Another Reason for Warming

More people, bigger cities:

Average temperatures across California rose slightly from 1950 to 2000, with the greatest warming coming in the state's big cities and mostly caused by urbanization -- not greenhouse gases -- authors of a study released on Wednesday said.

The study found that average temperatures in California rose nearly 2 degrees Fahrenheit (nearly one degree Celsius) in the second half of the 20th century, led by large urban centers such as San Francisco and Southern California.

"Everybody's talking about the carbon coming out of the SUV exhaust or the coal plant, but in the past 50 years in California the bigger impact has been urbanization and suburbanization," said Bill Patzert of NASA's Jet Propulsion Laboratory, one of the study's authors.

"Exhaust from the SUV is just piling on," he said. "One is heating you from above, the other is heating you from below."