What do the 'Climategate' hacked CRU emails tell us?

At a glance

What do you do if you cannot overturn well-established scientific theories by fair means? In the case of climate science deniers, you cheat. You play foul.

This is exactly what happened in November 2009. Sometime earlier, the email server at the Climatic Research Unit, University of East Anglia, was hacked (an illegal act in itself). A huge number of emails were stolen, sifted through and a selection was made available for download on a Russian server. The timing of the release was unsurprising, for early the following month the COP15 Climate Summit was due to be held in Copenhagen.

Selectively quoting parts of an email message removes all context. One of the most widely-quoted sentences, that will do nicely as an example, was as follows:

"I've just completed Mike's Nature 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."

Those fanatically promoting this conspiracy theory encouraged people to take such sentences at face value. The implication that was intended to be the take-home in this case was that climate scientists were covering up declining temperatures. However – and serial misinformers have a long track-record in this kind of thing - it means nothing of the sort. The people in that email were not talking about measured temperatures. Let's take a look at the context to find out what it really meant.

"Mike's Nature trick" referred to a technique described in a 1998 Nature paper, presenting a 600 year-long global temperature reconstruction by Michael Mann and colleagues. Michael is a palaeoclimate specialist who has for many years used tree-ring growth patterns in ancient wood to reconstruct conditions at the time those rings formed. The basic idea is that in cold, dry years, trees grow more slowly so their rings are relatively narrow and densely-spaced. In warm wet years, it's the opposite.

The "trick" is the technique of plotting recent instrumental data (i.e. weather observations) alongside the reconstructed tree-ring data for the time they overlap. It's a good way of checking if the reconstructed tree-ring data are representative and meaningful. They're no good for anything if they are not.

So, what does the “decline” refer to? It's also known as the 'divergence problem', a point on the timeline beyond which the reconstructed tree-ring data stop being representative and meaningful. This is a well-known issue in certain tree-ring datasets from specific places. What happens is that when plotted against instrumental temperature data, the reconstructed tree-ring data fall away – decline - below the instrumental data. This is a recent phenomenon that only showed up after about 1960. Prior to that, it hadn't been a problem.

Climate scientists started discussing the decline in the literature as long ago as 1995 – by which time they had many years of data showing that, where present, it stood out like a sore thumb. It seems to have been caused by an apparent loss in temperature-sensitivity with respect to certain species of trees growing in certain areas. Something had changed, making affected datasets unrepresentative of actual conditions.

All that ado about nothing. Just how much taxpayer's money was wasted on all the public inquiries that followed is anyone's guess. None were necessary. All you need to remember is that when it comes to climate science deniers, the difference between fair means and foul is at best blurred and more usually non-existent.

Please use this form to provide feedback about this new "At a glance" section. Read a more technical version below or dig deeper via the tabs above!

Further details

In November 2009, the servers at the University of East Anglia in Britain were illegally hacked and emails were stolen. When a selection of emails between climate scientists were published on the internet, a few suggestive quotes were seized upon by many claiming global warming was all just a conspiracy. A number of independent enquiries have investigated the conduct of the scientists involved in the emails. All have cleared the scientists of any wrong doing:

1. In February 2010, the Pennsylvania State University released an Inquiry Report that investigated any 'Climategate' emails involving Dr Michael Mann, a Professor of Penn State's Department of Meteorology. They found that "there exists no credible evidence that Dr. Mann had or has ever engaged in, or participated in, directly or indirectly, any actions with an intent to suppress or to falsify data". On "Mike's Nature trick", they concluded "The so-called “trick”1 was nothing more than a statistical method used to bring two or more different kinds of data sets together in a legitimate fashion by a technique that has been reviewed by a broad array of peers in the field."
2. In March 2010, the UK government's House of Commons Science and Technology Committee published a report finding that the criticisms of the Climatic Research Unit (CRU) were misplaced and that CRU’s "Professor Jones’s actions were in line with common practice in the climate science community".
3. In April 2010, the University of East Anglia set up an international Scientific Assessment Panel, in consultation with the Royal Society and chaired by Professor Ron Oxburgh. The Report of the International Panel assessed the integrity of the research published by the CRU and found "no evidence of any deliberate scientific malpractice in any of the work of the Climatic Research Unit".
4. In June 2010, the Pennsylvania State University published their Final Investigation Report, determining "there is no substance to the allegation against Dr. Michael E. Mann".
5. In July 2010, the University of East Anglia published the Independent Climate Change Email Review report. They examined the emails to assess whether manipulation or suppression of data occurred and concluded that "we find that their rigour and honesty as scientists are not in doubt."
6. In July 2010, the US Environmental Protection Agency investigated the emails and "found this was simply a candid discussion of scientists working through issues that arise in compiling and presenting large complex data sets."
7. In September 2010, the UK Government responded to the House of Commons Science and Technology Committee report, chaired by Sir Muir Russell. On the issue of releasing data, they found "In the instance of the CRU, the scientists were not legally allowed to give out the data". On the issue of attempting to corrupt the peer-review process, they found "The evidence that we have seen does not suggest that Professor Jones was trying to subvert the peer review process. Academics should not be criticised for making informal comments on academic papers".
8. In February 2011, the Department of Commerce Inspector General conducted an independent review of the emails and found "no evidence in the CRU emails that NOAA inappropriately manipulated data".
9. In August 2011, the National Science Foundation concluded "Finding no research misconduct or other matter raised by the various regulations and laws discussed above, this case is closed".

Just as there are many independent lines of evidence that humans are causing global warming, similarly a number of independent investigations have found no evidence of falsification or conspiracy by climate scientists.

Note: Updated on July 8, 2023 to correctly name CRU as the Climatic Research Unit.

The Skeptical Science website by Skeptical Science is licensed under a Creative Commons Attribution 3.0 Unported License.