 . . . March 1995
Nobody has anything good to say about smog. It burns our eyes, hurts our lungs, kills plants, acidifies lakes and spoils the view. Environmental activists and government officials have spent millions of dollars and written reams of legislation in an attempt to get rid of the stuff.
According to U-M atmospheric scientist and geologist James C. G. Walker, however, the blanket of air pollution that hangs over much of the industrialized world has a benevolent side. He says it could be protecting us from the full impact of other serious environmental threats, such as global warming and depletion of the ozone layer.
 "But by masking the effects of other environmental problems, it may be lulling us into an unmerited complacency. Environmental damage may already be worse than we think." A specialist in global change issues, Walker's laboratory is the Earth itself. He's interested in how human activities-particularly the widespread burning of fossil fuels like oil, natural gas and coal-are changing the global ecosystem in fundamental ways that scientists do not fully understand.
One of the more publicized and controversial changes is global warming. When we humans burn fossil fuels to run cars or power plants and factories, tons of carbon dioxide and other greenhouse gases rise into the atmosphere and remain there for hundreds of years. Although not as visible as the smoggy haze of air pollution, greenhouse gases are just as dangerous. Like the plastic cover on a greenhouse, they trap heat near the Earth's surface and prevent it from radiating into space. With no way to escape, the heat builds up and our planet gets warmer. If the trend continues, scientists say average temperatures could climb by 3 to 8 degrees F in the next 50 to 100 years-a level of warming unmatched since the end of the last Ice Age 10,000 years ago.  Since the beginning of reliable meteorological records about 1850, the amount of carbon dioxide in the atmosphere has climbed at a steady rate, but records show the warming effect of C02 has been erratic rather than steady, with periods of nearly constant temperatures separated by periods of rapid increase.
Walker wondered why global temperatures have not increased in a more uniform patterns and began looking for possible explanations. When he compared meteorological data and economic statistics, he noticed that "average global temperatures have increased during periods of economic recession and remained unchanged during periods of rapid economic growth.' "It occurred to me," he says, "that competition between warming greenhouse gases like carbon dioxide and cooling air pollutants like aerosols, soot, ash and smog might explain the erratic pattern in global warming. Perhaps the pollutants spewing into the atmosphere from factory smokestacks and automobile tailpipes are deflecting some incoming solar radiation and keeping the Earth cooler than it otherwise would be."
Clouds form when water droplets collect around tiny particles called cloud condensation nuclei, Walker explains. "The concentration of cloud condensation nuclei in industrial areas has increased in the 20th century as a result of industrial activity," he says. "So it's probably no coincidence that cloud cover is increasing, also." The more tiny pollutant particles in the atmosphere, the more water droplets in each cloud. This increases the tendency of clouds to reflect sunlight back into space and cool the Earth. Another important phenomenon related to competition between the warming effects of greenhouse gases and the cooling effects of industrial pollutants is how long they remain in the atmosphere. "Carbon dioxide remains in the atmosphere for hundreds or thousands of years," Walker points out, so if greenhouse gases were the only influence on global temperature, we would expect a smoothly climbing warming curve. But sources of the competing, cooling effect-air pollutants like sulfuric acid, aerosols and ash from smokestacks and tailpipes-stay in the atmosphere for just a few days or weeks. And until they're washed out of the atmosphere in precipitation, the hazy smog or clouds they produce block sunlight and cool the Earth. "During periods of rapid economic growth, when factories and power plants are expanding production, the quantity of pollutants in the air increases rapidly and produces a short-term cooling effect" Walker says. "When the economy slows down, the pollutant level drops off rapidly, leaving us exposed to the full impact of global warming again." Walker emphasizes that his idea about the relationship between global warming and the global economy is just that-an idea, not a valid scientific study. He does find it intriguing, however, to compare rates of warming with historical economic trends.
From the mid-1940s to the mid-1970s, there was little or no change in global average temperatures. "These were the years when nobody paid any attention to global warming, because the globe seemed to be cooling off-or at least not warming up," Walker says. "This also was a time of post-war economic boom, with renewed rapid growth in fossil fuel consumption and a corresponding surge in air pollution. Then came the oil crisis of the mid-1970s, escalating oil prices, the worldwide recession of the 1980s and a sharp drop in industrial productivity. "And what happened to global temperatures?" Walker asks "They just went rocketing up. Nearly all the hottest years on record were in the 1980s." If Walker is right about the relationship between global warming trends and the world's economy, what does he foresee in the world's future as it moves into a global economic recovery? "If the analysis is correct and the economy continues to pick up, then average global temperatures will not increase as fast as they have since the late 1970s," Walker says. When economic times are bad, factories shut down, reducing pollutant haze, and temperatures go through the roof. Confronted with heat waves, drought and massive hurricanes people worry about the environment again, but faced with the social and economic impact of widespread recession, have no resources left to invest in environmental problems. "We are in a race between environmental destruction and human attitudes on how to deal with it," Walker says. "It's not clear which side will win." Jim Walker has no idea how long the Earth can continue its delicate balancing act between greenhouse warming and pollutant cooling. He doesn't know how much time scientists have left to anticipate the effects global warming will have on life on Earth, or how much time policy-makers have to develop a plan to deal with it. But he does know one thing: The benefits of bad air are illusory and only temporary. Sally Pobojewski is the senior science writer for U-M News and Information Services.
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Scientists don't know why cloudiness is increasing, Walker says. "It might be some natural variability of the climate system," he speculates. "It might be a response to global warming. It might very plausibly be a response to increasing concentrations of dust, smog and haze in the atmosphere caused by industrial and agricultural activities."
From World War 1 (1914-18) until the end of World War II (1945), global temperatures increased rapidly. "This correlates with slower growth in the economy-especially with the factory closings during the Great Depression in the 1930s-and the corresponding drop in the amount of cooling pollutants spewed into the atmosphere."