Thursday, December 23, 2010

A Guide to Cherry-picking Temperature Data

With enough cherry-picking of the starting and ending years, you can usually get any temperature trend you want. Here is a nice diagram that shows that -- and that shows that you have to cherry-pick hard to cover up the long-term warming trend:

Here's how to read this diagram: the value plotted at point x = 1980, y = 60 corresponds to the 60-yr trend (°C/yr). (Note: trend is per year, not per decade.) The data source is Hadley's global temperature data (HadCRUT3).

This is from a new paper in BAMS: "Influence of choice of time period on global surface temperature trend estimates," Brant Liebmann et al, Bulletin of the American Meteorological Society, Nov 2010 p 1485. As the authors write:
It is evident that time segments of a few decades or shorter can exhibit either warming or cooling trends, while trends for longer segments are mostly positive, though quite weak compared to those present in shorter segments.

Here's an different presentation of the same data, that shows total temperature change for any interval, versus (as above) the trend:

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