As I have been very critical of Michael Mann and his colleagues at Real Climate, it is important for me to be very careful with this piece.
Analysis conducted on recently released information cast further questions on data used in recent papers that say global warming is worse than we thought--including Michael Mann's famous hockey stick chart. The analysis was conducted by Steve McIntyre and is published on his weblog Climate Audit. The conversation there is enlightening, interesting and sure to infuriate believers in AGW (anthropogenic global warming). I highly recommend a careful look at the various posts on the weblog relating to this issue.
The data, which the scientists had refused to release for a decade, came to light when the Royal Society of London demanded they archive their findings before publishing their latest paper. The data itself regards tree rings in northern Russia, in a place called Yamal. The controversy arises from the scientists' selection of 12 trees for analysis of temperatures over the past thousand or so years. The 12 trees were certainly not selected at random, and the data from these trees do not match data from many, many similar trees that were excluded from the study. Because the data from the Yamal series is weighted, which just means it is given more numerical importance than some other data, changes in Yamal have a disproportionate effect on all the research conducted by this team of scientists, going back to 1998. If McIntyre's criticism holds up, it will also have a follow-on effect on many other scientific publications that have referenced this work.
When data from other trees that appear to fit the criteria for analysis is examined, the dramatic temperature rise of the past 30 years disappears. When it is replaced by data from a set of trees called Schweingruber, one can see the difference with the naked eye:
We need to wait for more analysis and discussion, a point McIntyre has made repeatedly since he released his first examination. Although skeptics have already pounced on this as evidence of wrongdoing and/or incompetence, the full story isn't in yet.
Just one possible example of a reasonable explanation: If you were going to use trees as historical thermometers with a record of the past 1,000 years, you would want to choose trees whose most recent temperature records (the records are the tree rings--all things being equal, they grow a bit more when it is warm than when it is cold) agree with records from actual thermometer measurements over the past century. So this may be the reason they only selected 12 trees out of hundreds available to measure.
But if so, they'll have to explain why they didn't give their reasons in their publications--which is standard procedure. And they'll also have to explain something which strikes me as very odd. If you are selecting trees because they agree with recent records, how can you use them as evidence of recent temperatures--that's why you picked them.
The mere fact that they refused to release this data for years will make any explanation look made up post facto. Their total silence on Real Climate--the scientists in question are principal contributors to that weblog--and the silence of other climate activists may mean nothing for the moment. It would be wise not to be precipitate on this matter. But if it persists, it will be damning.
This group of scientists were excoriated for poor use of statistical analysis, and this criticism caused many to quit using Mann's Hockey Stick chart. The chart and the methods used to produce it were actually investigated by a Congressional Committee, which found that Mann et al had made serious errors and that their conclusions were not warranted. This is a separate, if related issue. It is, however, serious. If governments the world over have been making economic, social and political decisions based on data that does not stand up, the ramifications are huge.
But let's wait and see.