are the meteorological results of these forcings, including a large number of variable factors such as temperature, wind, rainfall, the probabilistic distribution of these, and extremes of weather.

The changes in energy balance as a result of changes in composition are calculated to be a few watts per square meter; some forcings are positive and others negative. Changes of this magnitude are a small, but finite, fraction of the average total energy flux into and out of the Earth's surface, which is approximately 200 W m-2. Nonetheless, changes of a percent or two in heat flux are calculated in some climate models to produce significant changes in key meteorological parameters such as temperature and rainfall.

In order to be confident in our ability to predict future climate, we must accurately quantify these forcings, use them to predict the response of the climate system, and verify that the response is accurate through comparison to data (e.g., the historical temperature record). Whereas observing systems are now in place and adequate for quantifying greenhouse forcings, those for quantifying the forcing by anthropogenic aerosols do not exist. Further, the inclusion of aerosols and their effects in climate models is highly simplified and may contain errors. An incorrect or uncertain calculation of forcing by anthropogenic aerosols could significantly alter the understanding of climate response because of the requirement that the climate response to any given forcing should not be much different from that allowed by comparison to historical data.

Thus, a great deal is at stake in correctly and accurately quantifying all climate forcings. Without this, it will be impossible to develop any reliable predictive capacity for climate responses.

In this report, we consider just one family of forcings—those resulting from aerosols. We conclude that there is substantial evidence that these forcings are significant compared to those by greenhouse gases (GHGs), but that the current quantification of them is much more uncertain than for forcings by GHG. We further conclude that the ability to adequately predict climate change will depend on reducing these uncertainties through an integrated research program.

ATMOSPHERIC AEROSOLS

An aerosol is defined as a suspension of solid or liquid particles in a gas. Atmospheric aerosols are ubiquitous and often observable by eye as dust, smoke, and haze. Particles comprising the atmospheric aerosol range in sizes from nanometers (nm) to tens of micrometers (µm), that is, from large clusters of molecules to visible flecks of dust. Most of the smallest particles (less than about 0.1 µm) are produced by condensation, either from reactive gases in the atmosphere (e.g., sulfur dioxide) or in high-temperature



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