Info

" Global mean and annual mean radiative forcing in W m 2. ''Adapted from Bernsten et al. (1997).

" Global mean and annual mean radiative forcing in W m 2. ''Adapted from Bernsten et al. (1997).

chemistry, physics, and atmospheric dynamics. Empirical relationships of the form of Eq. (R),

between the change in temperature at the earth's surface, A Ts, and the adjusted radiative forcing F.d (in W irT2) are often used (Hansen et al., 1997b). The value of 8 is model dependent, with typical values of 0.3-1.1 depending on the model and whether it includes the effects of feedbacks (Hansen et al., 1997b, 1997d). This range corresponds to changes in predicted global temperatures from f.5 to 4.5°C for a doubling of C02 (or the equivalent contributions from other greenhouse species).

As discussed in more detail in the following sections, some anthropogenic emissions are expected to cause positive radiative forcings whereas others are negative. Even though the two may be equal in magnitude, giving a net radiative forcing of zero, this does not mean that there will be no effects on climate. For example, negative radiative forcing caused by aerosol particles is expected to occur primarily over continents whereas the positive radiative forcing due to many greenhouse gases occurs globally. In addition, there are temporal differences, with the lifetime of particles typically being of the order of a week while those of many greenhouse gases are of the order of centuries. These differences can lead to impacts on climate, despite a net radiative forcing of zero. As a result of such considerations, radiative forcing is commonly used primarily for comparing the relative potential importance of various gases and particles on climate.

b. Absolute and Relative Global Warming Potentials

As was the case for ozone depletion potentials (see Chapter 13.B), the effects of greenhouse gases depend not only on the emissions but also on their lifetimes in the atmosphere (Ko et al., 1993). Global warming potentials (GWP) express the time-integrated radiative forcing due to the instantaneous emission of a fixed amount (usually f kg) of the gas of interest. Thus, a time-scale horizon (TH) that will be considered in assessing the radiative effects of the gas must be specified. Both absolute and relative GWPs have been put forth, where these are defined as follows:

Absolute

Absolute GWP = JTHagiiS[gasL dt (S)

Relative

Relative GWP =

In Eqs. (S) and (T), [gas] and [ref] represent the time-dependent concentrations of the gas of interest and the reference gas, respectively, which are assumed to decay with characteristic lifetimes or response times after the instantaneous injection of the pulse, and ax (units of W m~2 per ppb or ppm) is the radiative forcing of the gas or reference per unit increase in their atmospheric concentrations. The value of ax is assumed to be time-independent.

The reference gas often used for relative GWPs is C02 because it is the major greenhouse gas. As we have seen earlier, the cycling of C02 throughout the earth system is complex, occurs with different response times, and is not thoroughly understood in a quantitative manner at present. As a result, uncertainties in how it decays will be translated into uncertainties in the relative GWPs of other greenhouse gases.

There may, however, be some "cancellation of errors." For example, the concentration of atmospheric C02 ([ref], in Eq. (T)) depends in a nonlinear fashion on the amount of total dissolved inorganic carbon in the ocean surface layer because of the equilibria with water (see Chapter 8.B) so that relatively less atmospheric C02 can be taken up by the oceans as its atmospheric concentrations increase. This would leave relatively more C02 in the atmosphere, increasing its greenhouse effect. On the other hand, since the strongest infrared absorption bands of C02 are already saturated (vide supra), the radiative forcing (aco, in Eq. (T)) decreases as its concentrations increase.

Caldeira and Kasting (1993) show that these two factors largely cancel each other so that using C02 as the reference gas is still useful.

For the alternatives and proposed replacements for CFCs, CFC-11 has been used in some cases as the reference compound. In interpreting the GWPs, the reader should take note of which compound has been used as the reference.

Another index has been proposed as well, a forcing equivalent index (FEI) (Wigley, 1998), defined as

where A£gas(0 is the emissions reduction in the gas of interest calculated year-by-year that is needed to give the same change in radiative forcing as changes in C02 emissions, AEvo(.t).

Table 14.4 summarizes the estimated total direct radiative forcing calculated for the period from pre-industrial times to 1992 for C02, CH4, NzO, and 03 (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 Hemisphere. 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.

Also shown are the relative global warming potentials, using C02 as the reference and for the two time horizons of 20 and 100 years, respectively (IPCC, 1996). The apparently disproportionate effects of CH4, N20, and 03 relative to C02 are due to the fact that C02 was present from natural processes in large concentrations even in preindustrial times and is such a strong

TABLE 14.4 Direct Radiative Forcings and Global Warming Potential for the Major Greenhouse Gases" Relative to COz

Concentration

Preindustrial

0 0

Post a comment