Furthermore, the concentration will be even higher in the century that follows. To hold CO 2 concentrations below that level means that a long-term strategy to stabilize the atmosphere must lower emissions below current levels at the end of the next century. To a first approximation, the cumulative emissions reductions over the next century for a CO 2 concentration of 550 ppmv at the end of the next century are the same, independent of the annual pattern. This means that the least-cost strategy is the one which reduces emissions when it is cheapest to do so. There are several reasons that it is cheaper to reduce emissions in future years than it is to reduce them today.
Natural sinks have a longer period over which to remove emissions early in the period of analysis.
This implies higher cumulative emissions for trajectories which stabilize at a similar concentration but with larger near-term loading. The carbon cycle dividend can be substantial. Emission reduction technologies continue to evolve, and the costs of emissions reductions can be expected to decline with time. The IPCC Working Group II has identified a wide range of promising technologies which will reduce the future costs of emissions reductions relative to those available today. The further into the future that a burden must be undertaken, the smaller the resources that would have to be set aside today to undertake it.