FIGURE 14-47 Cloud droplet size distributions for stratiform clouds in the Denver area for urban and nonurban air masses (adapted from Alkezweeny et al., 1993).
Parungo and co-workers suggest this may be due to the indirect effects of increasing S02 emissions.
The indirect effect of aerosols on climate, which at present contributes a major uncertainty in understanding anthropogenic perturbations on climate, is a very active area of research. For some typical model treatments of this indirect effect and how it interacts with those due to other, simultaneous, perturbations, see, for example, Jones et al. (1994), Hansen et al. (1997a-d), C. C. Chuang et al. (1997), Lohmann and Feichter (1997), and Pan et al. (1998).
Figure 14.48 shows one assessment (Hansen et al., 1997d) of the contributions of anthropogenic emissions to the average global radiative forcing from preindus-trial times to the present as well as that due to changes in solar intensity over the past 200 years (see Section D.3). The contributions due to an increase in tropo-spheric 03 from preindustrial times to 1980 and that due to stratospheric ozone destruction from 1979 to 1995 are predicted to essentially cancel out. Three contributions due to changes in tropospheric aerosol particles are included. Desert aerosols give a positive radiative forcing because of their absorption of light discussed earlier, whereas sulfate and biomass particles scatter light, leading to a negative radiative forcing. The indirect effect of particles on clouds has very large uncertainties associated with it and is shown as — 1 W m~2 in Fig. 14.48. Finally, changes in vegetation are estimated to have contributed -0.2 W m~2, due the reduction in the area of forests, which are dark.
As discussed in IPCC (1996), the confidence level associated with these values ranges from high for the greenhouse gases to very low for tropospheric aerosols, and in particular for the indirect effects. For example, the calculations of Penner et al. (f 998) suggest a larger direct radiative forcing due to sulfate aerosol particles (-0.81 W itT2) than that shown in Fig. 14.48 and a global average contribution of +0.16 Wm"2 for fossil fuel black and organic carbon particles. The contribution of changes in the solar flux and uncertainties in this are discussed in Section D.3.
The uncertainties in the indirect effects on clouds are very large. As discussed earlier, increased CCN can alter the properties of clouds in several ways that can impact climate. Thus, they can lead to changes in cloud albedo and, in addition, alter the size distribution of cloud droplets, changing the precipitation rate and hence cloud lifetime. For example, Lohmann and Feichter (1997) carried out model studies of the indirect effects of sulfate on clouds and predicted increases in shortwave cloud forcing ranging from —1.4 to —4.8 W m~2. A significant portion of the effects was due to changes in cloud lifetime; for example, for the — f .4 W m~2 case, about 40% was attributed to changes in the cloud lifetime and 60% to changes in cloud albedo.
ft is important to note that such globally and annually averaged estimates of contributions to radiative forcing are not expected to be the sole measures of effects on climate. The inference may be mistakenly drawn that negative radiative forcing, e.g., through
tropospheric aerosol particles, may largely counterbalance the positive radiative forcing due to the greenhouse gases and hence there will be no net change in climate. This is not expected to be the case since the effects operate on different geographical and temporal scales. Thus, many of the greenhouse gases (e.g., C02, CH4, N20, and the CFCs) are sufficiently long-lived to be globally distributed. Their contributions to radiative forcing vary geographically, from about 3 W m~2 over hot regions such as the Sahara to ~0.6 W m~2 over the South Pole (National Research Council, 1996). Shorter-lived greenhouse gases such as 03 have much more spatial and temporal variability, with associated differences in their contributions to radiative forcing. However, the contribution of all greenhouse gases to radiative forcing operates both day and night since it involves their interaction with terrestrial radiation.
On the other hand, aerosol particles from anthropogenic activities tend to be concentrated over or near industrial regions in the continents. Because both the direct and indirect effects of particles are predominantly in terms of scattering solar radiation, their effects are expected primarily during the day.
For example, model studies by Sinha and Harries (1997) have explored a hypothetical case in which C02 is doubled to 710 ppm and the amount of tropospheric aerosol is increased about a factor of four, giving no net change in the predicted equilibrium surface temperature. However, even with a predicted net surface temperature change of zero, significant effects on climate are still predicted. The solar radiation at the surface at mid and low latitudes is predicted for this hypothetical case to decrease by as much as — 6 W m~2 in January. Similarly, the vertical distribution of the rate of total radiative heating is predicted to change by more than 4% at some altitudes, which would be expected to lead to changes in the lapse rate, potentially affecting atmospheric circulation processes.
In addition to the differences in geographical distribution of the greenhouse gases compared to the aerosol particles and the day-night differences, there are also differences in their temporal behavior. As discussed earlier, typical residence times for sulfate particles are about a week, whereas that of C02 is about 100 years. As a result, the impacts of sulfate aerosols are almost immediately manifested, whereas those due to C02 occur over decades to centuries (Schwartz, 1993).
Hansen and co-workers have carried out modeling studies that examine the effects of various perturbations, both anthropogenic and natural, on climate (Hansen et al., 1997a-c). The altitude and geographical location of the forcings are shown to be important determinants of the effects on climate, rather than simply the magnitude of the forcing. For example, the addition or removal of heat in the upper troposphere is partially compensated by changes in radiation to space, which does not occur close to the earth's surface (Hansen et al., 1997b).
In short, while net radiative forcing is a convenient means for examining the potential importance of various anthropogenic perturbations for climate, it cannot be used in an additive manner for gases and aerosol particles to predict the ultimate impacts.
b. Heterogeneous Chemistry Involving Climate Species
Another potential contribution of aerosol particles to global climate is that of heterogeneous chemistry. For example, particle surfaces could in principle destroy greenhouse gases such as ozone that are surface sensitive. Another example is the formation of greenhouse gases such as N20 on surfaces. Thus, nitrous acid (HONO), which is itself formed by heterogeneous reactions on surfaces, has been shown to react on acid surfaces to generate N20 by a mechanism that is not well understood (e.g., see Wiesen et al., 1995; Pires et al., 1996; and Pires and Rossi, 1997). Given the present lack of understanding of the reaction mechanism, it is not possible to assess the importance of such heterogeneous chemistry for N20 formation in the atmosphere. However, it does illustrate the potential for heterogeneous chemistry on aerosol particles to impact global climate through the effect on gas-phase species.
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