By P Gosselin on 14. December 2010

We know that more and more temperature stations have been sited in urban areas over the last few decades, and surely this has had an impact on the measurement of the globe’s temperature. But not to worry, say scientists, the raw data have been homogenized in order to compensate for this.

Elevated temperatures in urban environments. (Photo: NASA)

Have they compensated enough? Just how much uncertainty is in the land-based temperature record for the last 50 years? A new report from NASA shows some results that I think are surprising. 

Summer land surface temperature of cities in the Northeast were an average of 7C to 9C (13F to 16F) warmer than surrounding rural areas over a three year period, the new research shows. The complex phenomenon that drives up temperatures is called the urban heat island effect.

Dr. Marc Imhoff, leader of the study, says that urban areas located in forested regions have a much more pronounced urban heating effect than in desert areas. Adjustment is neither an easy nor accurate task (emphasis added):

However, accurate comparisons have long eluded scientists because ground-based air temperature sensors tend to be unevenly distributed and prone to local bias. The lack of quantifiable definitions for urban versus non-urban areas has also hindered comparisons

But thanks to satellites, progress is being made in understanding the urban heat island effect. Satellite data have been used to compare various urban settings. Ping Zhang, a scientist at Goddard and a lead author of the research:

“This, at least to our knowledge, is the first time that anybody has systematically compared the heat islands of a large number of cities at continental and global scales,”

Research by NASA scientists shows that compact urban areas have a higher urbean heat island effect that sprawling cities, and depend greatly on the surrounding environment, as mentioned above.

Taking the UHI effect has been the subject of much controversy over the years. This new study shows that it may have been signficnatly understated, at least in some areas.

Providence, R.I.; Washington, D.C.; Philadelphia, Pa.; Baltimore, Md.; Boston, Ma.; and Pittsburgh, Pa.; had some of the strongest heat islands of the 42 northeastern cities analyzed.

These 3 examples have all trended upwards over the last 30 or 40 years. The actual siting and history of each station would have to be examined individually. So one can imagine what a mess this is and how arbitrary all this could be. Here we can be thankful for satellites.

The warming indeed is man-made, but in a different sense. The urban warming is what has impacted people in the summertime, especially at night.

The U.S. Environmental Protection Agency estimates that, between 1979 and 2003, heat exposure has caused more than the number of mortalities resulting from hurricanes, lightning, tornadoes, floods, and earthquakes combined.

“It is the lack of cooling at nighttime, rather than high daytime temperatures, that poses a health risk,” said Benedicte Dousset, a scientist from the University of Hawaii who also presented data about heat islands at the AGU meeting.

Urban planners will have to take this into account when designing cities in the future. Trees, trees and more trees can help a lot. Meanwhile, all this leaves James Hansen with a pile of almost useless data. See full post and images here. H/T Marc Morano

Icecap Note: See this Icecap post under About Climate Change on Urban Heat Island Contamination. It is also discussed in detail in the body of this SPPI special report with Joseph D’Aleo and Anthony Watts with help of many other scientists and in some of the case studies. Also SPPI has posted this report by former NASA scientist Dr. Ed Long comparing raw and modified urban and rural stations.

The study also calls into serious question, the decisions by NOAA and CRU to exclude an urban adjustment (assume it can be accounted for with only an additional 0.05C/century uncertainty).