The hot concrete canyons in cities around the world don't skew the records of gradual warming this century, scientists have found. A new analysis of temperature stations worldwide shows that thermometers in rural areas have recorded the same long-term warming trend as all thermometers taken together, says a report in the 1 February issue of Geophysical Research Letters.
The study tests the impact of what are called "urban heat islands." Replacing trees and fields with buildings and parking lots creates local bubbles of heat, making city and suburban areas several degrees warmer on many days than a farm outside of town. The relentless growth of cities during the 20th century has caused some climatologists to wonder whether long-term records of "global warming" might instead be artifacts of this population shift.
Previous work suggested that wasn't true, but the new study is the most thorough yet. Climatologist Thomas Peterson of the National Climatic Data Center in Asheville, North Carolina, and his colleagues examined monthly average temperature records from 7280 gauges worldwide, many extending back more than a century. They classified 2290 stations as "rural" because they fell outside the boundaries of towns with more than 10,000 people on U.S. Department of Defense maps and lay within zones where satellites report little or no light at night.
The results were "startling," says Peterson: The rural stations showed a warming trend of 0.70°C per century, compared to 0.65°C for all stations. Moreover, the sawtooth patterns of warming and cooling from year to year were nearly identical between the two data sets. "The global climate temperature signal recorded at surface stations is amazingly robust and isn't impacted by urbanization in any significant fashion," Peterson concludes.
"This confirms my view that urban influences may be important on the local scale, but they only explain a small fraction of the [global] warming," says climatologist Phil Jones of the University of East Anglia in Norwich, England. Even skeptics of the greenhouse effect are impressed. "It's a very clever piece of work," says geographer Robert Balling of Arizona State University in Tempe. But, he adds, the study doesn't prove the relation between warming and emission of "greenhouse gases." Peterson agrees: "This paper looks only at observed instrumental records and answers a very narrow question."