Oct 152009

In the highly charged debate over climate change, overviews that are both balanced and informative can be hard to find.  Chip Knappenberger’s A Cherry-Picker’s Guide to Temper­ature Trends provides just such an overview of recent temperature trends. Knappenberger first charts all five main data sets*:


He then calculates trends from these data sets — using monthly data and going from September of each year through August 2009 — as simple linear least squares fits. He graphs each trend by starting year, with statistically significant trends (p < 0.05) being indicated by filled circles, and juxtaposes them to the average projected trend of the ensem­ble of climate models:


Thus the trend of RSS satellite data from September 1997 through August 2009, for instance, would be negative (and not statistically significant) while the trend of the GISS surface data from September 1998 through August 2009 would be positive, and marginally below model projections. Trends starting after 2004 are presumably too short to warrant inclusion.

Knappenberger uses this chart to identify cherry-picking by both camps — criticizing Stefan Rahmstorf and Richard Lindzen for selecting trends that happen to reinforce their respective positions. He also supplies examples for aspiring cherry-pickers:

Here are a few general statements that can be supported using my Cherry-Pickers Guide:

• For the past 8 years (96 months), no global warming is indicated by any of the five datasets.

• For the past 5 years (60 months), there is a statistically significant global cooling in all datasets.

• For the past 15 years, global warming has been occurring at a rate that is below the average climate model expected warming

And here are a few more specific examples that the seasoned cherry-picker could tease out:

• There has been no (statistically significant) warming for the past 13 years. [Using the satellite records of the lower atmosphere].

• The globe has been cooling rapidly for the past 8 years. [Using the CRU and satellite records]

Or on the other side of the coin:

• Global warming did not ‘stop’ 10 years ago, in fact, it was pretty close to model projections. [Using the GISS and NCDC records beginning in 1998 and 1999]

• Global warming is proceeding faster than expected. [Using the GISS record staring in 1991 or 1992—the cool years just after the volcanic eruption of Mt. Pinatubo]

I am sure the more creative of you can probably think of many others.

The best discussion of this article is by Lucia here.

It seems to me that recent temperature trends are important and bear watching. It is probably worth noting that Knappenberger’s chart is volatile, that the entire collec­tion of trends will tend to change as temperatures change.


GISS (Goddard Institute for Space Studies), NCDC (National Climate Data Center), and CRU (Climatic Research Unit) contain data compiled from surface records, while RSS (Remote Sensing Systems) and UAH (University of Alabama-Huntsville) contain data compiled from satellite observations of the lower atmosphere.

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