An Aerosol Tour de Forcing

aerosol haze According to the latest (2001) IPCC report, direct radiative forcing by anthropogenic aerosols cools the planet, but the forcing magnitude is highly uncertain, with a global, annual average between -0.35 and -1.35 W/m2 at the top of the atmosphere (TOA). The uncertainty of the total indirect effect is even larger. Aerosols eventually fall out of the atmosphere or are washed out by rainfall. The smaller particles having the largest radiative effect typically reside in the atmosphere for only a few days to a few weeks. This time is too short for them to be mixed uniformly throughout the globe (unlike CO2), so there are large regional variations in aerosol radiative forcing, with the largest effects predictably downwind of industrial centers like the east coast of North America, Europe, and East Asia. Consequently, aerosol effects upon climate are larger in particular regions, where they are key to understanding twentieth century climate change.

Aerosol concentrations have been measured downwind of sources over the past few decades, but the number of observing sites is limited and the analysis is laborious. Since the late 1970’s, satellite instruments have detected aerosols routinely with nearly global coverage. However, only the combined effect of all aerosols upon radiation impinging upon the satellite was originally measured. The original instruments couldn’t distinguish between dust and sulfate aerosols where both were present, over the Mediterranean or East Asia, for example. Recent instruments, like the Moderate Resolution Imaging Spectroradiometer (MODIS) measure radiation at multiple wavelengths. This allows particle size to be distinguished with greater confidence, which can be used with some assumptions to infer the aerosol species.

Range of forcing estimatesThe new generation of satellite instruments is at the heart of recent attempts to reduce the large uncertainty of direct radiative forcing by aerosols. Each of these studies provides an estimate of the most likely value, along with a range of uncertainty. Bellouin et al. (2005) in Nature arrive at TOA forcing of -0.8 ± 0.1 W/m2. While near the center of the range published by the IPCC, this estimate is noteworthy for its comparatively small uncertainty. Yet on the same day, Chung et al. (2005) published an article in the JGR, estimating based upon similarly extensive calculations that the forcing by aerosols at TOA is -0.35 ± 0.25 W/m2. A few months earlier, Yu et al. (2005) had estimated a more conciliatory value of -0.5 ± 0.33 W/m2. The wide range of estimates give some indication the difficulty of the problem.

Forcing estimates differ not only at TOA but also at the surface: Bellouin et al. predict that aerosols reduce the net radiation incident upon the surface by 1.9 ± 0.2 W/m2 compared to 3.4 ± 0.1 W/m2 for Chung et al. (2005). That is, Chung et al. estimate much greater atmospheric absorption. Because radiation into the surface is mainly balanced by evaporation, except within extremely arid regions, the discrepancy has implications for the supply of moisture to the atmosphere. Chung et al. estimate a much larger reduction in global rainfall by aerosols.

What are the sources of disagreement and uncertainty? Ideally, one would know the three-dimensional distribution of each aerosol species and its evolution throughout the year. One would also be able to distinguish natural and human fractions of each species. For sulfate aerosols, this means distinguishing droplets created by industrial sources, compared to biogenic sources. In addition, the ability of each particle to scatter radiation would be known as a function of its age and aggregation with other species (in the way that dust can be coated with sulfates when passing over industrial areas, for example). Many of these processes are included in aerosol models, but some of the key parameters are uncertain given limited observations.

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