More than a decade has passed since scientists and environmentalists began warning that industrial emissions into the Earth’s atmosphere could raise the planet’s temperatures to blistering levels, turn agricultural land into desert and even melt the polar ice caps. So far, there is little sign of any of that happening. Scientists have detected only small temperature increases over the past several decades—and some regions, including parts of eastern Canada, actually seem to have become cooler. Does that mean that global warning was a false alarm? No, say climatologists, but it may mean that something else is happening to at least partially offset the so-called greenhouse effect, the heat-trapping buildup of carbon dioxide (CO2) emissions in the atmosphere. According to some scientists, another atmospheric pollutant—sulphur dioxide—may be creating sulphate aerosols that alter the chemistry of clouds. The result: larger areas of the Earth are cast into shadow, more sunlight is bounced back into space—and the Earth grows cooler.
Increased cloud formation is not likely to cancel out the warming influence of the greenhouse effect, but it may be reducing it by as much as one-third. The aerosol theory could also help to explain why global temperature patterns are so unpredictable, with some regions of Asia and the north Pacific becoming warmer while parts of western Eu-
rope and eastern North America may be cooling slightly. According to Environment Canada, Alberta and Saskatchewan warmed by about Io C during the 1980s. But central Canada’s weather moderated by only about a quarter of a degree—and parts of northern Quebec and Labrador cooled by the same amount. What the unexpected weather patterns may show, says Henry Hengeveld, a Toronto-based senior adviser on climate change at Environment Canada, is “just how complex the unfolding of global warming will be. Every time we think we understand what is going on, something else comes along to change all our calculations.”
The aerosol theory developed after scientists noticed that records for the past 150 years show an average global temperature increase of about .5° C—significantly less than anticipated if the buildup of CO2 in the atmosphere is wrapping Earth in a cocoon of heated air. About five years ago, scientists suggested that sulphur dioxide, mainly from the burning of coal in power plants and factories, might be making the difference. Normally, cloud droplets are formed around particles of sea salt and other specks of naturally occurring material in the atmosphere. Now, thousands of tons of manmade sulphur dioxide, injected into the atmosphere every year, are forming tiny sulphate particles that provide a perfect nucleus for the formation of billions of extra water droplets, which then cluster into clouds.
There is growing evidence to support that
theory. A survey by scientists at the U.S. National Climatic Data Center in Asheville, N.C., last year showed that cloud cover over much of the industrialized northern hemisphere—where most sulphur dioxide is produced—has increased during the 20th century. In the United States and parts of southern Canada, cloud formation has increased by as much as 10 per cent since the mid-1950s.
In the meantime, scientists at Environment Canada, after examining data gathered by aircraft that fly into clouds to study their composition, concluded that sulphate concentrations produce clouds with unusually small and numerous water droplets. According to George Isaac, an Environment Canada cloud physicist, such clouds are more efficient at reflecting sunlight. As a result, says Isaac, they “scatter more sunlight back into space, which has a cooling effect” on Earth. A computer model at the U.S. National Center for Atmospheric Research in Boulder, Col., appears to support that contention. Since 1850, says Guy Brasseur, a senior scientist at the centre, the greenhouse effect has probably warmed the planet by an average of 2.5 watts per square metre—enough energy to heat the planet by about Io C. But the effect of cloud-generating sulphates in the atmosphere has reduced that number to about 1.5 watts per square metre. “So there is still warming,” says Brasseur, “but it is not as large globally as it would have been.”
Future weather patterns may reflect efforts by western nations to reduce CO2 and sulphur emissions. But experts predict that the fast-growing economies of China and India will generate new volumes of CO2 as well as sulphates. Those emissions will probably boost the greenhouse effect—while creating over the Asian giants the same kind of cooling cloud cover that now hovers above western industrialized nations.
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