Climate Change 2001:
Synthesis Report
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6.9 Reducing emissions of greenhouse gases to stabilize their atmospheric concentrations would delay and reduce damages caused by climate change.

6.10 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.

WGII TAR Sections 1.4.3, 18.8, & 19.5
6.11 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 6-2). 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 6-3 presents a summary of climate change risks or reasons for concern (see Box 3-2) juxtaposed against the ranges of global mean temperature change in the year 2100 that have been estimated for different scenarios.11

WGI TAR Section 9.3.3 & WGII TAR Sections, 5.2, 5.4, & 19.3-6
6.12 Comprehensive, quantitative estimates of the benefits of stabilization at various levels of atmospheric concentrations of greenhouse gases do not yet exist. While advances have been made in understanding the qualitative character of the impacts of future climate change, the impacts that would result under different scenarios are incompletely quantified. Because of uncertainty in climate sensitivity, and uncertainty about the geographic and seasonal patterns of 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 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 and regions, are not readily expressed in monetary or other common units. Because of these limitations, the benefits of different greenhouse gas 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.

WGII TAR Sections 19.4-5
Figure 6-3: Risks of climate change damages would be reduced by stabilizing CO2 concentration. The risks of adverse impacts from climate change are depicted for different magnitudes of global mean temperature change, where global mean temperature change is used as a proxy for the magnitude of climate change. Estimates of global mean temperature change by the year 2100 relative to the year 1990 are shown on the righthand side of the figure for scenarios that would lead to stabilization of the atmospheric concentration of CO2 , as well as for the full set of SRES projections. Many risks associated with warming above 3.5°C by the year 2100 would be avoided by stabilizing CO2 concentration at or below 1,000 ppm. Stabilization at a lower level would reduce risks further.White indicates neutral or small negative or small positive impacts or risks; yellow indicates negative impacts for some systems or low risks; and red means negative impacts or risks that are more widespread and/or greater in magnitude. The assessment of impacts or risks takes into account only the magnitude of change and not the rate of change. Global mean annual temperature change is used as a proxy for the magnitude of climate change, but impacts would be a function of, among other factors, the magnitude and rate of global and regional changes in mean climate, climate variability and extreme climate phenomena, social and economic conditions, and adaptation.

WGI TAR Section 9.3.3 & WGII TAR Section 19.8.2

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