Climate Change 2001: Synthesis Report

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Question 6

  1. How does the extent and timing of the introduction of a range of emissions reduction actions determine and affect the rate, magnitude, and impacts of climate change, and affect the global and regional economy, taking into account the historical and current emissions?
  2. What is known from sensitivity studies about regional and global climatic, environmental, and socio-economic consequences of stabilizing the atmospheric concentrations of greenhouse gases (in carbon dioxide equivalents), at a range of levels from today's to double that level or more, taking into account to the extent possible the effects of aerosols? For each stabilization scenario, including different pathways to stabilization, evaluate the range of costs and benefits, relative to the range of scenarios considered in Question 3, in terms of:
    • Projected changes in atmospheric concentrations, climate, and sea level, including changes beyond 100 years
    • Impacts and economic costs and benefits of changes in climate and atmospheric composition on human health, diversity and productivity of ecological systems, and socio-economic sectors (particularly agriculture and water)
    • The range of options for adaptation, including the costs, benefits, and challenges
    • The range of technologies, policies, and practices that could be used to achieve each of the stabilization levels, with an evaluation of the national and global costs and benefits, and an assessment of how these costs and benefits would compare, either qualitatively or quantitatively, to the avoided environmental harm that would be achieved by the emissions reductions
    • Development, sustainability, and equity issues associated with impacts, adaptation, and mitigation at a regional and global level

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 the rise in sea levels. Future climate change is determined by historic, current, and future emissions. Differences in projected temperature changes between scenarios that include greenhouse gas emission reductions and those that do not tend to be small for the first few decades but grow with time if the reductions are sustained.

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 the year 1990 levels, within a few decades, about a century, or about 2 centuries, respectively, and continue to decrease steadily thereafter (see Figure SPM-6). 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. 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. Q6.4

There is a wide band of uncertainty in the amount of warming that would result from any stabilized greenhouse gas concentration. This results from the factor of three uncertainty in the sensitivity of climate to increases in greenhouse gases.4 Figure SPM-7 shows eventual CO2 stabilization levels and the corresponding range of temperature change estimated to be realized in 2100 and at equilibrium.

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

  1. CO2 emissions: The time paths of CO2 emissions that would lead to stabilization of the concentration of CO2 in the atmosphere at various levels are estimated for the WRE stabilization profiles using carbon cycle models. The shaded area illustrates the range of uncertainty.
  2. CO2 concentrations: The CO2 concentrations specified for the WRE profiles are shown.
  3. Global mean temperature changes: Temperature changes are estimated using a simple climate model for the WRE stabilization profiles. 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 SRES scenarios. The dashed lines show the temperature changes projected for the S 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 uncertainty for each stabilization case at the year 2300. The diamonds on the righthand side show the average equilibrium (very long-term) warming for each CO2 stabilization level.Also shown for comparison are CO2 emissions, concentrations, and temperature changes for three of the SRES scenarios.




Q6 Figure 6-1

Emission reductions that would eventually stabilize the atmospheric concentration of CO2 at a level below 1,000 ppm, based on profiles shown in Figure SPM-6, and assuming that emissions of gases other than CO2 follow the SRES A1B projection until the year 2100 and are constant thereafter, are estimated to limit global mean temperature increase to 3.5°C or less through the year 2100. Global average surface temperature is estimated to increase 1.2 to 3.5°C by the year 2100 for profiles that eventually stabilize the concentration of CO2 at levels from 450 to 1,000 ppm. Thus, although all of the CO2 concentration stabilization profiles analyzed would prevent, during the 21st century, much of the upper end of the SRES projections of warming (1.4 to 5.8°C by the year 2100), it should be noted that for most of the profiles the concentration of CO2 would continue to rise beyond the year 2100. The equilibrium temperature rise would take many centuries to reach, and ranges from 1.5 to 3.9°C above the year 1990 levels for stabilization at 450 ppm, and 3.5 to 8.7°C above the year 1990 levels for stabilization at 1,000 ppm.5 Furthermore, for a specific temperature stabilization target there is a very wide range of uncertainty associated with the required stabilization level of greenhouse gas concentrations (see Figure SPM-7). The level at which CO2 concentration is required to be stabilized for a given temperature target also depends on the levels of the non-CO2 gases. Q6.6
Sea level and ice sheets would continue to respond to warming for many centuries after greenhouse gas concentrations have been stabilized. The projected range of sea-level rise due to thermal expansion at equilibrium is 0.5 to 2 m for an increase in CO2 concentration from the pre-industrial level of 280 to 560 ppm and 1 to 4 m for an increase in CO2concentration from 280 to 1,120 ppm. The observed rise over the 20th century was 0.1 to 0.2 m. The projected rise would be larger if the effect of increases in other greenhouse gas concentrations were to be taken into account. There are other contributions to sea-level rise over time scales of centuries to millennia. Models assessed in the TAR project sea-level rise of several meters from polar ice sheets (see Question 4) and land ice even for stablization levels of 550 ppm CO2-equivalent. Q6.8

Reducing emissions of greenhouse gases to stabilize their atmospheric concentrations would delay and reduce damages caused by climate change.

