Greenhouse Gases: Aerosols

    Fuel combustion, and to a lesser extent agricultural and industrial processes, produce not only gases but also tiny solid and liquid particles called aerosols that remain suspended in the atmosphere. Although aerosols are not considered greenhouse gases, they do affect global warming in several ways.

   Diesel engines and some types of biomass burning produce black aerosols such as soot, which absorb the Sun’s energy and therefore contribute to warming. Conversely, coal-fired power plants burning high-sulfur coal emit sulfate aerosols, which are light-colored aerosols that reflect incoming solar energy back to space. In this way, they have a cooling effect. Natural aerosols that also have a cooling effect are produced during volcanic eruptions and the evaporation of seawater. Aerosol particles also have an indirect cooling influence by acting as “seeds” for the condensation of water vapor into cloud masses. In general, the amount of solar energy reflected back to space is greater on cloudy days.

    Overall, aerosols may roughly offset the net warming influence of non-carbon dioxide greenhouse gases, half through their direct cooling effect and half through their indirect cooling effect. However, considerable uncertainty in aerosol processes means that their cooling influence could be much larger or much smaller. Aerosols are one of the least-understood factors in climate change and their effects are still being debated. Scientists are more certain, however, about the net effect of all greenhouse gas and aerosol emissions, which is estimated to be roughly equal to the warming influence of carbon dioxide alone.