In November 2009, over 1,000 private emails between climate change scientists were stolen and published online. The uproar that followed briefly shook the public's faith in global warming science, and prompted investigations that debunked sceptics' allegations that the mails showed the planet wasn't warming. Yet still the scientists have questions to answer

What is "climategate"?

It is the controversy over a set of over 1,000 private emails and many other documents that were stolen or leaked from the University of East Anglia's (UEA) Climatic Research Unit (CRU) in November 2009. All the emails involved CRU staff, principally the CRU head Phil Jones, but in correspondence with many of the world's leading climate scientists, including the main researcher behind the "hockey stick" graph, Michael Mann. CRU's speciality was reconstructing records of the Earth's past temperatures from thermometer data and "proxy" such as tree-ring measurements.

Why did it cause such a storm?

Selected contents of the emails were used by some to suggest that scientists had been manipulating or hiding data, had been working together to frustrate people requesting access to the data and to prevent journal papers they disagreed with from appearing. Climate sceptics believed they had found the "smoking gun" which confirmed their belief that global warming was not happening. You can read five key emails here, with commentary from Fred Pearce. The impact of the emails was amplified because they were released just weeks before the major UN climate change summit in Copenhagen, at a time when the threat of climate change and the need for global action was at the forefront of public attention.

Do the emails show that climate change science is a sham and that the world is not warming?

No they don't. The claims made by Sarah Palin, among many others, have been comprehensively debunked.

So what are the issues raised?

Two main issues: access to scientific data and how scientific research is reviewed. One of the fundamental principles of scientific inquiry is that research is independently checked to see if different researchers get the same results. That requires access to the raw data and the raw data behind CRU's temperature record was not publicly available. It had been painstakingly put together over many years from many different data sources, and corrections had been applied to account for known biases. The emails show the scientists were very reluctant to share this data with people they saw as only wanting to make trouble, and appeared to work to block requests filed under Freedom of Information law and even suggested deleting information to prevent it being released.

Scientific knowledge is accredited through the process of peer review, when scientists check each others work before publication in a journal. The emails appear to show climate scientists working together to try to stop papers critical of their work being published.

Have these claims been investigated?

In total, there are four investigations. The first to report was by the House of Commons science and technology select committee, though some critics suggested the imminent general election has led it to be rushed. It questioned Phil Jones and others in person and concluded that the reputation of the CRU scientists was intact but that there were issues over access to data, the blame for which it laid at the door of UEA. Next was a short report chaired by Lord Oxburgh, to examine the "integrity" of the research, though the exact brief is contested. It also absolved the scientists and said making the data public was the responsibility of those who collected it, often national weather services around the world.

The third, and biggest enquiry, was set up by UEA to operate independently under Sir Muir Russell and will be published on 7 July. Finally, there is an ongoing Norfolk police enquiry into how the emails became public, but there have been no recent developments. In addition, the environmental journalist Fred Pearce conducted an in-depth investigation for the Guardian.

What did the Muir Russell review conclude?

It found the scientists had not fudged their results or silenced critics. But it found serious shortcomings in the openness with which they worked, posing a risk to the credibility of UK climate science and indicating a transformation in the way science has to be conducted in this century.

How does "climategate" relate to the flaws in the IPCC report?

The controversies are largely separate but overlapped in time. The UN's advisory body on climate change science, the Intergovernmental Panel on Climate Change, was forced to admit that it had mistakenly included a statement in its landmark 2007 report that all Himalayan glaciers could melt entirely by 2035. Intense scrutiny in the aftermath found only a few further minor errors in the thousands of pages of the report. But the combination of these two controversies brought fierce pressure to bear on climate science in the aftermath of the Copenhagen summit, which failed to seal a global deal.

There are some direct links between the two, however, as the emails discuss IPCC work, in particular in relation to the "hockey stick graph".

What impact has "climategate" had?

In the short term, opinion polls showed small shifts towards more sceptical positions, though the failure of politicians at Copenhagen and a very cold winter in the northern hemisphere may equally have been the cause. Overall, the UK public largely believes the planet is warming and that human activities, and the greenhouse gases they produce are to blame. Some climate scientists have said things will never be the same again and that a new openness more appropriate to the internet age will result from the leaking of the emails. In the longer term, whether it will have an effect on global efforts to cut greenhouse gas emissions remains to be seen.

What was later released in November 2011?

On 22 November 2011, a fresh tranche of private emails exchanged between leading climate scientists throughout the last decade was released online. Their release came less than a week before the UN climate talks are due to resume in Durban, and appear to be genuine. Over 200,000 encrypted emails were posted on the internet, along with 5,000 publically-available emails and a "background and context" file highlighting some of the excerpts. Prof Michael Mann, director of the Earth System Science Centre at Penn State University, who is quoted in the batch of released emails described the release as "truly pathetic".