Climate email hackers had access for more than a month

Computer hackers who broke into the Climatic Research Unit (CRU) server at the University of East Anglia had access to its systems for more than a month.

The full data – covering 1,000 emails and 3,000 documents in which the most recent document and email is dated 12 November – came to wider notice when a copy was posted on a web server in Russia on 19 November.

But a month earlier a BBC weatherman who had expressed doubts about climate change on his blog was sent a sample of the email exchanges, suggesting the hackers already had access to the private system.

The university declined to answer questions about the setup and security of the computers used by CRU scientists, but security experts say there are only three tenable explanations for how the server was hacked: a determined break-in by an external hacker; that one of the CRU or university systems was accidentally "compromised" by a computer virus or other "malware"; or it was an "inside job" by a disaffected member of university staff. The latter is viewed as the least likely.

Climate change deniers have seized on the disclosures, claiming they proved that the scientists had colluded to manipulate climate data and that they called into question the evidence for human-driven global warming.

Leading scientific bodies and governments have dismissed the charges, insisting there is clear evidence that humans are to blame for global warming.

The first leak occurred after 9 October, when one of the BBC's regional weathermen, Paul Hudson, wrote an article arguing that for the last 11 years there had not been an increase in global temperatures. On 12 October he was forwarded a "chain of emails", including some which subsequently appeared in the hacked documents. Last night the BBC confirmed Hudson had been forwarded emails written by two of the scientists, but refused to disclose the source.

"Paul spotted that these few e-mails were among thousands published on the internet following the alleged hacking of the UEA computer system," said a BBC spokesman.

After sending Hudson the sample, nothing more emerged from the hackers for a month. Then early on 17 November someone hacked into the RealClimate website, used by climate scientists to explain their work. Using a computer in Turkey, they uploaded a zip file containing all 4,000 emails and documents. But within a couple of minutes Gavin Schmidt, the website's co-founder, realised something was wrong and shut down the site. The file had been online for 25 minutes but had not been picked up.

On 19 November the hackers used a computer in Saudi Arabia to post a link on The Air Vent – a website popular with climate change sceptics – pointing to a fresh copy of the zip file, this time stored on a Russian web server. At that point it was finally picked up by blogs and news organisations around the world.