Subject to uncertainties concerning the "climate sensitivity", future anthropogenic climate change is determined by the sum of all positive and negative radiative forcings arising from all anthropogenic greenhouse gases and aerosols, and not by the level of CO2 alone. The forcing scenarios considered here use the sum of the radiative forcings of all the trace gases (CO2, CH4, ozone (O3), etc.) and aerosols. The total forcing may be treated as if it came from an "equivalent" concentration of CO2 Therefore, the "equivalent CO2" concentration is the concentration of CO2 that would cause the same amount of global mean radiative forcing as the given mixture of CO2, other greenhouse gases, and aerosols.

1990 2000 2010 2020 2030 2040 2050 2060 2079 2080 2090 2100 Year

Figure 2. (a) Projected global mean temperature when the concentration of CO2 is stabilized following the S profiles and the WRES50 and 1 000 profiles. CH4, N2O and SO2 emissions are assumed to remain constant at their 1990 levels and halocarbons follow an emissions scenario consistent with compliance with the Montreal Protocol. The climate sensitivity is assumed to be the mid-range value of 2.5°C. For comparison, results for the IS92a, c and e emissions scenarios are shown for the year 2100. The values are shown relative to 1990; to obtain the anthropogenic change from pre-industrial times, a further 0.3-0.7°C should be added; (b) As for (a), but for global sea level change using central ice-melt parameters.

Because the effects of greenhouse gases are additive, stabilization of CO2 concentrations at any level above about 500 ppmv is likely to result in atmospheric changes equivalent to at least a doubling of the pre-industrial CO2 level.

Temperature and sea level projections depend on the assumed climate sensitivity, the target and pathway chosen for CO2 concentration stabilization, and the assumed scenarios for other greenhouse gases and aerosol forcing. For the mid-range climate sensitivity of 2.5°C, global mean temperature increases from 1990 for reference stabilization cases, in which the emissions of non-CO2 gases and SO2 are assumed to remain constant at their 1990 levels, range from 0.5 to 2.0°C by the year 2100 (Figure 2). For increases from pre-industrial times, 0.3 to 0.7°C should be added. Rates of temperature change over the next fifty years range from 0.1 to 0.2°C/decade. Projections of sea level rise from 1990 to 2100 range from 25 to 49 cm (Figure 2), for mid-range climate sensitivity and ice-melt parameter values. Temperature and sea level projections are sensitive to assumptions about other gases and aerosols.

This paper is presented in terms of the temperature and sea level changes that might result from different greenhouse gas stabilization levels. However, it would be possible, given further work, to deduce the greenhouse gas stabilization levels required to meet specific policy objectives in terms of temperature or sea level change targets, which are more readily related to climate change impacts.

Impacts of climate change

A great deal is known about the potential sensitivity and vulnerability of particular systems and sectors; both substantial risks and potential benefits can be identified. Currently, however, our ability to integrate this information into an assessment of impacts associated with different stabilization levels or emissions trajectories is relatively limited.

While the regional patterns of future climate change are poorly known, it is clear that the altered patterns of radiative forcing associated with anthropogenic emissions will alter regional climates noticeably, and will have different effects on climate conditions in different regions. These local and regional changes include changes in the lengths of growing seasons, the availability of water, and the incidence of disturbance regimes (extreme high temperature events, floods, droughts, fires and pest outbreaks), which, in turn, will have important impacts on the structure and function of both natural and human-made environments. Systems and activities that are particularly sensitive to climate change and related changes in sea level include: forests; mountain, aquatic and coastal ecosystems; hydrology and water resource management (including the cryosphere); food and fibre production; buman infrastructure and buman health.

Impacts are not a linear function of the magnitude and rate of climate change. For some species (and hence systems).

thresholds of change in temperature, precipitation or other factors may exist, which, once exceeded, may lead to discontinuous changes in viability, structure or function. The aggregation of impacts to produce a global assessment is not currently possible because of uncertainties regarding regional climate changes and regional responses, the difficulty of valuing impacts on natural systems and human health, and issues related to both interregional and intergenerational equity.

The ultimate concentration of greenhouse gases reached in the atmosphere, as well as the speed at which concentrations increase, is likely to influence impacts, because a slower rate of climate change will allow more time for systems to adapt. However, knowledge is not currently sufficient to identify clear threshold rates and magnitudes of change.

Mitigation costs of stabilizing CO2 concentrations

Factors that affect CO2 mitigation costs include:

budget" is an important determinant of mitigation costs. Lower stabilization targets require smaller carbon budgets, which require a greater degree of intervention.

