Claim: Cities have grown during the 20th century and their asphalt, concrete and buildings absorb much more heat than vegetated countryside. This biases temperature measurements - this "Urban Heat Island Effect" accounts for much or all of global warming, not CO2.

Why This Claim is Wrong: This effect is and has long been very well understood and continues to be factored into studies of global temperatures. The denialist claim is disingenuous in the extreme and aims itself at the naive, who don't realize how obvious and for how long this effect has been understood by climate scientists. For example, the IPCC report in 2007 finds that urban heat island effects have been determined to have negligible influence (less than 0.0006 °C per decade over land and zero over oceans) on these measurements, yet the climate denialists ignore this and continue their conspiracy theories in the press, counting on the public to NOT have read the IPCC report. Here's an easily digested video summary of the story. There isn't, and never was, a "urban heat island" issue with the data. Here's the latest graph showing global temperatures using all data, and also using only rural weather stations. For the entire 160 year period, there is no difference (with the exception of the Civil War time - questionable data-taking in the heat of battle?) between the temperature set with, or without, the urban weather stations.

Update 2012: This is a good place to mention the work of retired physics Professor Richard Muller, formerly of UC Berkeley. Muller was an astronomer, but under the funding of the Koch brothers oil money, he decided to put himself into the climate fray. He clearly enjoys being a contrarian in general (I've heard him speak at UCSC). He had been a critic of the climate science conclusions, although he admits he had not actually studied climate science. This shoot-from-the-hip approach can make you an endearing maverick... or it can make you look like a fool. After too much of the latter, he decided to pull together some bright people and re-analyze available historical temperature data and assess the case for biases, including the urban heat island effect - the "BEST" project, and read it here as well. His results on the global temperature trend is closely confirming the work of actual climate scientists, to his surprise, and to the outrage of high profile blogger climate denialist and Fox News TV weatherman Anthony Watts (wikipedia's entry). While this can be considered a good step in the right direction, Muller's motives are under question, as he continues to misrepresent climate scientists and climate science in public and advances the agenda of the fossil fuel Koch Industries who fund him, as detailed by climatologist Michael Mann here. Peter Sinclair put together this video on the Richard Muller story. I'm glad to see Muller finally siding with the long standing evidence, as he's had a checkered past in his fairness to climate scientists and their work (see here, and an updated video here (Muller begins 10 minutes in).

Update 2013: New temperature data over the past century which does not use thermometers but natural systems, re-confirms the steep (in geologic time scale terms) rise in temperature seen in weather station data (Anderson 2013 in Geophysical Research Letters).

In Short: This is and was a well-understood issue which was properly calibrated in the original work. Even if one does not calibrate out the suspect weather stations, numerous studies verify that there is, and was, no "urban heat island" effect in the global temperature data. Remember, even if a weather station is in a hotter urban setting, it is the TREND in temperatures over time which is the signature of global warming.


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