An Aerosol Tour de Forcing

Guest commentary from Ron Miller and Dorothy Koch (NASA GISS)

Scientists have confidence in a result to the extent that it can be derived by different investigators. Their confidence is increased if different techniques lead to the same conclusion. Concurrence provides evidence that the conclusion does not depend upon assumptions that occasionally are insufficiently supported. In contrast, two articles published last December on the same day arrive at very different and incompatible estimates of the effect of human-made aerosols on the radiative budget of the planet (Bellouin et al., 2005; Chung et al., 2005). They follow an earlier estimate published last year, (which included Dorothy as a co-author) that was in the middle (Yu et al., 2005). Aerosols are important to climate partly because their concentration is increased by the same industrial processes that increase the atmospheric concentration of greenhouse gases; yet aerosols generally oppose greenhouse warming. Because aerosols cause respiratory and other health problems and acid rain, they have been regulated more aggressively than greenhouse gases. Concentrations of some aerosols have decreased over the United States and Europe in recent decades as a result of environmental laws, although an increase has been observed in many thrid world regions, where economic development is a priority. In the twenty-first century, aerosol levels are anticipated to drop faster than greenhouse gases in response to future emission reductions, which will leave greenhouse warming unopposed and unmoderated.

Each published calculation of aerosol radiative forcing was a tour de force for integrating a wide variety of measurements ranging from absorption of radiation by individual particles to satellite estimates of aerosol amount. The disparate results emphasize the complexity and difficulty of the calculation. But let’s start at the beginning….

Aerosols are solid particles or liquid droplets that are temporarily suspended within the atmosphere. Naturally occurring examples are sea spray or sulfate droplets, along with soil particles (dust) eroded by the wind. During the twentieth century, natural sources of sulfate aerosols were overwhelmed by the contribution from pollution, in particular from the burning of fossil fuels. The number of soot particles in the atmosphere was increased by industry and the burning of forests to clear land for agriculture. Sulfate aerosols are reflective and act to cool the planet. Soot particles are also reflective, but can absorb sunlight and cause warming. Soot production is greater if combustion occurs at low temperatures, as with cooking fires or inefficient power generation. Aerosols also scatter longwave radiation, although this is significant only for larger aerosols like soil dust, and is neglected by all three of the studies discussed here.

In addition to their ability to scatter radiation and change the net energy gain at the top of the atmosphere (the ‘direct’ effect), aerosols modify the reflectance and lifetime of clouds (the ‘indirect’ radiative effects). Aerosols act as nuclei for the condensation of water vapor, resulting in the distribution of water over a larger number of cloud droplets compared to condensation in clean air. This increases the cloud’s ability to reflect sunlight, while increasing the number of droplet collisions required to form a raindrop large enough to fall out of the cloud, effectively increasing the cloud lifetime. Observations and models provide a weaker constraint upon the size of the indirect effects, so the studies discussed here confine themselves to calculating only the direct radiative effect of anthropogenic aerosols.

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