CURRENT debate on policies for limiting climate change due to greenhouse-gas emissions focuses on whether to take action now or later, and on how stringent any emissions reductions should be in the near and long term. Any reductions policies implemented now will need to be revised later as scientific understanding of climate change improves. Here we consider the effects of a sequential-decision strategy (Fig. 1) consisting of a near-term period (1992-2002) during which either moderate emissions reductions (achieved by energy conservation only) or aggressive reductions (energy conservation coupled with switching to other fuel sources) are begun, and a subsequent long-term period during which a least-cost abatement policy is followed to limit global mean temperature change to an optimal target DELTA-T*. For each policy we calculate the global mean surface temperature change DELTA-T(t) using a simple climate/ocean model for climate sensitivities DELTA-T2x (the response to doubled CO2 concentrations) of 4.5, 2.5, 1.5 and 0.5-degrees-C. The policy beginning with moderate reductions is less expensive than that with aggressive reductions if DELTA-T* > 2.9, 2.1, 1.5 and 0.9-degrees-C respectively; otherwise, the aggressive-reductions policy is cheaper. We suggest that this approach should assist in choosing realistic targets and in determining how best to implement emissions reductions in the short and long term.
