The Future

As seen throughout this chapter, the parameters controlling climate are extremely varied and complex, with multiple feedbacks between them. Some possible future scenarios based on the state of the science as of about 1995 are described in the IPCC (1996) document. Figure 14.63 shows one model estimate for temperature changes due only to direct radiative forcing by C02 from f990 to 2f00 based on three scenarios (Wigley, 1998). While as we have seen, many gases contribute to radiative forcing, the calculations in Fig. 14.63 are based on expressing these changes in terms of equivalent C02 reductions. Note that this does not take into account other contributing factors such as aerosol particles which may contribute in the opposite

Year

FIGURE 14.63 Calculated temperature changes relative to 1990 for existing policies (—), for a 5% decrease in equivalent C02 as required by Kyoto protocol from 1990 to 2010 followed by no further emissions reductions (•••), and for further reductions of 1% per year (compounded) from 2010 to 2100 (---) assuming a climate sensitivity of 2.5 K for a doubling of C02 (adapted from Wigley, 1998).

Year

FIGURE 14.63 Calculated temperature changes relative to 1990 for existing policies (—), for a 5% decrease in equivalent C02 as required by Kyoto protocol from 1990 to 2010 followed by no further emissions reductions (•••), and for further reductions of 1% per year (compounded) from 2010 to 2100 (---) assuming a climate sensitivity of 2.5 K for a doubling of C02 (adapted from Wigley, 1998).

direction. Hence, Fig. 14.63 should be taken as illustrative of the effects on direct radiative forcing by gases and not the net result of all contributing factors and feedbacks.

Some of complexities of climate are reflected in the natural variability, observed through such "proxies" as the ice core and tree-ring records. However, there are reasons, based on our current state of scientific understanding, to believe that anthropogenic activities that have resulted in changes in atmospheric composition may affect climate locally, regionally, and globally. Despite the uncertainties in the deconvolution of anthropogenic and natural contributions to the relatively recent observed global climate changes, the weight of scientific opinion at the end of the twentieth century is reflected in the IPCC (1996) summary:

Nevertheless, the balance of evidence suggests that there is a discernible human influence on global climate.

Elucidating this influence remains a challenge for the twenty-first century.

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