Climate Change 2001:
Working Group III: Mitigation
Other reports in this collection Tolerable Windows and Safe Landing Approaches

Considerable work since the SAR has explored the implications for global emissions of GHGs of a set of constraints on a variety of associated phenomena. This vein of research is referred to as the tolerable windows and/or safe landing (TWSL) approach. See, for example, Alcamo and Kreileman (1996a, 1996b) and Swart et al. (1998) for early work on the safe landing approach and Toth et al. (1997) for early work on the tolerable windows approach. The approach seeks to limit the emissions time-paths with implications for the near term and long term. While the tolerable windows and safe landing analyses differ somewhat in the detail of their implementation, they are similar in approach. We consider the safe landing approach first. In a multimodel exercise four constraints on emissions trajectories are considered: temperature change since 1990, maximum decadal rate of temperature change, sea level rise between 1990 and 2100, and maximum rate of sea level change. In addition, a limit on the rate of reduction of emissions is set.

Criteria Low Medium High
Change in temperature from 1990 1.0°C 1.5°C 2.0°C
Decadal change in temperature 0.10°C 0.15°C 0.20°C
Change in sea level 20cm 30cm 40cm
Decadal change in sea level 2cm 3cm 4cm
Maximum reduction in emissions 2% 3% 4%

The safe landing interval is the range of emissions, given in CO2 equivalent emissions (Ceq), in 2010. This range is 7.6–11.9GtCeq; 1990 emissions were 7.10GtC, and approximately 9.8GtCeq, equivalent, defined in terms of CO2, CH4, and N2O only (Pitcher, 1999). Emissions for Annex I nations can be derived by subtracting the anticipated non-Annex I emissions from the global total.

Figure 10.4:
Emission corridors under different assumptions on delaying reduction measures and equity principles.

Results from the analysis depend strongly on the constraints and model sensitivities. The tolerable windows approach (Toth et al., 1997, 1999; Bruckner et al., 1999; Petschel-Held et al., 1999) is formulated as a type of extended and generalized cost–benefit analysis for which two kinds of normative inputs are required. First, with the help of climate-impact response functions that depict reactions of climate-sensitive socioeconomic and natural systems to climate change forcing, social actors can specify their willingness to accept a certain amount of climate change in their own jurisdiction. Second, the same social actors reveal their willingness to pay for climate change mitigation in terms of acceptable burden-sharing principles and implementation schemes internationally, as well as in terms of tolerable utility, consumption, or Gross Domestic Product (GDP) loss in their own jurisdiction. An integrated climate-economy model (e.g., Integrated Assessment of Climate Protection Strategies - ICLIPS) can then determine whether there exists a corridor of emission paths over time that keeps the climate system within the permitted domain.

If the corridor does not exist, a willingness to accept more climate change can be specified (e.g., as a result of resource transfers to increase the adaptive capacity in the most constraining region or sector on the impact side). Alternatively, willingness to pay for emission reductions can be increased or more cost-reducing flexibility instruments can be allowed on the mitigation side. If the corridor does exist, it can be perceived as the room to manoeuvre for global climate policy over the long term. The tolerable windows approach leaves the specification of climate-change mitigation regimes up to decision makers involved in climate-change policy making at the global and national levels. The primary goal of the ICLIPS integrated assessment model (IAM) is to determine the implications of different equity principles in burden sharing and of various implementation mechanisms on the existence and shape of the emission corridor. Nevertheless, the model can also produce cost-effective emission paths.

The German Advisory Council on Global Change (WBGU) proposed two climate change constraints based on geohistorical arguments: the tolerable magnitude of climate change is set to 2°C compared to the pre-industrial era4 and the rate of temperature increase should not exceed 0.2 °C per decade. On the cost side, it is assumed that to reduce GHG emissions at a rate faster than 4%/yr would be economically too painful to implement. These constraints are used to illustrate the application of the tolerable windows approach. The results presented here are based on an extended atmospheric chemistry–climate model. In addition to CO2, the model also includes CH4, N2O, chlorofluorocarbons (CFCs), and aerosols. One simplifying assumption is that all GHG emissions are reduced at the same rate, except for CFCs, which follow the IPCC IS92a scenario paths. For simplicity, energy-related global CO2 emissions are presented in Figure 10.4.

Figure 10.4(a) presents the basic emission corridor for the WBGU window. It follows from the mathematical formulation of the model that at least one permitted emission path passes through any arbitrary point in the corridor. However, not every arbitrary path within the corridor is necessarily a permitted path. If emissions follow the upper boundary of the corridor in the first few decades after 1995, for example, this would entail a sharp turnaround and persistent emission reductions at the maximum annual rate (4%/yr) for many decades to come.

How do near-term emissions affect the available flexibility over the long-term? The scenario presented in Figure 10.4(b) shows this. Here it is assumed simply that CO2 emissions follow the baseline path according to the IPCC IS92a scenario until 2010. The result is a much narrower corridor: it implies that the likelihood of a fast turnaround of emissions and persistent reductions at relatively higher rates (3%–4%/yr) is significantly higher.

The next analysis illustrates the implications of a fairness principle for the Annex I emission corridor. The assumption is that GHG emissions by non-Annex I countries follow the baseline path and these countries start emission reductions only when their per capita emissions reach those of Annex I levels on the basis of their 1992 populations. The resultant Annex I corridor is presented in Figure 10.4(c). Obviously, the result is a relatively narrow corridor.

Figure 10.4(d) shows the resultant emission corridor if the above two assumptions about future emissions are combined. This implies that the world community follows the baseline emission path until 2010 and reduction obligations will be distributed between the Annex I and non-Annex I countries according to the case in Figure 10.4(c). The result for Annex I countries emissions through the first half of the 21st century looks like a straightjacket rather than an emission corridor with ample choice.

Importantly, Annex I corridors in Figures 10.4(c) and 10.4(d) reflect the rigid implementation of emission quotas that result from the specified equity principle. No cost divergence is considered between Annex I and non-Annex I. The difference between Figures 10.4(a) and 10.4(c) corridors indicates the potential to reduce abatement costs if Annex I countries are allowed to “buy” part of the non-Annex I corridor. The economic value of this transaction is the subject of many detailed energy-economy models (see Section 10.4).

It is clear that all these emission corridors are associated with the global climate window as specified by the Council. It is beyond the scope of this analysis to discuss arguments for and against whether the 2°C increase in global mean temperature above the pre-industrial level and the rate of temperature increase at no more than 0.2°C per decade are preferred or realistic propositions. The objective for the tolerable windows approach is to provide an assessment framework that can help test any climate protection proposal formulated through selected climate attributes. The computed emission corridors, nevertheless, can assist in deciding the magnitude and urgency of the policy measures associated with them, and/or trigger rethinking the originally proposed climate change targets. The presented example also shows how equity concerns can be analyzed in the tolerable windows approach, albeit in a terse form.

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