The cost of stabilizing CO2 concentrations also depends on the cost of fossil fuels relative to carbon-free alternatives. The cost of meeting a stabilization target generally increases with the cost difference between fossil fuels and carbon-free alternatives. A large cost differential implies that consumers must increase their expenditures on energy significantly to reduce emissions by replacing fossil fuels with carbon-free alternatives. The cost difference between unconventional fossil fuels and carbon-free alternatives is likely to be smaller than the difference between conventional oil and gas and carbon-free alternatives. If oil and gas still contribute significantly to the energy mix at the time when global CO2 emissions must be reduced consistent with a given stabilization target, transition costs will be higher than if oil and gas compose a small part of energy use. While the cost premium for carbon-free alternatives is likely to be smaller for higher stabilization levels, we cannot predict how this cost differential will change over time. Since,

(b) The concentration target and route to stabilization, which determine the carbon budget available for emissions;

(c) The behaviour of the natural carbon cycle, which influences the emissions carbon budget available for any chosen concentration target and pathway;

(d) The cost differential between fossil fuels and carbon-free alternatives and between different fossil fuels;

(e) Technological progress and the rate of adoption of technologies that emit less carbon per unit of energy produced;

tion levels, the net effect on the transition costs for different stabilization levels is not clear.

A given concentration target may be achieved through more than one emission pathway. Emissions in the near-term may be balanced against emissions in the long-term. For a given stabilization level, there is a "budget" of allowable accumulated carbon emissions and the choice of pathway to stabilization may be viewed as a problem of how to best (i.e., with the greatest economic efficiency and least damaging impacts) allocate this carbon budget over time. The differences in the emissions path for the same stabilization level are important because costs differ among pathways. Higher early emissions decrease the options to adjust emissions later on.

(8) The degree of international cooperation, which determines the extent to which low cost mitigation options in different parts of the world are implemented; and

(h) Assumptions about the discount rate used to compare costs at different points in time.

The costs of reducing emissions depend on the emissions “baseline”, i.e., how emissions are projected to grow in the absence of policy intervention. The higher the baseline, the more carbon must be removed to meet a particular stabilization target, thus the greater the need for intervention. The costs of emissions reductions are also sensitive to the concentration stabilization target. As a first approximation, a stabilization target defines an amount of carbon that can be emitted between now and the date at which the target is to be achieved (the "carbon budget”). The size of the "carbon

ture retirement is apt to be costly. To avoid premature retirement, mitigation efforts can be spread more evenly over time and space. The cost of any stabilization target can be reduced by focusing on new investments and replacements at the end of the economic life of plant and equipment (i.e., at the point of capital stock turnover), which is a continuous processes.

The cost of a stabilization path also depends on how technology affects the cost of abating emissions at a point in time and over time. In general, the cost of an emission pathway increases with the amount of emissions that must be abated at any point in time. The technological changes needed to lower the cost of abating emissions will require a mix of measures. Greatly increased government research and development, removal of market barriers to technology development and dissemination, explicit market supports, tax incentives and appropriate emission constraints will probably act together to stimulate the technology needed to lower the costs of stabilizing atmospheric CO2

concentration.

With regard to mitigation costs, a positive discount rate lowers the present value of the costs incurred. This is because it places a lower weight on investments made in the future. Indeed, the further in the future an economic burden (bere, emission reductions) lies, the lower the present value of costs. In a wider context, discounting reduces the weight placed on future environmental impacts relative to the benefits of current energy use. Its use makes serious challenges, such as rapid switching of energy systems in the future, seem easy in terms of present dollars and may affect consideration of intergenerational equity.

Integrating information on impacts and mitigation costs

This report provides a framework for integrating information on the costs, benefits and impacts of climate change. Concentration stabilization profiles that follow "business-asusual" emissions for periods of a few to several decades should not be construed as a suggestion that no action is required for those periods. In fact, studies suggest that even in those cases of business-as-usual emissions for some period of time, actions must be taken during that time to cause emissions to decline subsequently. The strategies for developing portfolios of actions leading to immediate or eventual reductions below business-asusual are discussed below.

Numerous policy measures are available to facilitate adaptation to climate change, to reduce emissions of greenhouse gases, and to create technologies that will reduce emissions in the future. If expressed in terms of CO2 equivalent or total radiative forcing, a given stabilization level can be met through various combinations of reductions in the emissions of different gases and by enhancing sinks of greenhouse gases. Governments must decide both the amount of resources to devote to this issue and the mix of measures they believe will be most effective. IPCC WGIII (1996)2 states that significant "no-regrets"3 measures are available. Because no-regrets policies currently are beneficial, the issues facing governments are how to implement the full range of no-regrets measures and whether, and if so, when and how far to proceed beyond purely no-regrets options. The risk of aggregate net impacts due to climate change, consideration of risk aversion, and the application of the precautionary principle provide rationales for action beyond no-regrets.

2 Hereafter referred to as SAR WGIII.

3 "No regrets" measures are those whose benefits, such as reduced energy costs and reduced emissions of local/regional pollutants, equal or exceed their cost to society, excluding the benefits of climate change mitigation.