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Spring 2005 Vol. 5 No. 1

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©Jeff Koegel What's Driving Climate Change?

Scientists know from temperatures observed at the Earth's surface that the planet is warming. There are factors that drive this warming, as well as others that cause cooling. Carbon dioxide in the atmosphere, for example, increases temperatures by absorbing infrared radiation emitted by the Earth's surface, radiation that would otherwise escape to space, in what we know as the greenhouse effect. Small droplets and dust found in the atmosphere after large volcanic eruptions, on the other hand, reflect sunlight back to space before it ever reaches the planet, thereby having a cooling effect. In addition to pollution and volcanoes, other "forcings" of the climate include changes in land use and variation in the amount of energy received each year from the sun.

But how do scientists quantify the effect of a particular forcing on temperatures? Traditionally they have relied upon a concept known as "radiative forcing," a disturbance in the energy balance at the top of the atmosphere resulting from an external driver, such as industrial activity. For solar variability and many greenhouse gases, there is a direct relationship between top-of-the-atmosphere radiative forcing and fluctuations in surface temperature.

A new report from the Research Council says radiative forcing is an observable quantity that is relatively easy to compute. It also gives policy-makers an extremely useful tool for making decisions about climate change. In particular, it provides a way to compare the impact of different pollutants on global warming.

The concept has some limitations, however. By only diagnosing the change in global mean surface temperature, for example, the regional impact of some industrial pollutants and changes in land use, such as deforestation, may be underestimated. And it offers little information on other types of climate change, such as precipitation levels or ecological functioning.

The report says scientists should examine how forcings behave at different altitudes and in different regions of the world. Further research also is needed to quantify how forcings affect climate other than through temperature change.

In addition, the report says that more needs to be known about the role airborne particles play in the creation of clouds, which not only affect rainfall but also reflect sunlight. It is important to understand such effects because pollutants -- including some that cause cooling and others that cause warming -- are likely to decrease with tougher air-quality regulations. The report emphasizes that an expanded understanding will give governments a better idea of how pollution-control strategies impact climate.   -- Bill Kearney

Radiative Forcing of Climate Change: Expanding the Concept and Addressing Uncertainties. Committee on Radiative Forcing Effects on Climate, Climate Research Committee, Board on Atmospheric Sciences and Climate, Division on Earth and Life Studies (2005, approx. 225 pp.; ISBN 0-309-09506-9; available from the National Academies Press, tel. 1-800-624-6242; $37.00 plus $4.50 shipping for single copies).

The committee that wrote the report was chaired by Daniel J. Jacob, Gordon McKay Professor of Atmospheric Chemistry and Environmental Engineering, Harvard University, Cambridge, Mass. The study was funded by the U.S. Climate Change Science Program.

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Copyright 2005 by the National Academy of Sciences