Claim: "Aerosols Are Not Even Considered in Climate Models"
Why this Claim is Wrong: This was another stunner when I first heard it in Fall '09, from an instructor who should know better - note again the direct quotation marks. Of course aerosols are considered in

climate models, and have been for many years. This graph at left is from Hansen et al. (2007), which shows climate forcings due to the sun, snow coverage, land use, water vapor, ozone, well-mixed GHG's (methane, H2O, CO2, NO2...) and several different aerosol types, all using the Goddard Institute for Space Science's Climate Model E. Note that most aerosols act to cool climate by reflecting some sunlight back out into space before it can reach the ground. Only greenhouse gases have a strong upward and accelerating positive temperature forcing, and for fully 70 years now greenhouse gases have dominated all other effects except temporarily during volcanic eruptions, in line with observed temperatures.

Aerosols have both a direct climate effect, in absorbing and reflecting visible and infrared radiation, and an indirect effect in seeding cloud formation. Generally, more aerosols (including human-caused sulfates) result in more cloud condensation nuclei and more but smaller cloud droplets. These are better reflectors and less likely to rain out, and thus they tend to reflect sunlight from hitting the surface. We certainly want to understand the detailed climate effects of aerosols better than we now do, as currently we need a mixture of basic physics and fitting to observed data to parameterize the behaviors under different relevant conditions. But note that even with the unusually frequent volcanic stratospheric aerosol injections since 1960 (including Mt. Agung, Mt. St. Helens, and Mt. Pinatubo) - volcanic eruptions being powerful net cooling forcings - yet this is precisely the time when global temperatures have actually accelerated upward. While the aerosol forcing curves are likely to move a bit as we gain better understanding and ability to model, even if we're off by 100% on each aerosol curve, it won't come close to changing the net result and net conclusion - aerosols are not the reason for global warming. Man-made greenhouse gases are. The contribution of aerosols to climate is net cooling. While the uncertainty bars are indeed large, but they do not change the conclusion that anthropogenic GHG's dominate climate change, and this is expected to become more so as cleaner technologies reduce man-made aerosol pollution.

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