O.C. warming

Researchers say loss of orange groves, boom in population have created an environment that retains heat.

By Gary Robbins

May 11, 2004

You know that our freeways have become ever more crowded. And that houses and condos have popped up virtually everywhere. And that open space has become scarce.

But were you aware that a byproduct of these changes means our weather has slowly and subtly become significantly warmer?

The average nighttime low temperature in Santa Ana increased by 7 degrees from 1949 to 2002 and will continue to rise, according to a new analysis by the Jet Propulsion Laboratory in Pasadena.

The warming is the result of an extraordinary boom that saw the county's population soar from 216,000 to almost 3 million people during that same period. In the process, much of the region was covered by concrete, asphalt, lawns and golf courses, all of which absorb and retain heat to varying degrees, researchers say.

The temperature also rose because tens of thousands of trees - many of them orange trees, the citrus for which our county is named - were cut down to accommodate the growth. Trees are mini-cooling systems, generating water vapor that's invisible to the naked eye.

"In the past 50 years, Orange County has had an extreme makeover, and the result has been a big heat up," says William Patzert, the JPL climatologist who did the climate analysis.

"And the temperature will continue to rise because Southern California's population is going to triple over the next half-century."

Patzert's study shows that Santa Ana's average nighttime low temperature- computed on an annual basis-is now 53.3 degrees, or 7 degrees higher than it was in 1949, back when the county had 68,000 acres of orange groves. (We now have about 116 acres.)

The city's daytime high has risen about 1 degree, to 75.3 degrees, over the same half century or so, based on readings taken at fire station No. 5 on Walnut Street.

This isn't unique to Santa Ana or Orange County. Scientists have long known that such cities as Tokyo, Mexico City and Los Angeles are "urban heat islands" whose concrete and asphalt absorb the warmth of the sun and hold much of it through the night. In more humid areas, such as Houston and Atlanta, the phenomenon causes warm, unstable air to rise and appears to contribute to the formation of some storms, says the National Aeronautics and Space Administration.

Scientists also suspect that the urban heat island effect also has contributed to the deadly heat waves that have afflicted Chicago and other cities. The problem is so serious that researchers are looking for ways to make buildings and other structures better reflect the sun so that the heat radiates from the Earth's surface.

Such environmental engineering hasn't been a major issue in Orange County, partly because much of the region benefits from onshore winds during summer afternoons. And the nighttime temperatures are rarely oppressive.

"If the temperature isn't rising by a lot, it probably isn't going to have a whole lot of effect on humans," said F. Sherwood Rowland, the University of California, Irvine, researcher who won a Nobel Prize for his work in atmospheric chemistry. "But it does mean that there's more demand for cooling."

Which, ironically, could contribute, to an unknown degree, to warmer temperatures. As Patzert notes, "People turn on the air conditioners, which makes their homes and cars cool, but the conditioners produce heat."

But Patzert does see a danger.

"We've been in a drought for the past five years that most people haven't noticed because we import most of our water. But the environment notices the change. The warming helps make the landscape drier. And that increases the fire danger."