Published December 11, 2009 by Sean
Originally posted at http://www.houlihane.co.uk/blog.
Steve McIntyre has a detailed analysis of what has been described in the recent news as ‘a trick‘ to present temperature data in a certain way. His analysis is probably too detailed for many people to understand, so I have tried to provide a simple synopsys.
In 1999 the IPCC was preparing it’s Third Assessment Report on climate. Some of the climategate emails discuss a graph which was provided for publication in this report. From the evidence collated by Steve, it seems that the IPCC authors were concerned that the Report should provide a very clear view of unprecedented warming. Rather than simply collating evidence, they seem to have been selecting evidence which supported the message which they wished to convey to policy makers.
An early draft of this graph is shown here.
Of key significance is the Biffra reconstruction (yellow line) which seems to diverge from the measured temperature records about 50 years ago. At the time, there was a degree of concern amongst the scientists that this was an important problem with the reconstruction, and a reluctance to adjust the data so this was not such a visible problem.
They seemed keen to provide a graph which would be acceptable to the IPCC, and some of their discussions are now available and are discussed in Steve’s post. In conclusion, they appear to have truncated the problematic series, and spliced or grafted it in some way to the instrumental series. There is a subtle side effect here, the whole series moves down because the overlap with instrumental temperatures is now different.
This version seems to have been acceptable to the IPPC, and is now part of history.
I also show below an overlay of the two versions, showing how in a short period of time the expression of our knowledge of historical temperatures has been changed. Since these data are only shown in printed graphs, it is impossible to cleanly reconstruct the comparison – I have had to manually emphasise the green line which represents the modified Biffra series (clearly there has been some re-claibration of this series, it is not even of the same shape.
Filed under Science