Pollution Is Changing Clouds, Researchers Find : Environment: Study also says sunlight and heat could be reflected back into space, potentially cooling the planet.

The lumbering C-130 skims perilously close to the ocean waves, collecting data suggesting that pollution may be altering the clouds above--and ultimately Earth’s climate.

Researchers think smog is creating clouds where none existed and increasing the brightness of normal clouds. That, in turn, reflects sunlight and heat back into space, potentially cooling the planet.

“It’s unlikely that would be enough to force us into an ice age, but it’s certainly likely to cool the climate significantly,” said project scientist Phil Durkee.

“The best models we have seen indicate this could be on the same scale as what has been predicted for global warming--only in the opposite direction,” he said.

The Monterey Area ShipTrack Experiment grew out of mysterious bright white lines spotted crisscrossing the ocean cloud cover in the first satellite photos shot three decades ago.

The lines were eventually traced to ships. But smokestacks failed to produce enough visible smoke to explain the tracks, some of which stretched hundreds of miles.

Attention soon centered on invisible microscopic particles spewing from the smokestacks, which allow moisture in the atmosphere to precipitate more quickly. In effect, smog apparently seeds the air, creating clouds.

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And if narrow ship tracks had such a striking effect, the far more extensive pollution over land could potentially change vast stretches of the Earth’s cloud cover.

The MAST experiment, a joint U.S.-British effort operated out of the Naval Postgraduate School near Monterey, is testing those theories. Participants included five U.S. Navy ships, a high-flying ER-2 from the National Aeronautics and Space Administration, a Royal Air Force C-130 and a smaller C-131 from the University of Washington.

Packed with instruments ranging from exotic lasers to common wind vanes, the British C-130 zigzagged behind the unsuspecting freighter Hanjin Barcelona as it steamed toward Long Beach.

“Fly down the plume,” ordered Douglas Taylor of the Meteorological Research Flight facility in Farnsborough, England. The plane banked into the invisible trail of smokestack exhaust behind the vessel. Inside the plane’s computer center, meteorologist Jonathan Taylor peered into instruments showing particle sizes and amounts, as well as cloud reflectivity. One plume, he said, is 15 miles wide.

The ship tracks are minor contributors to global cloud pollution, but their sharp definition and location in relatively unpolluted skies at sea makes them ideal for study.

“They act as a really nice, controlled experiment--we have a nice clean cloud, then the ship makes it dirty,” said Taylor. “But the findings relate to pollution anywhere.”

The graphs jumped as the C-130 crossed the freighter’s invisible pollution plume. Cloud reflectivity soared.

The findings have a military purpose as well, participants reluctantly acknowledged. Knowing which ship engines produce which ship tracks could be useful in a naval confrontation. But most participants are interested in the broader climate implications.

Back at the operations center, they leafed through satellite photos showing the Hanjin Barcelona’s ship track off the California coast. Durkee pointed to a brighter swatch of clouds just off Monterey Bay.

“There are ships under there, but the coastal clouds are already so polluted from land sources that they don’t leave ship tracks,” he said.

The British meteorologist flipped to a photo of his homeland that showed clouds turning far brighter as they reached England’s polluted coastal cities. Los Angeles-area shots show the same, said Durkee, with power plants leaving especially long trails.

In fact, they say, the cloud cover over the entire Northern Hemisphere is brighter, or more reflective, than in the less developed Southern Hemisphere.

Exactly how the change will affect Earth, they add, is uncertain.