Over-Estimating Global Warming

Anthony Watts and others have made available the results of their study showing that the mis-siting of weather stations is responsible for about half of the reported warming in the United States over the past 30 years.  Weather stations are supposed to be put in grassy areas, away from concrete and other substances that absorb significant heat and re-radiate it.  It turns out that many of the nation's weather stations have been located near blacktop roads, on south-facing walls and otherwise are found in  places that show localized  higher-than-actual temperatures.  The result is that the nation's weather reporting system is influenced by the urban heat island (UHI) effect, where cities, with more concrete, blacktop and black tar roofs, soak up more heat than rural areas.  Relying on temperature reports from these areas reflects the effects of increasing urbanization, not changes in temperature or climate.

Here's the summary:

A reanalysis of U.S. surface station temperatures has been performed using the recently WMO-approved Siting Classification System devised by METEO-France’s Michel Leroy. The new siting classification more accurately characterizes the quality of the location in terms of monitoring long-term spatially representative surface temperature trends. The new analysis demonstrates that reported 1979-2008 U.S. temperature trends are spuriously doubled, with 92% of that over-estimation resulting from erroneous NOAA adjustments of well-sited stations upward. The paper is the first to use the updated siting system which addresses USHCN siting issues and data adjustments.

The new improved assessment, for the years 1979 to 2008, yields a trend of +0.155C per decade from the high quality sites, a +0.248 C per decade trend for poorly sited locations, and a trend of +0.309 C per decade after NOAA adjusts the data. This issue of station siting quality is expected to be an issue with respect to the monitoring of land surface temperature throughout the Global Historical Climate Network and in the BEST network.

You can see Mr. Watts' draft paper, and some of the comment on it, here. The continuing discussion is found here.