Radiative forcing direct effects

Table 14.4 summarizes the estimated total direct radiative forcing calculated for the period from preindustrial times to 1992 for C02, CH4, N20, and O, (IPCC, 1996). The estimate for CH4 includes the effects due to its impacts on tropospheric ozone levels or on stratospheric water vapor, both of which are generated during the oxidation of methane. That shown for 03 is based on the assumption that its concentration increased from 25 to 50 ppb over the Northern Flemi-sphere. The total radiative forcing due to the increase in these four gases from preindustrial times to the present is estimated to be 2.57 W m 2. 

It should be noted that the magnitude of the predicted forcing is quite sensitive to treatment of relative humidity (RH) in the model because of the effects on particle size and optical properties (e.g., Haywood and Shine, 1995 Haywood and Ramaswamy, 1998 Ghan and Easter, 1998 Haywood et al., 1998a Penner et al., 1998). For example, in the calculations by Penner et al. (1998), when the particle properties were held fixed at the values for 90% RH for 90-99% RH, the predicted direct radiative forcing for sulfate particles decreased from - 1.18 W m-2 to -0.88 W m 2 for the Northern Hemisphere and from -0.81 to -0.55 W m-2 globally. 

Ghan, S. J., and R. C. Easter, Comments on A Limited-Area-Model Case Study of the Effects of Sub-Grid Scale Variations in Relative Humidity and Cloud upon the Direct Radiative Forcing of Sulfate Aerosol, Geophys. Res. Lett., 25, 1039-1040 (1998). 

Liao, H., and J. H. Seinfeld, Effect of Clouds on Direct Aerosol Radiative Forcing of Climate, J. Geophys. Res., 103, 3781-3788... 

Such a complicated interactivity of processes can both directly and indirectly affect formation of the atmospheric greenhouse effect. Derwent et al. (2001) described a global 3-D Lagrangian chemistry transport model (STOCHEM) which reproduces chemical processes including MGC transport and can be used to reproduce interrelated fields of TO and methane concentration (Johnson et al., 2002) under conditions of emission to the atmosphere of short-lived TO precursors such as CH4, CO, NOx, and hydrogen. At the same time, the radiative forcing (RF) of NOx emissions depends on the location of emissions near the surface or in the upper troposphere, in the Northern or Southern Hemisphere. For each short-lived MGC/... 

FIGURE 7 (a) Radiative forcing from greenhoitse gases, sulfate aerosols (direct and indirect effect),... 

Today, the anthropogenic emissions of SO, primarily from fossil fuel combustion, largely dominate the sulfur flux into in the atmosphere on the global scale. Climate models have determined the corresponding direct and indirect impacts on radiative forcing, but large uncertainties remain in these estimates. In fact, predictions of future climate need to account not only for the effects of sulfate aerosols, but also for the contributions of mineral dust, black carbon, organic carbon, and sea salt. The current view is that atmospheric particles should be treated as multicomponent, mul-... 

Direct radiative forcings affect directly the Earth s radiative balance for instance, added C02 absorbs infrared radiation. Indirect forcings lead to a radiative imbalance by first altering some component of the climate system that then leads to a change in radiative fluxes. An example of an indirect effect is increasing aerosol levels that produce clouds with smaller drops clouds with smaller drops are not as prone to produce precipitation, so the clouds persist longer and reflect and absorb more radiation. 

The N2O molecule is effective at retaining long-wave radiation with a relative radiative forcing 280 times that of a CO2 molecule. Despite this the relatively low atmospheric N2O concentration results in a contribution of only 5-6% of the present day greenhouse effect with a direct radiative forcing of about 0.1 Wm. In the stratosphere N2O reacts with oxygen to produce NO radicals, which contribute to ozone depletion. 

It has become clear only recently that the atmospheric sierosol plays an important role for the climate on earth. It is common to distinguish between direct and indirect effects of the aerosols on the climate. Aerosols effect directly the radiation balance of the earth due to scattering and absorption of electromagnetic radiation (radiative forcing). On the other hand they influence the physics and chemistry of the atmosphere as condensation nuclei for cloud droplets and their chemical reactions with atmospheric trace gases. Though these indirect aerosol effects are difficult to quantify, they are at least as important as the direct radiative forcing. An especially important and complex example for the indirect influence of aerosols on the chemistry and radiation balance of the earth is the role of stratospheric aerosol particles on the polar ozone depletion, which is discussed in more detail below. 

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