the last 20 years (ie, from 1981 onwards) and from 1961 for Keith's to hide the decline."

This sentence, in particular, has been leapt upon by sceptics as evidence of manipulating data, but the credibility of the email has not been verified. The scientists who allegedly sent it declined to comment on the email.

"It does look incriminating on the surface, but there are lots of single sentences that taken out of context can appear incriminating," said Bob Ward, director of policy and communications at the Grantham Research Institute on Climate Change and the Environment at the London School of Economics. "You can't tell what they are talking about. Scientists say 'trick' not just to mean deception. They mean it as a clever way of doing something - a short cut can be a trick."

In another alleged email, one of the scientists apparently refers to the death of a prominent climate change sceptic by saying "in an odd way this is cheering news".

Ward said that if the emails are correct, they "might highlight behaviour that those individuals might not like to have made public." But he added, "Let's separate out [the climate scientists] reacting badly to the personal attacks [from sceptics] to the idea that their work has been carried out in an inappropriate way."

The revelations did not alter the huge body of evidence from a variety of scientific fields that supports the conclusion that modern climate change is caused largely by human activity, Ward said. The emails refer largely to work on so-called paleoclimate data - reconstructing past climate scenarios using data such as ice cores and tree rings. "Climate change is based on several lines of evidence, not just paleoclimate data," he said. "At the heart of this is basic physics."

Ward pointed out that the individuals named in the alleged emails had numerous publications in peer-reviewed scientific journals. "It would be very surprising if after all this time, suddenly they were found out doing something as wrong as that."

Professor Michael Mann, director of Pennsylvania State University's Earth System Science Centre and a regular contributor to the popular climate science blog Real Climate, features in many of the email exchanges. He said: "I'm not going to comment on the content of illegally obtained emails. However, I will say this: both their theft and, I believe, any reproduction of the emails that were obtained on public websites, etc, constitutes serious criminal activity. I'm hoping the perpetrators and their facilitators will be tracked down and prosecuted to the fullest extent the law allows."

When the Guardian asked Prof Phil Jones at UEA, who features in the correspondence, to verify whether the emails were genuine, he refused to comment.

The alleged emails illustrate the persistent pressure some climatologists have been under from sceptics in recent years. There have been repeated calls, including Freedom of Information requests, for the Climate Research Unit to make public a confidential dataset of land and sea temperature recordings that is "value added" by the unit before being used by the Met Office. The emails show the frustration some climatologists have had at having to operate under such intense, often politically motivated, scrutiny.

Prof Bob Watson, the chief scientific advisor at the Department for the Environment, Food and Rural Affairs said, "Evidence for climate change is irrefutable. The world's leading scientists overwhelmingly agree what we're experiencing is not down to natural variation."

"With this overwhelming scientific body of evidence failing to take action to tackle climate change would be the wrong thing to do – the impacts here in Britain and across the world will worsen and the economic consequences will be catastrophic."

A spokesman for Greenpeace said: "If you looked through any organisation's emails from the last 10 years you'd find something that would raise a few eyebrows. Contrary to what the sceptics claim, the Royal Society, the US National Academy of Sciences, Nasa and the world's leading atmospheric scientists are not the agents of a clandestine global movement against the truth. This stuff might drive some web traffic, but so does David Icke."

• This article was amended on Wednesday 25 November 2009. A unit was garbled: the files amount to 160MB of data, not 160MbB. This has been corrected.