The climatic, environmental, and socio-economic consequences of greenhouse gas emissions were assessed in Question 3 for scenarios that do not include any climate policy interventions. These same issues are addressed here in Question 6, but this time to assess the benefits that would result from a set of climate policy interventions. Among the emission reduction scenarios considered are scenarios that would achieve stabilization of CO₂ concentrations in the atmosphere. The role of adaptation as a complement to mitigation and the potential contributions of reducing emissions to the goals of sustainable development and equity are evaluated. The policies and technologies that might be used to implement the emission reductions and their costs are considered in Question 7.

The greater the reductions in emissions and the earlier they are introduced, the smaller and slower the projected warming and rise in sea levels. Future climate change is determined by historic, current, and future emissions. Estimates have been made of the global mean temperature and sea-level rise effects of a 2% per year reduction in CO₂ emissions by developed countries over the period 2000 to 2100, assuming that developing countries do not reduce their emissions.⁶ Under these assumptions, global emissions and the atmospheric concentration of CO₂ grow throughout the century but at a diminished rate compared to scenarios that assume no actions to reduce developed country emissions. The effects of the emission limit accrue slowly but build with time. By the year 2030, the projected concentration of CO₂ in the atmosphere is reduced roughly 20% relative to the IS92a scenario of unabated emissions, which diminishes warming and sea-level rise by a small amount within this time frame. By the year 2100, the projected CO₂ concentration is reduced by 35% relative to the IS92a scenario, projected global mean warming reduced by 25%, and projected sea-level rise reduced by 20%. Analyses of CO₂ emission reductions of 1% per year by developed countries indicate that the lesser reductions would yield smaller reductions in CO₂ concentration, temperature change, and sea-level rise. Actions such as these taken now would have a greater effect at the year 2100 than the same emissions reductions implemented at a later time.

Reductions in greenhouse gas emissions and the gases that control their concentration would be necessary to stabilize radiative forcing. For example, for the most important anthropogenic greenhouse gas, carbon cycle models indicate that stabilization of atmospheric CO₂ concentrations at 450, 650, or 1,000 ppm would require global anthropogenic CO₂ emissions to drop below year 1990 levels within a few decades, about a century, or about 2 centuries, respectively, and continue to decrease steadily thereafter (see Figure 6-1). These models illustrate that emissions would peak in about 1 to 2 decades (450 ppm) and roughly a century (1,000 ppm) from the present (see Table 6-1). Eventually CO₂ emissions would need to decline to a very small fraction of current emissions. The benefits of different stabilization levels are discussed later in Question 6 and the costs of these stabilization levels are discussed in Question 7.

There is a wide band of uncertainty in the amount of warming that would result from any stabilized greenhouse gas concentration. Estimates of global mean temperature change for scenarios that would stabilize the concentration of CO₂ at different levels, and hold them constant thereafter, are presented in Figure 6-1c. The uncertainty about climate sensitivity yields a wide range of estimates of temperature change that would result from emissions corresponding to a selected concentration level.⁷ This is shown more clearly in Figure 6-2, which shows eventual CO₂ concentration stabilization levels and the corresponding range of temperature change that is estimated to be realized in the year 2100 and at long-run equilibrium. To estimate temperature changes for these scenarios, it is assumed that emissions of greenhouse gases other than CO₂ would follow the SRES A1B scenario until the year 2100 and that emissions of these gases would be constant thereafter. Different assumptions about emissions of other greenhouse gases would result in different estimates of warming for each CO₂ stabilization level.