Greenhouse gas emission reduction (mitigation) actions would lessen the pressures on natural and human systems from climate change. Slower rates of increase in global mean temperature and sea level would allow more time for adaptation. Consequently, mitigation actions are expected to delay and reduce damages caused by climate change and thereby generate environmental and socio-economic benefits. Mitigation actions and their associated costs are assessed in the response to Question 7. Q6.10

Mitigation actions to stabilize atmospheric concentrations of greenhouse gases at lower levels would generate greater benefits in terms of less damage. Stabilization at lower levels reduces the risk of exceeding temperature thresholds in biophysical systems where these exist. Stabilization of CO2 at, for example, 450 ppm is estimated to yield an increase in global mean temperature in the year 2100 that is about 0.75 to 1.25°C less than is estimated for stabilization at 1,000 ppm (see Figure SPM-7). At equilibrium the difference is about 2 to 5°C. The geographical extent of the damage to or loss of natural systems, and the number of systems affected, which increase with the magnitude and rate of climate change, would be lower for a lower stabilization level. Similarly, for a lower stabilization level the severity of impacts from climate extremes is expected to be less, fewer regions would suffer adverse net market sector impacts, global aggregate impacts would be smaller, and risks of large-scale, high-impact events would be reduced.

Figure SPM-7: Stabilizing CO2 concentrations would lessen warming but by an uncertain amount. Temperature changes compared to year 1990 in (a) year 2100 and (b) at equilibrium are estimated using a simple climate model for the WRE profiles as in Figure SPM-6. The lowest and highest estimates for each stabilization level assume a climate sensitivity of 1.7 and 4.2°C, respectively. The center line is an average of the lowest and highest estimates.








Q6 Figure 6-2

Comprehensive, quantitative estimates of the benefits of stabilization at various levels of atmospheric concentrations of greenhouse gases do not yet exist.Advances have been made in understanding the qualitative character of the impacts of climate change. Because of uncertainty in climate sensitivity, and uncertainty about the geographic and seasonal patterns of projected changes in temperatures, precipitation, and other climate variables and phenomena, the impacts of climate change cannot be uniquely determined for individual emission scenarios. There are also uncertainties about key processes and sensitivities and adaptive capacities of systems to changes in climate. In addition, impacts such as the changes in the composition and function of ecological systems, species extinction, and changes in human health, and disparity in the distribution of impacts across different populations, are not readily expressed in monetary or other common units. Because of these limitations, the benefits of different greenhouse gas emission reduction actions, including actions to stabilize greenhouse gas concentrations at selected levels, are incompletely characterized and cannot be compared directly to mitigation costs for the purpose of estimating the net economic effects of mitigation. Q6.12

Adaptation is a necessary strategy at all scales to complement climate change mitigation efforts. Together they can contribute to sustainable development objectives.

Adaptation can complement mitigation in a cost-effective strategy to reduce climate change risks.  Reductions of greenhouse gas emissions, even stabilization of their concentrations in the atmosphere at a low level, will neither altogether prevent climate change or sea-level rise nor altogether prevent their impacts. Many reactive adaptations will occur in response to the changing climate and rising seas and some have already occurred. In addition, the development of planned adaptation strategies to address risks and utilize opportunities can complement mitigation actions to lessen climate change impacts. However, adaptation would entail costs and cannot prevent all damages. The costs of adaptation can be lessened by mitigation actions that will reduce and slow the climate changes to which systems would otherwise be exposed. Q6.14-15
The impact of climate change is projected to have different effects within and between countries. The challenge of addressing climate change raises an important issue of equity. Mitigation and adaptation actions can, if appropriately designed, advance sustainable development and equity both within and across countries and between generations. Reducing the projected increase in climate extremes is expected to benefit all countries, particularly developing countries, which are considered to be more vulnerable to climate change than developed countries. Mitigating climate change would also lessen the risks to future generations from the actions of the present generation. Q6.16-18

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