Albedo change by anthropogenic aerosols: Where uncertainty does and does not matter

Modern climate-change theory holds that the relatively stable climate of the Holocene is being altered (or "forced") by human activities. The two primary forcings are heat-trapping by anthropogenic greenhouse gases (GHGs) and albedo increase by anthropogenic aerosols. These have opposing effects on global-mean surface temperature. Thus, accurate quantification of both forcings would seem to be required in order to interpret the causes of temperature change to date and, in turn, assess the validity of climate models and of global warming forecasts. Unfortunately, while human-induced heat-trapping is known to within about 10 percent, human- induced albedo change can only be roughly estimated. As stated in the latest Intergovernmental Panel on Climate Change (IPCC) report, current uncertainties in aerosol forcing lead to a factor-of-four uncertainty in total anthropogenic forcing. However, because GHGs are accumulating while aerosols are not, the relative uncertainty in projected anthropogenic forcing is far less. For example, we will show that IPCC's A1F1 emission scenario (population stability by mid-century with continued economic growth, improvements in efficiency, and reliance on fossil fuels) results in an anthropogenic forcing by 2100 of 9 W/m2 with an uncertainty of only about 20 percent. In short, aerosol forcing uncertainty degrades our abilities to interpret 20th century warming, test climate models, and forecast future warming, but it does not significantly degrade our ability to forecast future forcing. Given this, we suggest that forcing projections be used directly as a gauge of human impact - for example, by comparison to known natural forcings. A forcing of 9 W/m2 is at the upper end of the estimated forcing associated with the previous glacial-to-interglacial transition.