If a [weather station] site is initially chosen because it meets all the qualifications for observing temperature, there is little about the site that could change to develop a cool bias. Almost all the changes will result in a warming trend from the original, ideal setting.
Natural changes such as the growth of trees and shrubs, reduce the clear sky radiation, resulting in a warming trend. Man made changes, such as increased building and paving in and around the site, also results in a warming trend that is unrelated to any potential climate change. Finally, deterioration of the shelter housing the instruments also leads to an artificial warming.
Any correction of these potential warming factors simply returns the site to its initial, ideal state. The only way to get an artificial cooling is to start with a less than ideal setting for recording air temperature and improve it. While this may have happened in a few locations, it is obvious that the gradual degradation of recording sites is the norm.
The calculations of the temperature increase due to increasing CO2 are theory, which can only be verified with actual, accurate data. Those who claim that the accuracy of the data is not relevant are, in effect, defending a theory against reality, which is faith, not science.
I know that supporters of the AGW theory get very upset when they are accused of behaving in a religious fashion, instead of behaving like scientists. To avoid this, I suggest they start behaving like scientists and support the effort to obtain the best data possible.
Found here: http://www.norcalblogs.com/watts/climate_change/
Thanks.