Global Climate Model Estimates of the Total Anthropogenic Aerosol Effect

The total anthropogenic aerosol effect, as defined here, consists of the direct effect, semi-direct effect, indirect cloud albedo effect, and cloud lifetime effect for warm clouds. The total anthropogenic aerosol effect is obtained by calculating the difference between a multiyear simulation with present-day aerosol emissions and a simulation representative for preindustrial conditions, in which anthropogenic emissions are turned off. It should be noted that the representation of the cloud lifetime effect in global climate models is essentially one of changing the autoconversion of cloud water to rainwater.

The radiative forcing that results from the indirect cloud albedo effect attributable to anthropogenic aerosols is estimated from global models as -0.7 W m-2, with a 90% confidence range of -0.3 to -1.8 W m-2 (Forster et al. 2007). Feedbacks that result from the cloud lifetime effect, semi-direct effect, or aerosol-ice cloud effects can either enhance or reduce the cloud albedo effect. Climate models estimate the total aerosol effect (direct plus indirect effects) on the TOA net radiation since preindustrial times to be -1.2 W m-2, with a range of -0.2 to -2.3 W m-2 (Figure 23.5 and Denman et al. 2007). The range of the total aerosol effect from different models cannot easily be compared to the range of the indirect cloud albedo effect alone because different model simulations entered these various compilations.

All models agree that the total aerosol effect is larger over the northern hemisphere than over the southern hemisphere (Figure 23.5), consistent with emissions of anthropogenic aerosols and precursor gases being much greater in the northern hemisphere. This effect has not been seen, however, in satellite data (Han et al. 1998; Schwartz 1988), suggesting that either dynamic influences on the liquid water path mask such an effect or that the models do not represent aerosol-cloud interactions realistically. The values of the northern hemisphere total aerosol effect vary between -0.5 and -3.6 W m-2; in the southern hemisphere they range between slightly positive to -1.1 W m-2; and the average southern/northern hemisphere ratio is 0.3. Estimates of the ocean/ land partitioning of the total aerosol effect vary from 0.03 to 1.8, with an average value of 0.7. Although the combination of ECHAM4 model results with POLDER satellite estimates suggests that the total aerosol effect should be larger over oceans, combined estimates of the LMD and ECHAM4 models with MODIS satellite data reach the opposite conclusion. The average total aerosol effect over the ocean of -1 W m-2 agrees with estimates between -1 to -1.6 W m-2 from AVHRR/POLDER (Denman et al. 2007).

Estimates of the total aerosol effect from global climate models are generally larger than those estimated from inverse approaches, which constrain the indirect aerosol effect to be between -0.1 and -1.7 W m-2 (Anderson et al. 2003; Hegerl et al. 2007). The estimated total anthropogenic aerosol effect is now lower than was stipulated in IPCC's Third Assessment Report and

Northern Hemisphere

Southern Hemisphere

Northern Hemisphere

Southern Hemisphere

Global

□ Sulfate + Black carbon (BC) B Sulfate + Organic carbon (OC)

D Sulfate + BC + OC on water and ice clouds ^ Combined GCM and satellite results

□ Mean and standard deviation of all results

Figure 23.5 Total anthropogenic aerosol effect (direct, semi-direct and indirect cloud albedo and lifetime effects) in 12 global climate models and two determinations from satellite observations in global mean, over the northern and southern hemispheres, over oceans and over land, and the ratio over oceans/land. Anthropogenic aerosol effect is defined as the change in net radiation at TOA from preindustrial times to the present day resulting from anthropogenic emissions of aerosols and aerosol precursors. Patterns denote different anthropogenic species whose forcings were examined and the cloud types affected; all are for water clouds except as indicated.

this is attributable to improvements in cloud parameterizations. Still, large uncertainties remain.

The influence of aerosols on evapotranspiration and precipitation is also quite uncertain, with model results for the change in global average precipitation ranging from almost no change to a decrease of 0.13 mm day1 (5 cm yr1), with much greater changes locally. Decreases in precipitation are larger when the atmospheric GCMs are coupled to mixed-layer ocean models, where the sea surface temperature and, hence, the evaporation from the ocean is also allowed to vary, than in models in which the sea surface temperature is held constant (Denman et al. 2007). The decrease in evapotranspiration results primarily from decreases in solar radiation at the surface, as a result of increased

Ocean

Land

-1—I—I—I—I—I—I—I—I—I—I—I—I—r-

-i—I—I—I—I—I—I—I—I—I—I—I—I—r

Ratio ocean/land

—\—I—I—I—I—I—I—I—I—I—I—I—I—r-

□ Sulfate + Black carbon (BC) H Sulfate + Organic carbon (OC)

□ Sulfate + BC + OC on water and ice clouds ^ Combined GCM and satellite results

□ Mean and standard deviation of all results

Figure 23.5 (continued) Vertical black lines denote ±1 standard deviation in cases of multiple simulations and/or results. Modified after Denman et al. (2007).Vertical black lines denote ±1 standard deviation in cases of multiple simulations and/or results. Modified after Denman et al. (2007).

aerosol optical depth and optically thicker clouds. The decrease in solar radiation at the surface is then partly balanced by a decreased latent heat flux, which results in a reduced global mean precipitation rate (e.g., Liepert et al. 2004).

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