Over the past several decades, industrial countries have made major strides in cleaning up pollutants roiling from smokestacks.
But some researchers now say this progress could have a troubling side effect - accelerating the pace of global warming.
The reason: Tiny pollutant particles, once airborne, can reflect sunlight back into space, easing temperatures in what is known as aerosol cooling. By cleaning up industrial pollution, countries are reducing the effect of this cooling.
Nobody is recommending that nations halt efforts to curb pollution.
Still, when this factor is taken into account, global warming could outpace the level now forecast by climatologists, a team of European climate scientists reports in Thursday's edition of the journal Nature. Already, climate estimates sponsored by the United Nations foresee average temperatures rising by about 10 degrees Fahrenheit by 2100.
The study doesn't represent a formal forecast, but the team says it highlights the importance of pinning down the impact of twin changes in the atmosphere: As aerosol pollutants play an increasingly small role in affecting temperatures, the effect of changes in greenhouse-gas levels will play an even more significant role.
"Scientists must pay more attention to this issue as they do more complex modeling, and the public needs to be aware of the implications," says Meinrat Andreae, who led the research.
The team made calculations based on a simplified set of equations describing how the climate system works. Indeed, other researchers liken the effort to a somewhat sophisticated back-of-the-envelope calculation scientists often perform to see if a phenomenon is worth a second look.
The results, published a week before the G-8 summit in Scotland, are likely to add urgency to British Prime Minister Tony Blair's efforts to keep climate change a headline issue.
But Mr. Blair has been unable to convince President Bush to support mandatory targets and timetables for emissions reductions. The White House's preferred approach instead focuses on reducing the US economy's carbon intensity - the amount of carbon emitted per unit of economic output - by 18 percent over 10 years. It relies heavily on research and market forces to encourage adoption of technologies to achieve its goals.
The European research team's call for more research also comes as the United Nations sponsored Intergovernmental Panel on Climate Change (IPCC) is in the midst of preparing its next set of climate-change reports, set for release in 2007. Researchers had to submit results to journals by mid-May - deadlines that suggest a flurry of new studies relating to the global climate will come out over the next year.
Researchers are aiming to narrow uncertainties linked to climate sensitivity and man-made aerosols, because sensitivity determines how much warming could be expected as concentrations of carbon dioxide and other heat trapping "greenhouse" gases in the atmosphere grow. Even from the effects of what humans already have pumped skyward, many researchers believe the atmosphere's temperature will increase by 2 degrees by 2100. That number would rise with continued greenhouse emissions.
If the impact of aerosol cooling is small and the climate is less sensitive to CO2 increases than current estimates hold, warming might fall toward the low end of projections by the IPCC's science working group. If cooling has been large and the climate is very sensitive, warming could exceed their projections.
Uncertainties about aerosol cooling and climate sensitivity to greenhouse gasses remain large. But as new studies have emerged, the importance of these two factors has appeared to grow, says Dr. Andreae, who heads the biogeochemistry department at the Max Planck Institute for Chemistry in Mainz, Germany. Aerosol cooling may have been greater in the past than we believed, and the climate may be more sensitive to greenhouse-gas accumulation.
While the European team's broad description of cooling and sensitivity is the latest warning, the devil is in the details, other climate scientists say.
For example, the team's calculations appear to be based in part on the assumption that all of the warming since the 1850s is due to human activities. The IPCC, in contrast, has been willing to attribute "most" of the warming over the past 40 years. The difference seems small, only a few tenths of a degree. But it leads to a lower climate sensitivity than Andreae's team calculates, notes Theodore Anderson, a climate researcher at the University of Washington in Seattle.
Moreover, ice core data from the last glacial maximum some 20,000 years ago place limits on the climate's sensitivity, notes Gavin Schmidt, a climate researcher at NASA's Goddard Institute for Space Studies in New York. This leads to a climate system less responsive to changing CO2 levels than the European team's model implies - and in line with estimates that underlie current climate-change projections.
In addition, all pollutants are not created equal, both researchers add. Sulfur-based aerosols from burning fossil fuels tend to reflect sunlight back into space, acting as an atmospheric coolant. But other aerosols, such as black-carbon soot, warm the atmosphere. The relative contribution of each is highly uncertain. But their offsetting effects could lead to weaker aerosol cooling than Andreae's team suggests. Moreover, aerosols have indirect but important effects on clouds that the model fails to capture.
If nothing else, the effort highlights the importance of a set of satellite missions aimed at solving the cloud-aerosol problems. Two satellites are set for launch later this year that will round out a suite of five satellites dubbed "the A Train." The satellites orbit Earth in a line so their unique instruments can cover the same swaths of the atmosphere. The satellites fill wide gaps in measurements of aerosols, clouds, their movements, and their interactions with each other and with climate, helping to zero in on the climate's true sensitivity to a doubling of CO2 concentrations.