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budgets of trace gases and aerosols. Thinning of the stratospheric ozone layer yields a higher UV flux into the troposphere, thereby accelerating photooxidation processes.

Understanding the complex interactions between tropospheric chemistry and global change presents a formidable scientific challenge, which can be addressed only by close cooperation between scientific disciplines, tight interaction between observation and modeling, and broad international cooperation.

2 J Introduction

In the public mind, "global change" has become almost synonymous with "global warming" or "climate change," a narrow reduction of the original meaning. Although there is no doubt that the possibility of climate change is of great concern to the Earth's population, we must not forget that we are living in a period when almost all components of the Earth System are undergoing change. The chemical composition of the atmosphere is being perturbed at a vast scale by human activities, The terrestrial biota are modified by land use change, hiomass burning, deforestation, and species extinction. Marine life is impacted by overfishing, eutrophication, and pollution. There is a tendency to see these issues as independent environmental problems, each grabbing the public's attention for some time, and each demanding a specific solution.

This approach obscures the fact that all these phenomena arc occurring simultaneously and within the same "Earth System," As a result, they interact with one another, reinforcing or damping each other, or changing each other's temporal evolution. This view is reflected in the "Bretherton" diagram (Figure 2.1), which shows the complex linkages between human activities, physical climate system, and biogeochemical cycles.

It is especially important to examine the Earth System for possible feedbacks, which amplify the effect of perturbations. It is well established that increasing temperatures result in changes in ice albedo, atmospheric water vapor content, and cloudiness, changes that in turn act to increase temperature. If additional positive feedbacks exist, they would add to known feedbacks and, because of the extremely nonlinear behavior at higher gains, could have disproportionately large effects (Lashof et al., 1997).

In this chapter, I explore some of these interactions among human activities, atmospheric chemistry, climate, and ecology, using selected examples or case studies. I proceed from the (relatively) simple to the more complex, keeping in mind that exploring any of the issues addressed here in its full depth and complexity is well beyond the frame of an overview chapter such as this.

2.2 The "Simplest" Case: Anthropogenic Halogcnatcd Hydrocarbons

The clearest evidence of a global change in the Earth System is the changing composition of the atmosphere, particularly the increasing concentrations of some long-lived trace gases emitted by industrial activities. This was first documented for CO? by the long-term measurements made by C. D. Keeling at Ma una Loa Station on Hawaii (Bacastow et al., 1985; Keeling et al., 1995) and subsequently for numerous

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other trace gases such as methane (CH4), nitrous oxide (N?0), carbon monoxide (CO), and numerous halogenated hydrocarbons (I loughton et al., 1996, and references therein).

The latter class of compounds includes species such as the chlorofluorocarbons (CFCs, Freons), methyl chloroform (trichlorocthanc), and the partially halogenated chlorofluorocarbons (HCFCs), which are exclusively human-made and have no natural sources. Because they are produced industrially, often by relatively few manufacturers, accurate records exist of the amounts and times at which they were produced and released into the environment. Most anthropogenic halogenated hydrocarbons have no significant biological sinks and are resistant to hydrolysis in aquatic systems, so their only major sink is photochemical breakdown in the atmosphere. For most substances, this sink follows first-order kinetics, that is, its rate is a linear function of the trace gas concentration.

As a result of the well-characterized and simple source and sink functions of these gases, their concentration in the atmosphere as a function of space and time coordinates can be relatively easily understood and modeled. Figure 22 illustrates this behavior with the example of the temporal record of methyl chloroform, a substance that has no known natural sources and is removed almost exclusively by reaction with tropospheric OH. The use of methyl chloroform is severely restricted by the Montreal Protocol, and consequently its production declined sharply around 1990. This resulted in a reversal of its atmospheric trend in 1991, from an average increase of 4.5 ± 0.1%/year to a decline of about 14%/year in 1995/1996, This behavior can be described in an atmospheric model and used to deduce both its weighted-mean atmospheric lifetime (4.8 years) and the global-mean OH concentration (Montzka et al., 1996; Prinn et ah, 1995).

Although we have used these compounds as examples of the simplest case, with minimal feedback processes, wc should note that even here some complications might arise, if any of these substances could change the stratospheric ozone density to such a degree that it would significantly alter the UV flux into the troposphere, and thus the tropospheric OH abundance, it could influence its own lifetime. Although it has been

Figure 2.2. Temporal evolution of the atmospheric mixing ratio of methyl chloroform (CHjCQj) in the troposphere over the Northern Hemisphere (from kurylo et al., 1999), This version of the diagram has been produced by the Larth System Science Education Program, Universities Space Research Association, Whitelaw, Wisconsin. See also color plate section.

Figure 2.2. Temporal evolution of the atmospheric mixing ratio of methyl chloroform (CHjCQj) in the troposphere over the Northern Hemisphere (from kurylo et al., 1999), This version of the diagram has been produced by the Larth System Science Education Program, Universities Space Research Association, Whitelaw, Wisconsin. See also color plate section.

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argued that significant changes in tropospheric OH have not occurred over the time scale represented in Figure 2.2 (Prinn et al., 1995), this issue is not without controversy {Krol et al,, 1998), This issue is discussed in more detail below.

2.3 A More Complex Case: CO2

In a way, CO? represents the opposite extreme from the preceding case: It is subject to strong and complex biogeochemical interactions, and its anthropogenic sources are only minor perturbations on the natural fluxes. The annual fluxes of CO2 in and out of the terrestrial and marine biota make up some 150 Pg C per year (1 Pg — 10g), whereas the emission from fossil fuel combustion and cement manufacturing accounts for "only** about 6 Pg year K Yet it is this small increment added to the large biogenic fluxes of CO? that is responsible for most of the growth of CO? concentration in the atmosphere and for about half of the greenhouse gas effect. The rest of the atmospheric CO2 increase is due to the effect of tropical deforestation, which moves carbon out of the "long-lived terrestrial biomass" reservoir into the atmosphere.

To understand and predict the atmospheric abundance of CO2, we need a thorough understanding of all the complex biogeochemical interactions that control its transfer between the Earth's compartments, including the deep and shallow oceans, the marine and terrestrial biota, the sediments and soils, and so on. There are a large number of known feedbacks between climate and the cycles of carbon and the "nutrient" elements (N, P, and S), and quite likely an even greater number are still unexplored (see, for example, Lashof et ah, 1997). Consequently, CO? is probably the most "interesting" trace gas to a biogcochemist. To an atmospheric chemist, however, it is "boring," because it does not undergo any relevant chemical reactions in the atmosphere. Therefore, I do not address the global carbon cycle in any detail in this chapter but restrict myself to these few short remarks.

It may be worthwhile, however, to emphasize one point here, to which we will come back several times in the following sections: the importance of the Tropics in understanding global change. The Tropics arc the part of the globe with the most rapidly growing population, the most dramatic industrial expansion, and the most rapid and pervasive change in land use and land cover. The Tropics contain also the largest standing stocks of terrestrial vegetation and have the highest rates of photosynthesis and respiration (Houghton and Skole, 1990; Raich and Potter, 1995). It is therefore likely that changes in tropical land use will have a profound impact on the global carbon cycle in future decades (Houghton et aL, 1998),

2.4 Trace Gases w ith Very Complex Source and Sink Patterns: CH4, N20

After CO?, methane is the most important greenhouse gas, and there is unequivocal evidence that its atmospheric concentration is increasing because of human activities. In an effort to understand this increase, a large effort has gone into elucidating the budget of this trace gas.

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