Aerosols reactions can impact global warming assessment, yet they are often not a significant factor in models

Some so-called greenhouse gases trap heat and boost the planet's surface temperature, which keeps the Earth habitable.  Past climatic events have shown that temperatures can move into ranges that would be harmful to current biota, including humans.  Key drivers of such changes are CO2 concentrations, methane concentrations, and (to some degree) the Earth's orbital patterns.

Though most climate simulations on which much climate change assessment depends include the direct, heat-trapping effects of atmospheric constituents, few account for how the presence of GHG's either increases or decreases atmospheric concentrations of planet-cooling aerosols.  These effects are generally understood by the scientific community, but are not well quantified.

Aerosols, particles so small that they remain suspended in the atmosphere, can come from natural sources such as volcanoes and sea spray.  However, they also form in chemical reactions involving the gases emitted by cars, trucks, and facility smokestacks.  Light-colored aerosols, such as sulfate droplets, scatter sunlight and reflect some of it back into space, cooling Earth's surface just as natural clouds do.  Other particulates, dark in color, may contribute to atmospheric warming.

Recently, researchers modified a NASA climate model to consider chemical reactions among major atmospheric constituents and the resulting effect on aerosol concentration.  Their report reveals that some greenhouse gases have a substantially stronger warming effect than previously recognized because they take part in reactions that destroy aerosols, while others actually tend to boost concentrations of cooling aerosols.  Thus, an accurate climate model requires assessment of the impact on aerosol concentration due to the presence of other constituents in the atmosphere.

Analyses using the revised model suggest that the aerosol-stifling power of methane and carbon monoxide considerably boosts the planet-warming effect of these gases.  Previous studies have shown that a kilogram of methane, over the course of a century, warms Earth about 25 times more effectively than a kilogram of carbon dioxide does.  But add in methane's hydroxyl-consuming effect, and its planet-warming potential jumps to 28 times that of CO2.

Similarly, carbon monoxide's greenhouse warming potential rises from 2.2 times to 3.3 times that of CO2 when its hydroxyl-consuming effect is considered.  If the inhibiting influence of these two gases on the formation of planet-cooling clouds is also incorporated into the model, their greenhouse effect increases even further.

The revised model also suggests that various nitrogen oxides produced by fossil-fuel burning tend to increase aerosol concentrations through a complex series of reactions, thereby tripling their cooling power.  NOx emissions are increasing in developing countries, but still play a relatively minor role in the overall global greenhouse effect.

What is the impact on policy of this assessment?  Because many greenhouse gases also trim aerosol concentrations, it will be necessary to cut emissions even further to keep Earth's average temperature from increasing 2 degrees Celsius above pre-Industrial Revolution levels.

Thus, policymakers not only have to think about greenhouse gases, but about pollution too.  This suggests that recent efforts to prevent EPA from regulating GHG emissions, separate and apart from regulating air pollution, makes no sense.  One has to control both not only for the sake of human health, but also to control climate change.

The report on the revised model can be found at http://www.sciencemag.org/cgi/content/short/326/5953/672.