| 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 1.4.3.5,
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 |
