Page 151

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.

Implications

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