Fighting gas with gas

For the moment, let's put aside the debate between global warming skeptics and true believers. Let's assume the earth is getting hotter, and man-made greenhouse gases are the reason.

But what then? It is one thing to admit the reality of global warming. It is another to take the economically painful steps required to turn down the planet's thermostat. The U.S. Energy Information Administration predicts North American carbon dioxide emissions will be 34% higher in 2010 than in 1990, while the 1997 Kyoto Protocol demands that they be 7% lower than 1990 levels. Canadian analysts estimate that reaching our Kyoto targets could lop $40-billion off Canada's gross domestic product. In the United States, the cost could be as high as half a trillion dollars. Politicians are loath to accept such a cost. They would likely be thrown out of office. No one knows how global warming will affect Western nations, and few voters are willing to impoverish themselves for the sake of conjecture.

Proponents of the 1997 Kyoto Protocol argue that reducing greenhouse gas emissions is the only way to stop global warming. But scan the recent report from the Intergovernmental Panel on Climate Change's first working group, and another, far cheaper, mechanism suggests itself. Though there is evidence that the emission of greenhouse gases, such as carbon dioxide and methane, cause the atmosphere to heat up, there is also evidence that another class of pollutants called "aerosols" -- microscopic airborne particles or droplets -- produce the opposite effect. Like aerosols produced by volcanic eruptions, certain man-made aerosol emissions create a cooling effect because they reflect incoming solar radiation.

The IPCC estimates the magnitude of this cooling effect at about a fifth of the warming effect produced by man-made greenhouse gases. But this estimate includes only the "direct" solar-absorption effects of cooling aerosols. It does not include any measure of what is known as the "indirect" effects, which might be much greater. Microscopic airborne particles act as cluster points for the condensation of tiny water droplets. The presence of aerosols in the atmosphere helps produce more cloud droplets and, as a result, more opaque cloud structures that repel a greater share of incoming solar radiation. No one knows how large the indirect effect is. But IPCC scientists believe it may be pronounced enough to cancel out most of the warming effect of man-made greenhouse gases.

Of course, aerosols cause their own problems. Some carbon-based aerosols -- "black carbon" of the type spewed out by buses -- actually contribute to warming. And most man-made cooling aerosols originate with sulfur dioxide, which is a harmful pollutant in its own right and a cause of acid rain. Moreover, aerosol particles typically remain in the atmosphere for only short periods of time (as compared with carbon dioxide, which has an atmospheric lifespan of about a century) and its cooling effects are typically confined to the geographical area near the source of emission. Plus, roughly half the sulfates in the atmosphere are from power stations and factories, which also happen to be a major source of carbon dioxide. This means it is difficult to decouple production of warming pollutants from production of their cooling cousins.

But recent scientific research in the field of cloud formation suggests it may one day be possible to produce more reflective clouds without ramping up production of sulfur dioxide. In the June 15 issue of Science magazine, a group of researchers report that the trace presence of certain gases such as nitric acid and ammonia can cause the formation of smaller water droplets, and conclude that "a minute amount of soluble gas can exert a profound effect on the number of [liquid] droplets. ... [C]louds or fogs ... may exist even though the droplets have not undergone traditional [formation] and even though the ... humidity never exceeds 100%. Such 'pollution clouds' have a higher droplet ... concentration ... than 'clean clouds.'"

In other words, if man-made greenhouse gases are the disease, then man-made aerosols may one day be the cure.

The science of seeding clouds with sulfate aerosols and trace gases is in its infancy. But it may one day be possible to stabilize the world's warming trend by adjusting the mix of chemicals released into the atmosphere. The logistical problems associated with such a plan would be immense. But they would likely be far more tractable than the political problems associated with implementation of the Kyoto protocol, or any other agreement that requires Westerners to tighten their belts.