Radiative forcing indirect effects

Wang, W.-C., H. Mao, IS. A Isaksen, J. S. Fuglestvedt, and S. Karlsdottir, 1996 Indirect Effects of Increasing Atmospheric Methane on the Radiative Forcing Through Climate-Chemistry Interactions. Proceedings of XVIII Quadrennial Ozone Symposium-96, September 12-21,1996, L Aquila, Italy. 

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/... 

The impact of secondary aerosols on indirect radiative forcing is the most variable and is the least understood [3]. The reasons why the indirect effect of secondary aerosols is so difficult to describe is that it depends upon [1] (1) a series of molecular-microphysical processes that connect aerosol nucleation to cloud condensation nuclei to cloud drops and then ultimately to cloud albedo and (2) complex cloud-scale dynamics on scales of 100-1000 km involve a consistent matching of multiple spatial and time scales and are extremely difficult to parameterize and incorporate in climate models. Nucleation changes aerosol particle concentrations that cause changes in cloud droplet concentrations, which in turn, alter cloud albedo. Thus, macro-scale cloud properties that influence indirect forcing result from both micro-scale and large-scale dynamics. To date, the micro-scale chemical physics has not received the appropriate attention. 

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. 

Methane oxidation leads to a net loss of OH in the atmosphere, thereby lengthening the lifetime of CH4 itself (we will discuss this later in this chapter). It is estimated that this longer lifetime increases the radiative forcing of CH4 by 25-35% over that in the absence of this feedback effect. Methane oxidation also leads to tropospheric O3 this indirectly increases the greenhouse effect by another 30-40% through the effect of the added O3 itself. Finally, increases in CH4 also indirectly lead to further climate forcing by increasing stratospheric H20 (about 7% of CH4 is oxidized in the upper troposphere). 

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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