- 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?
- 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,
- 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
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.
- 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.
- CO2 concentrations: The CO2 concentrations
specified for the WRE profiles are shown.
- 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.
|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.
Reducing emissions of greenhouse gases to stabilize their
atmospheric concentrations would delay and reduce damages caused by climate
|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
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.
|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.
Adaptation is a necessary strategy at all scales to complement
climate change mitigation efforts. Together they can contribute to sustainable
|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.
|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.