Dr. Roy Spencer, Principal Research Scientist at the University of Alabama in Huntsville, analyzed the records of weather stations in the U.S. Historical Climatology Network (USHCN) which NOAA touts as the official U.S. climate record. He found that almost all of the reported warming since 1973 can be accounted for merely by the adjustments to the record made by NOAA.
Spencer also analyzed the British CRUTem3 and International Surface Hourly (ISH) data. His main conclusions are:
1) The linear warming trend during 1973-2012 is greatest in USHCN (+0.245 C/decade), followed by CRUTem3 (+0.198 C/decade), then my ISH population density adjusted temperatures as a distant third (+0.013 C/decade)
2) Virtually all of the USHCN warming since 1973 appears to be the result of adjustments NOAA has made to the data, mainly in the 1995-97 time frame.
3) While there seems to be some residual Urban Heat Island (UHI) effect in the U.S. Midwest, and even some spurious cooling with population density in the Southwest, for all of the 1,200 USHCN stations together there is little correlation between station temperature trends and population density.
4) Despite homogeneity adjustments in the USHCN record to increase agreement between neighboring stations, USHCN trends are actually noisier than what I get using 4x per day ISH temperatures and a simple UHI correction.
When “global warming” only shows up after the data are adjusted, one can understand why so many people are suspicious of the adjustments.
See his post on the matter here.
Spencer has also examined the surface temperature records according to the population density near the stations. He finds “clear evidence of an urban heat island effect on temperature trends in the U.S. surface station network” which produce a warming bias to station records.
Among his findings:
1) Essentially all of the +0.20 deg. C/decade average warming trend over the U.S. in the last 40 years computed from the CRUTem3 dataset (which the IPCC relies upon for its official global warming pronouncements) evaporates after population adjustment (no claim is made for countries other than the U.S.)
2) Even without any adjustments, the ISH data have a 20% lower warming trend than the CRUTem3 data, a curious result since the CRUTem3 dataset is supposedly adjusted for urban heat island effects.
Comment: These analyses may explain, in part, why the official surface temperature record does not agree with the record from satellites. It also implies that recent warming is not nearly as dire as some would have us believe, nor does the real temperature reflect any major influence by carbon dioxide emissions.
NOAA is not the only agency “adjusting” the temperature record. NASA’s GISS has been doing that for U.S. and global temperatures also (for instance see here.)
All the adjustments tend to make older temperatures, for example, those in the 1930s, colder while making all more recent temperatures warmer. This is statistically improbable if the adjustments were to be justified by some physical conditions. This whole process is troubling because it suggests corruption within the government-funded scientific establishment whose results apparently have to be politically correct rather than scientifically correct.