A new study presented to the American Geophysical Union (AGU) documents that the concentration of concrete, large buildings, and other human activities artificially raises urban temperatures in such cities as Atlanta and Houston by an average of 10 degrees on hot summer days. The study supports a wide body of evidence suggesting ground-based temperature readings do not provide reliable evidence of significant global warming.
On December 17, just one week after the AGU study was released, the New York Times nevertheless reported 2003 was the third warmest year on record, and that the three hottest years on record all have occurred during the past six years. The misleading Times story, based on temperature readings taken at ground-based weather stations, conflicts with satellite measurements of the Earth’s lower atmosphere, which show very little, if any, warming since measurements began in 1979.
Recent studies on urban heat island effects appear to reconcile the ground-based measurements with the satellite data.
For example, in a recent article in Remote Sensing of the Environment, a research journal for environmental scientists, Rice University Department of Physics and Astronomy researcher David Streutker analyzed two sets of infrared temperature measurements for the city of Houston, Texas.
By comparing ground-based and satellite temperature readings, Streutker demonstrated that over the course of 12 years, between 1987 and 1999, the Houston urban heat island effect increased nearly a full degree Celsius. Urban population growth, rather than any external warming, explained the rise in temperatures in and around Houston.
Similarly, a study recently published in Australian Meteorological Magazine documented that the urban heat island effect artificially raises temperature readings in towns as small as 1,000 people.
According to the Center for the Study of Carbon Dioxide and Global Change, “Changes in population, which have generally been positive nearly everywhere in the world over this period, could easily explain” why ground-based temperature readings, usually taken in and near cities, show an apparent warming trend that is not substantiated by other data.
The new AGU study bolsters the evidence that surface-based temperature readings reflect localized human population growth rather than any significant increase in global temperatures. “The majority of evidence is pointing to some sort of urban modification,” said Daniel Rosenfield of Hebrew University.
To affect surface temperature readings, “How big does a city need to be?” pondered NASA research meteorologist Marshall Shepherd. “The answer is still out there.”
James M. Taylor is managing editor of Environment & Climate News. His email address is [email protected].