Role of Greenhouse Gases

2.4.1 The greenhouse effect

The greenhouse effect is a natural feature of the climate system. In fact, without the atmosphere (and hence the greenhouse effect), the earth's average temperature would be approximately 33°C colder than it is currently. The earth/atmosphere system balances absorption of solar radiation with emission of longwave (infrared) radiation to space. The earth's surface primarily absorbs most of the shortwave solar radiation from the sun, but it also reradiates some of this radiation as longwave radiation (Fig. 2.3). Energy is lost before reaching the surface of the earth through reflection from clouds and aerosols in the atmosphere. Little is directly absorbed by the atmosphere, which is relatively transparent to shortwave radiation. Also, an average of about 30% is reflected off the earth's surface.

The atmosphere is more efficient at absorbing longwave radiation, which is then both emitted upward toward space and downward toward the earth. This downward emission serves to heat the earth further. This further warming by reradiated longwave radiation from the atmosphere is known as the greenhouse effect. The amount of longwave radiation that is absorbed and

Fig. 2.3. The earth's radiation and energy balance. The net incoming radiation of 342 W m-2 is partially reflected by clouds and the atmosphere, or at the surface. Some of the heat absorbed at the earth's surface is returned to the atmosphere as sensible and latent heat. The remainder is radiated as thermal infrared radiation and most of that is absorbed by the atmosphere, which in turn emits radiation both up and down; this produces the greenhouse effect. (Source: Kiehl and Trenberth, 1997.)

Fig. 2.3. The earth's radiation and energy balance. The net incoming radiation of 342 W m-2 is partially reflected by clouds and the atmosphere, or at the surface. Some of the heat absorbed at the earth's surface is returned to the atmosphere as sensible and latent heat. The remainder is radiated as thermal infrared radiation and most of that is absorbed by the atmosphere, which in turn emits radiation both up and down; this produces the greenhouse effect. (Source: Kiehl and Trenberth, 1997.)

then reradiated downward is a function of the constituents of the atmosphere. Certain gases in the atmosphere are particularly good at absorbing longwave radiation and are known as the greenhouse gases. These include water vapour, carbon dioxide (CO2), methane (CH4), some chlorofluorocarbons (CFCs) and nitrous oxide (N2O) (see Chapter 3, this volume).

If the make-up of the atmosphere changes and the result is an increase in concentrations of the greenhouse gases, then more of the infrared radiation from earth will be absorbed by the atmosphere and then reradiated back to earth. This changes the radiative forcing of the climate system and results in increased temperature of the earth's surface. Such perturbations in the radiation balance of the earth system are known as changes in the radiative forcing, and the factors that affect this balance are known as radiative forcing agents (Shine et al., 1990). One of the ways in which the effect of greenhouse gases is measured is by determining their radiative forcing. This can be viewed as a measure of their relative ability to alter the climate.

2.4.2 Current concentrations of greenhouse gases

This section discusses the major greenhouse gases and their relative contribution to the current radiative forcing of the atmosphere (Fig. 2.4), compared

Fig. 2.4. Estimates of global and annual average radiative forcing (W m-2) attributable to changes in greenhouse gases and aerosols, and the solar flux from 1850-1990. The height of a rectangular bar indicates either the best or mid-range estimate of the forcing; the vertical lines in the bars represent the uncertainty range. (Source: Schimel et al., 1996.)

Fig. 2.4. Estimates of global and annual average radiative forcing (W m-2) attributable to changes in greenhouse gases and aerosols, and the solar flux from 1850-1990. The height of a rectangular bar indicates either the best or mid-range estimate of the forcing; the vertical lines in the bars represent the uncertainty range. (Source: Schimel et al., 1996.)

with pre-industrial times (1850). Because of human activities, atmospheric concentrations of greenhouse gases (except water vapour) have increased considerably since the beginning of the industrial revolution. Sources of these gases include fossil fuel burning, tropical deforestation, biomass burning, chemical industrial activities, and agricultural activities.

Carbon dioxide is the must abundant of the greenhouse gases. Concentrations of CO2 [CO2] have increased from about 280 parts per million by volume (ppmv) in the pre-industrial period to 358 ppmv in 1995, with a rate of increase of 1.6 ppmv per year (Fig. 2.2). The concentration by mid-1998 was 368 ppmv. Major sources of CO2 emissions include burning of fossil fuels and production of cement. Tropical deforestation contributes to [CO2] increase by removing vegetation, which is one of the major sinks of CO2 (Weubbles and Rosenberg, 1998).

On a per molecule basis, methane is a more effective greenhouse gas than CO2. The current globally averaged concentration is 1.72 ppmv and is increasing at the rate of 1% per year. For reasons that are not completely understood, the rate of increase in methane concentration decreased in the early 1990s, but has returned to a higher level of increase in the past few years. Methane is produced in rice culture, ruminant fermentation, landfills, and through losses during gas production and distribution and coal mining.

Nitrous oxide is even more efficient than methane in absorbing longwave radiation. Its mean concentration in 1990 was about 311 parts per billion by volume (ppbv) and has been increasing by about 0.2-0.3% per year. Its pre-industrial level was about 275 ppbv. Sources of the increase include fertilized soils that are used for crop production, biomass burning, industrial processes and feed lots. Hence, this is the greenhouse gas, along with methane, that is most strongly associated with agricultural activities (Schimel et al., 1996; see also Chapter 3, this volume).

Chlorofluorocarbons (CFCs) are inordinately efficient greenhouse gases with a relatively long lifetime of about 100 years. Used primarily as propellants and refrigerants, they are perhaps better known for their role in the destruction of ozone in the stratosphere rather than as greenhouse gases. The CFCs 11 and 12 currently have the largest concentrations in the atmosphere (0.27 and 0.50 ppbv, respectively). Their concentrations have increased, but at a diminishing rate in the later 1990s. However, these and other CFCs accounted for 15% of the increase in radiative forcing since 1900 and contributed nearly 25% of the increased forcing in the 1980s (Houghton et al., 1990). The continued phasing out of CFCs, as per the Montreal Protocol, indicates that these will become less significant greenhouse gases over time. They are being replaced by halogenated hydrocarbons (HCFCs), which, although still having some capacity as greenhouse gases, have less capacity than the CFCs they are replacing. Whether or not these changes are significant in the future depends on how large their emissions become.

The depletion of stratospheric ozone has been a problem because it has allowed increased ultraviolet radiation to reach the earth's surface. However, the increase in tropospheric ozone is also problematic, since tropospheric ozone acts as a greenhouse gas and is also a pollutant that affects humans, plants and animals. Tropospheric ozone contributed more than N2O to the positive forcing of the climate system during the 20th century.

The relative contribution of these different gases to the change in radiative forcing from pre-industrial times to the present, as well as that of some other external forcing agents, such as variations in solar activity, is presented in Fig. 2.4. Clearly, the largest contributor to the positive forcing has been the increase in greenhouse gases. In order of importance, they are CO2, CH4, the halocarbons and N2O (see Chapters 12 and 18, this volume).

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