A Greenhouse Forcing and Temperature Rise Estimation Procedure
This chapter provides a compact estimation procedure for projecting greenhouse warming. It can be applied to a broad but simple family of emission scenarios and to either of two postulated climate sensitivities. Using this procedure, one can determine (1) the increment in the radiative forcing from 1990 to 2030 that would accompany the scenario under examination and (2) the equilibrium global mean temperature increase that would be consistent with that increment and the chosen forcing. Because of limitations in our understanding of the oceans, it is not possible to incorporate a simple estimation procedure for time-dependent (transient) climatic changes. But since the focus here is on the long-term implications of human actions, transient effects are less important than equilibrium effects.
Although emissions will undoubtedly continue after the year 2030 and would contribute to further climate change, the procedure presented in this chapter is limited to the next few decades. It is limited in this way both because of the difficulty of producing credible projections beyond that period and because of the panel's emphasis on practical actions that can be undertaken now. Certainly, actions or inaction during this period will have important longer-term implications that deserve attention in broader considerations of a sustainable environment. In addition, this interval was selected because 2030 is roughly the time at which the IPCC's high scenario (sometimes referred to as "business-as-usual") suggests that an equivalent doubling of the preindustrial CO2 concentration may occur if minimal (or no) actions are taken to limit the recent, precontrol rates of increase of greenhouse gas emissions (Intergovernmental Panel on Climate Change, 1990). Beyond 2030, various approximations that are made may also become less valid.
The procedure takes into account the rate at which, following its emission
into the atmosphere, each greenhouse gas is removed from the atmosphere (e.g., by transport to the oceans or by chemical reaction), but no account is taken of the extent to which one greenhouse gas (including ozone and water vapor in the stratosphere) may be affected by or introduced as a by-product of a chemical reaction that depletes another. In particular, in accord with the uncertainties attending the fate of CO2 emissions (Emanuel et al., 1989), the procedure is approximately consistent with the observations of the past century; i.e., approximately 60 percent of the CO2 emissions introduced into the atmosphere are removed promptly, and the remaining 40 percent contribute to long-term (i.e., several century) enhancement of the CO2 concentration. The current concentrations, current emission rates, and lifetimes of the most important of the greenhouse gases that were considered are given in Table 17.1, and projected concentrations are shown in Figure 17.1. The radiative forcing associated with each of these gases is depicted as a function of its concentration level in Figure 17.2.
As indicated in Chapter 18, the Effects Panel agrees that it is plausible to expect that the increase in the equilibrium global mean temperature of our climatic system that might be implied by an equivalent CO2 doubling would
TABLE 17.1 1990 Atmospheric Concentrations, Emissions, and Lifetimes of Key Greenhouse Gases
lie between 1° and 5°C (1.8° to 9°F). Accordingly, the change from its 1990 value of the equilibrium global mean temperature associated with the 2030 concentration of each of the greenhouse gases is calculated for the 1° sensitivity and for the 5° sensitivity.
The values of the forcing and of the change in equilibrium global average temperature that have been compiled in accord with the foregoing description can be extracted for any chosen scenario from Figure 17.3.
Future changes in concentration are based on changes in emissions from their 1990 baseline values. Table 17.1 lists the 1990 baseline atmospheric concentrations and estimated emissions, subdivided into natural and anthropogenic sources. These estimates are quite close to the IPCC estimates, differing slightly because of updated estimates of lifetimes and because of calibration of the model used in representing the carbon budget.
Figure 17.1 shows the atmospheric concentrations in the year 2030 resulting from linear changes in 1990 emissions over the period from 1990 to 2030. Generally, percentage changes from +100 percent to -100 percent encompass the plausible scenarios. However, CO2 emissions could, under some scenarios more than double over the next 40 years (e.g., see Trabalka,
1985), and emissions of hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs) that are being introduced as replacement compounds for those CFCs being controlled under the Montreal Protocol could increase by 1,000 percent or more from present emission levels.
The effects of the changes in emissions can then be converted into changes in the radiative forcing as a function of concentration. Figure 17.2 shows the relationships based on the formulae for estimating changes in radiative forcing as selected by IPCC, which are generally in reasonable agreement with the other results (Intergovernmental Panel on Climate Change, 1990). Note that, among the simplifications, no account is taken here of the radiative effects of the CO2 and stratospheric H2O that would result from chemical destruction of CH4 and no allowance is made for changes in stratospheric or tropospheric ozone. In addition, the radiative forcings caused by the different species are not completely equivalent (Wang et al., 1991), but the approximations here are adequate for this comparative analysis.
Based on these relations, Figure 17.3 relates changes in radiative forcing to possible changes in emissions over the period 1990 to 2030 for CO2, CH4, N2O, CFCs controlled by the Montreal Protocol (CFCs 11, 12, 113, 114, and 115), and halocarbons not controlled by the Montreal Protocol (including HCFC-22, CH3CCl3, and CCl4). As indicated above, these estimates do not account for the climate-chemistry couplings involving species that are not directly emitted (e.g., ozone changes, stratospheric water vapor, and CH4 conversion to CO2), which will likely cause noticeable changes, but not so large as to change the general character of the curves.
