Last week I commented on a pre-publication draft of a new study of historical temperature records which essentially confirms what all previous studies have shown: the Earth appears to be getting warmer. The work was performed by the Berkeley Earth Surface Temperature group, which cheekily abbreviates to BEST. There has been a fair amount of discussion on-line of the early results of the Berekely study.
One of their first results was the re-discovery of the fact that the urban heat island (UHI) effect can be easily compensated for. As discussed here on sceptical science, it appears that the UHI effect may indeed have been over-compensated.
While discussing the implications of the work with my colleagues on the Steering Committee of the Surface Temperature Initiative, my attention was drawn to some areas in which the new temperature estimates differed. There are some large historical differences, but most importantly there are differences for the last decade. The BEST results for this period are shown alongside other estimates below.
My colleagues were surprised to see that the BEST team have compared their data with the HadCRU temperature estimate. This is a compilation of CRUTEM3 (a global land-surface temperature estimate) and HadSST2 (a global ocean-surface temperature estimate) merged at coastal cells. So comparing with HadCRU is not a fair comparison. On general principles, one would expect more rapid changes in land surface temperature than sea surface temperature.
This explains most of the difference between the BEST release and HadCRU T3 shown in the graphs and explains why the series looks so different in their curve. However the Berekely curve shows more dramatic warming even than CRUTEM3 the Climate Research Unit’s Global Land Surface Mean Temperature estimate:
The BEST estimate shows current warming at ≈0.029 °C per year compared to the CRU estimate ≈ 0.009 °C per year. This is a large and significant differences. For example, extrapolating over a century, it is the difference between a catastrophic 2.9 °C, or a more amenable 0.9 °C.
The differences above are merely a blink-of-an-eye in terms of climate change. But they cover a period spanning many electoral cycles, and whether we like it or not, such data can affect key government decisions. And decisions made now can have climatic consequences for decades or even centuries to come. The role of the Surface Temperature Initiative is to provide tools to allow scientists to understand how their estimates can be so different, and to decide which one is closest to the correct answer.
Thanks to Peter Thorne for detailed corrections.