Pollution is Shifting Rain Patterns in Sierra and Worldwide

Reprinted from the San Francisco Chronicle

By Don Thompson

June 10, 2004

As he flew high above the snowy Sierra Nevada this spring, atmospheric scientist Veerabhadran Ramanathan saw confirmation of what he'd both hoped and feared to see: Big, dark storm clouds that weren't producing any rain. Air pollution appears to be altering rainfall patterns in the Sierra and around the world, said Ramanathan of the Scripps Institution of Oceanography in La Jolla.

It's the latest fallout from an exploding human population that over the last half-century has pushed untold tons of smog, soot and ash into the atmosphere, he said Thursday at the nation's first state-sponsored global warming research program.  

The vast sprawl of Los Angeles, with its millions of cars pumping greenhouse gases into the air, are in effect driving away the very rainfall its population needs to survive as pollution forces rain to fall outside the state. Instead of accumulating as snow in California's Sierra, a smaller amount lands in neighboring Nevada.

Ramanathan co-led a 1999 study that reported the existence of a vast "brown cloud" of pollution, dust and chemicals that he believes is slowing solar evaporation from the oceans and leading to a net reduction in world rainfall. It's part of a band of pollution encircling the globe, helping produce a 10-20 percent reduction in the amount of sunlight reaching the earth over the last 50 years. The phenomenon cools the earth's surface, but heats the middle atmosphere.

Minuscule flecks of black carbon make up perhaps only 10 percent of the pollution cloud, but play a dominant role in altering the way the atmosphere behaves, Ramanathan said. The dark particles absorb solar radiation and scatter sunlight, helping produce that characteristic haze that today coats not only cities like Los Angeles but once-pristine areas like Yosemite National Park downwind. They also form the nucleus that attracts cloud moisture into water droplets. Clouds are getting thicker and darker because they retain more moisture, adding to the darkening effect on the earth below.

When enough moisture accumulates around natural dust particles -- clouds of which have been circling the globe for eons -- the droplets fall as rain. But Ramanathan said the carbon specks are often too small to produce drops big enough to hit the ground.

He was among scientists reporting the first results from the California Energy Commission's Climate Change Center. Researchers affiliated with the center are only beginning to develop computer models that can predict trends down to the regional level, a scale small enough to help state policy-makers.

So far, their projections don't provide much good news. Levels of carbon dioxide will double from historical levels by mid-century, pushing up temperatures across the state but particularly inland. The greatest increase will be at the highest altitudes -- the mountains that hold the snow pack containing more than a third of California's drinking and irrigation water.

Warmer water spilling into the ocean and more intense wind-driven waves pounding the Northern California coast could alter the nutrient-rich coastal waters and affect the area's sea life, said Lisa C. Sloan of the Climate Change and Impacts Laboratory at the University of California, Santa Cruz. Precipitation is likely to increase in the northern third of the state, her models show, transforming grasslands to scrubland and oak woodlands to conifer forests. But rainfall is likely to decrease in Southern California, where it's needed most, Sloan said: "This is where the population is, and this is where the giant sucking sound for the water is."  

The number of annual heat waves -- three consecutive days of high temperatures -- doubled for Los Angeles and quadrupled for San Francisco under her models, with accompanying health problems from heat, disease, ozone and asthma. An even more pessimistic model by Norm Miller of the Lawrence Berkeley National Laboratory predicts Los Angeles heat waves could increase three- to six-fold, and double in Sacramento. His modeling shows an even greater loss in crucial Sierra snow.  

All this comes as California's population is predicted to keep growing, adding to the demand for increasingly scarce water. Jay Lund, an environmental engineer at the University of California, Davis, went so far as to compare California to the mythical Tantalus, doomed to a thirsty eternity in which rising water threatened to drown him, only to recede out of reach each time he stooped to drink. His computer model, though still rudimentary, incorporates more than a million research variables and takes seven days to run a given set of projections.

He concluded that adapting to climate change will cost California billions of dollars, cause severe dislocations particularly for agriculture and the state's water resources, but nonetheless is affordable for a state with a gross annual state product well over $1 trillion.

"It's not going to cause the collapse of civilization in California," Lund said. "This adaptation, however, will be very challenging."