In this chapter we explored a range of scenarios for future CO2 emissions drawing upon the extensive literature on this subject. Models of the socioeconomic system are coupled with those of the carbon cycle to determine future emissions under alternative assumptions about population and income growth, the cost and availability of existing and future energy-producing and -using technologies, and the key determinants of the carbon cycle.
Most scenarios suggest that in the absence of a constraint on emissions, atmospheric CO2 concentrations will continue to rise well beyond current levels, highlighting a gap between business-as-usual CO2 emissions and those required to stabilize concentrations at levels currently under consideration. To eliminate this gap will require the development and deployment of a new generation of energy technologies.
Of course, technology development is but one of the options for dealing with global climate change. As pointed out in Metz et al. 2001, climate policy requires a portfolio of responses. The challenge facing today's policy makers is to arrive at a prudent hedging strategy in the face of climate-related uncertainties. Among the options are
• immediate reductions of greenhouse gas emissions,
• investments in actions to assist human and natural systems in adapting to any climate change that should occur,
• continued research to reduce uncertainties about how much change will occur and what effects it will have, and
• R&D on energy supply and end-use technologies to reduce the costs of limiting greenhouse gas emissions.
The issue is not one of "either-or" but one of finding the right blend of options. Policy makers must decide how to divide greenhouse insurance dollars among these competing needs.
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