Long-Term, L.A. Feels the Warmth

Los Angeles has heated up about 5 degrees since they started keeping records in 1878, but you wouldn’t have known it during the summer.

The summer that ended officially last Thursday was about 2 1/2 degrees cooler than what currently is considered normal, and during the years since 1998, temperatures in Los Angeles have averaged well below those in the eight previous years.

“But that’s only temporary,” said Bill Patzert, a climatologist at the Jet Propulsion Laboratory in La Canada Flintridge. “In the next 10 to 20 years, it’s going to heat up again.”

Indeed, the long-term trend in the Southland is increasingly higher temperatures. Climatologists largely blame urbanization, which traps moisture that tends to retain heat better than the dry chaparral it replaced, and global warming, which plays a lesser but important role. The temporary cooling we’re now experiencing is the result of cyclical cooling of Pacific Ocean temperatures.

Data compiled by Patzert show that average yearly temperatures have climbed from about 62 degrees in 1878 to about 67 today.

He said the steady increase has paralleled Los Angeles’ steady increase in population, from less than 10,000 in the 1870s to more than 3.7 million today.

During that period, an arid semidesert has been converted in large part to a lush greensward of faithfully irrigated gardens, parklands and landscaped business properties. Because moist ground holds heat better than dry ground, the warmth absorbed by the ground during the day is better retained at night.

“Daytime temperatures are about 3 degrees warmer than they used to be, but nighttime temperatures are about 7 degrees warmer,” Patzert said. “So the most significant thing is how much warmer the nights are.”

Although less of a factor than urbanization, the phenomenon known as global warming also has contributed to the overall temperature increase, Patzert said.

Scientists generally accept the view that the release of carbon dioxide through the large-scale burning of fossil fuels creates a “greenhouse effect” that traps warmth, leading to a gradual increase in temperatures throughout much of the world.

Like the ascent of a mountain range, the average temperature climb since 1878 has been steady, but it’s been marked by peaks and valleys.

There was a leveling off between 1890 and the early 1920s, a peak in the late ‘20s and late ‘30s, a dip during the ‘40s and early ‘50s, a peak in the late ‘50s, another dip in the ‘60s and ‘70s, a strong peak in the ‘80s and most of the ‘90s and an abrupt dip since 1997.

Patzert said these peaks and valleys appear due to a cyclical phenomenon, not yet fully understood, in the surface temperatures of the north Pacific Ocean.

This phenomenon, the Pacific Decadal Oscillation, is separate from the better-known El Nino/La Nina phenomenon, which frequently brings heavier-than-normal rain to Southern California during its El Nino phase. Both phenomena involve surface ocean temperatures, but Pacific Decadal Oscillation cycles tend to last 10 to 20 years, as opposed to El Nino/La Nina’s one to two years.

“In the ‘80s and ‘90s, due largely to the PDO, the ocean was warmer than normal,” Patzert said. “Now, it’s cooler.”

When the water is cooler, the air over it tends to be cooler too, and temperature differential between the hot inland desert air and the cool ocean air tends to be greater. Cool air is denser than warm air, so air tends to flow from cooler areas to warmer areas.

With a temperature differential often approaching 40 degrees during July, August and early September, the onshore flow of cool, damp ocean air was especially persistent this year. The same thing happened last year.

On many days, the dank air piled up against the coastal slopes above Los Angeles as clouds and fog. Midday sunshine usually burned away the overcast in downtown. But along the coast, it often stayed cloudy and cool all day.

Eric Boldt, a meteorologist with the National Weather Service in Oxnard, said he agreed with Patzert’s analysis, although he said “it’s sometimes hard to point a finger” at what caused a specific temperature to rise or fall.

Cooler periods tend to be drier than warm periods, Patzert said. Between fall 1998 and fall 2004, an average of 11.44 inches of rain fell on downtown Los Angeles, almost 4 inches below normal. The season of 2001-02, with 4.42 inches, was the driest on record.

Then, between July 1, 2004, and June 30, 2005, 37.25 inches of rain fell on the city, the most since the record of 38.18 inches was set in 1883-84.

It was an anomaly, and none of the climatologists saw it coming.

“This was the one that Mother Nature throws at you to keep you humble,” Patzert said.