Synthesis Report - Question 6

Climate Change 2001: Synthesis Report

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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 projected rate and magnitude of warming and sea-level rise can be lessened by reducing greenhouse gas emissions.



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.

WGI TAR Section 3.7.3

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.

WGI TAR Section 9.3.3

Figure 6-1: Stabilizing CO2 concentrations would require substantial reductions of emissions below current levels and would slow the rate of warming.

a) CO2 emissions: The time paths of CO2 emissions that would lead to stabilization of the concentration of CO2 in the atmosphere at 450, 550, 650, 750, and 1,000 ppm are estimated for the WRE stabilization profiles using carbon cycle models. Lower CO2 concentration levels would require an earlier reversal of emissions growth and earlier decreases to levels below current emissions. The shaded area illustrates the range of uncertainty in estimating CO2 emissions corresponding to specified concentration time paths, as represented in carbon cycle models. Also shown for comparison are CO2 emissions for three of the SRES scenarios (A1B, A2, and B1), which do not include greenhouse gas emission limits.

b) CO2 concentrations: The CO2 concentrations specified for the WRE profiles gradually approach stabilized levels that range from 450 to 1,000 ppm. Also shown for comparison are estimates of CO2 concentrations that would result from three of the SRES projections of emissions (A1B, A2, and B1).

c) Global mean temperature changes: Global mean temperature changes are estimated for the WRE stabilization profiles using a simple climate model tuned in turn to each of several more complex models.Estimated warming slows as growth in the atmospheric concentration of CO2 slows and warming continues after the time at which the CO2 concentration is stabilized (indicated by black spots) but at a much diminished rate. It is assumed that emissions of gases other than CO2 follow the SRES A1B projection until the year 2100 and are constant thereafter.This scenario was chosen as it is in the middle of the range of the SRES scenarios. The dashed lines show the temperature changes projected for the S profiles, an alternate set of CO2 stabilization profiles (not shown in panels (a) or (b)). The shaded area illustrates the effect of a range of climate sensitivity across the five stabilization cases.The colored bars on the righthand side show, for each WRE profile, the range at the year 2300 due to the different climate model tunings and the diamonds on the righthand side show the equilibrium (very long-term) warming for each stabilization level using average climate model results. Also shown for comparison are temperature increases in the year 2100 estimated for the SRES emission scenarios (indicated by red crosses).

WGI TAR Sections 3.7.3 & 9.3.3, & IPCC TP3

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