Figure 17.3 also shows (with asterisks) the expected changes in emissions for CO2, CH4, and N2O assuming a scenario similar to the IPCC high scenario. The asterisk on the CFC curve, for example, indicates a 50 percent reduction, even though stricter controls have recently been agreed upon under the Montreal and London Protocols.
To provide an indication of the potential climatic importance of the change in radiative flux, temperature change multipliers have been used to produce the vertical coordinates on the right-hand side of the figure. The commitment to future warming (i.e., the expected equilibrium temperature increase) that would occur as a consequence of emissions from 1990 to 2030 can be derived by taking the product of the multiplier and the climate sensitivity to a CO2 doubling. This product, for two different climate sensitivities, is displayed on the vertical coordinate on the right-hand side of Figure 17.3. Thus, if the sensitivity is 1°C (1.8°F), the CO2 contribution to future warming assuming constant emissions (0 percent change) is about 0.24°C (0.43°F). The estimates for climate sensitivity span a rather wide range, indicated in the figure by including coefficients for climate sensitivities of 1° and 5°C (1.8° to 9°F). This wide range of climate sensitivity estimates creates a large range in possible temperature changes from 1990 to 2030, demonstrating
the importance of uncertainties created by our limited understanding of oceanic behavior and other phenomena.
Although the full warming due to emissions from 1990 to 2030 will not occur until a few decades thereafter, there will also be warming during the period from emissions prior to 1990. In addition, the continuing emissions beyond 2030 will lead to further warming over the longer term.
The results of this analysis offer several points for consideration in attempting to optimize a greenhouse gas limitation policy.
1. The slope of the N2O curve in Figure 17.3 is so flat that even large changes in emissions would have a relatively minor effect over this period. This assumes, however, that the ozone interactions with N2O are small. In any case, continued emissions of N2O, over the long term, will not lead to a significant increase in warming despite its long lifetime.
2. Once CFC emissions are reduced by 50 percent, little more is gained (with respect to their greenhouse warming effect) by further reduction in the period to 2030, although a CFC buildup would continue to occur at this level of emissions. (In addition, the effects of CFCs on ozone need to be considered.) Clearly, however, a failure to implement the Montreal Protocol would have a substantial warming effect (as pointed out by Hansen et al., 1989). Unless emissions of the uncontrolled CFCs increase substantially (and they might), their greenhouse warming effect will be relatively modest over this period, although continued emissions would allow an additional concentration buildup and the associated forcing. The uncontrolled CFCs do not generally have long lifetimes.
3. Strong controls on CH4 emissions, though perhaps difficult to implement, would produce a large effect. (Note that the potential for additional CH4 emissions from CH4 hydrates now tied up in permafrost has not been included.)
4. Carbon dioxide is clearly the major factor and has the steepest slope and the potential to lead to the largest temperature changes. Note, however, that 25 to 50 percent reductions in CO2 emissions over the period 1990 to 2030 will still lead to rather substantial increases in the radiative flux (and ultimately in temperature change).
Summing the radiative flux changes assuming no change in emissions (already a rather stringent measure) produces a flux increase of about 1.6 W/m2. This, when added to the 2.45 W/m2 already experienced since 1765 (or the 1.95 W/m2 since about 1900), indicates that the climate will have been committed to the radiative equivalent of a CO2 doubling (4.4 W/m2) by about 2030 or a little later. Any increases in emission rates will only
make the changes greater. In addition, although beyond the time horizon of this report, continued emissions beyond 2030 will further increase the projected temperature change.
The extent to which changes in radiative forcing will be significant to society depends on the climate sensitivity and the consequent climatic impacts on human activities and natural systems. Given that past climates have varied substantially as a result of comparable forcings and that ecosystems under such conditions were quite different than at present, however, this schematic analysis suggests that significant climate change will be very difficult to avoid, although its rate of onset may be slowed.
Emanuel, W. R., G. G. Killough, W. M. Post, H. H. Shugart, and M. P. Stevenson. 1989. Computer Implementation of a Globally Averaged Model of the World Carbon Cycle. TR010. Washington, D.C.: Carbon Dioxide Research Division, U.S. Department of Energy.
Hansen, J., A. Lacis, and M. Prather. 1989. Greenhouse effect of chlorofluorocarbons and other trace gases. Journal of Geophysical Research 94:16417–16421.
Intergovernmental Panel on Climate Change. 1990. Climate Change: The IPCC Scientific Assessment, J. T. Houghton, G. J. Jenkins, and J. J. Ephraums, eds. New York: Cambridge University Press.
Trabalka, J. R., ed. 1985. Atmospheric Carbon Dioxide and the Global Carbon Cycle. DOE/ER-0239. Washington, D.C.: U.S. Department of Energy.
Wang, W.-C., M. P. Dudek, X.-Z. Liang, and J. T. Kiehl. 1991. Inadequacy of effective CO2 as a proxy in simulating the greenhouse effect of other radiatively active gases. Nature 350:573–577.