CO2 limits may initially worsen warming.

[CO.sub.2! limits may initially worsen warning

Policies designed to control fossil-fuel emissions might temporarily hasten the greenhouse warming before ultimately limiting the global temperature rise, according to calculations by climate researcher Tom Wigley. Yet that possibility should not deter efforts to control greenhouse-gas emissions, he says.

Wigley, of the University of East Anglia in Norwich, England, says the real message of his findings is that success will not come easily. "It might take decades for even a strong policy to produce some noticeable response," he says.

Wigley's calculations spotlight a highly uncertain arena in climate-change scenarios: the influence of sulfur dioxide (SN: 8/25/90, p.118). Like carbon dioxide, sulfur dioxide is produced by the combustion of fossil fuels. But while carbon dioxide gas traps heat, sulfur dioxide turns into tiny sulfate droplets that reflect sunlight back toward space. These sulfate "aerosols" also cool the Earth's surface indirectly by making clouds more reflective.

Scientists don't know the strength of such cooling effects, especially the effect on clouds. But if sulfate aerosols have an important influence, policies that limit fossil-fuel use would exert two opposing forces on the climate by reducing emissions of both the warning gas and the cooling gas.

To investigate the outcome of that tug-of-war, Wigley calculated how various pollution controls would affect the carbon dioxide "forcing" and the sulfate aerosol "forcing." His study, detailed in the Feb. 7 NATURE, represents the first attempt to quantify the impact of both direct and indirect aerosol effects.

Because carbon forcing appears to dominate aerosol forcing, a policy that cuts emissions would eventually limit a temperature rise. But Wigley found that the aerosol effect would delay the climate's response to any emissions control strategy and would reduce the overall effectiveness of such policies.

Since the cooling power of sulfate aerosols remains unknown, Wigley tested a range of cases. In a scenario where aerosols exerted considerable effect, fossil-fuel limitations enhanced greenhouse warming for more than three decades before beginning to slow the temperature rise. That's because carbon dioxide stays in the atmosphere for more than 100 years, while aerosols fall out within days. Thus, controls would rapidly reduce the aerosol cooling, and only later begin to curb the carbon dioxide warming, he found.

Although the sulfate aerosol effect might appear to represent an ameliorating force, "it cannot be considered to be a good thing," Wigley maintains.

Because industrial centers in the Northern Hemisphere produce the most sulfur pollution, the aerosol effect could throw the world's climate off balance by cooling the north more than the south. Although this might limit an increase in average global temperatures, the hemispheric imbalance could significantly alter weather patterns around the world, possibly producing a situation "as severe as what we might be heading for with the plain greenhouse effect," says Wigley.

Atmospheric scientist Robert J. Charlson agrees. "It would be a fundamental mistake to think that the aerosols in any way balance the greenhouse forcing," he says.

Charlson, of the University of Washington in Seattle, views aerosol's influence on clouds as a priority for future climate research. Investigators must study not only pollution-generated aerosols but also natural ones, he says.

Wigley adds, however, that unanswered questions about aerosols should not hold up negotiations on an international climate treaty, which formally began in Chantilly, Va., last week. Rather, he says, "the possible effects of fossil-fuel-derived sulfate aerosols should be seen as further reason for implementing controls on fossil-fuel use."