Looking at the dilemmas covered in previous sections, the following conclusions emerge:

• a carefully crafted portfolio of mitigation, adaptation, and learning activities appears to be appropriate over the next few decades to hedge against the risk of intolerable magnitudes and/or rates of climate change (impact side) and against the need to undertake painfully drastic emission reductions if the resolution of uncertainties reveals that climate change and its impacts might imply high risks;
  
• the nature of the climate change problem requires that mitigation action at any level needs to start in the near term, as well as the development of appropriate adaptation strategies;
  
• emission reduction is an important form of mitigation, but the mitigation portfolio includes a broad range of other activities, including investments to develop low-cost non-carbon energy, and to improve energy efficiency and carbon management technologies to make future CO₂ mitigation inexpensive;
  
• timing and composition of mitigation measures (investment in technological development or immediate emission reductions) is highly controversial because of the technological features of energy systems, and the range of uncertainties involved with, for example, their impacts of climate change;
  
• international flexibility instruments help reduce the costs of emission reductions, but they raise a series of implementation and verification issues that need to be balanced against the cost savings;
  
• while there is a broad consensus on Pareto optimality as an efficiency principle, there is no agreement on the best equity principle for burden sharing. Efficiency and equity are important concerns in negotiating emissions limitation schemes, and they are not mutually exclusive. Therefore, equity will play an important role in determining the distribution of emissions allowances and/or within compensation schemes that follow emissions trading resulting in a disproportionately high level of burden to certain countries. Finally, it is more important to rely on politically feasible burden-sharing rules than to select one specific equity principle.

Finally, a series of potential large-scale geophysical transformations that might exert a major influence on the desired level of stabilization have been identified and examined more closely in recent years. These imply thresholds that humanity might decide not to cross because the potential impacts or even the associated risks are considered to be unacceptably high. Little is know about these thresholds today. Most recent results and the implications of the possibility of such thresholds are summarized in Chapter 19 of WGII (IPCC, 2001b). Nevertheless, currently estimated “danger zones” are in the domain of high stabilization levels for most threshold events.

Considering the special combination of features of the climate problem listed at the beginning of this chapter, it is obvious that no “once forever” solution exists. Making long-term commitments in any area where retraction is possible is problematic. Making decisions that entail long-term and possibly irreversible consequences due to long delays, inertia and similar system properties is even more difficult, especially under severe uncertainties. Therefore, as emphasized in this chapter, the most promising approach to climate policy is sequential decision-making. This process involves a regular reassessment of the long-term climate risks (net damages from a given magnitude of climate change) and their management objectives (climate or GHG concentration stabilization) in the light of newly available information. Short-term strategies are then crafted so that both GHG emissions and the underlying socioeconomic processes (resource use, technologies) evolve in a direction which makes future course corrections in any direction the least expensive. The current structure of the international climate regime is formulated in this vein: the UNFCCC provides some, albeit vague, guidelines for long term stabilization objectives while short-term goals are settled in and implemented under protocols for each budget period.

The analytical tools to support the above decision-making processes need to handle this double feature. They should provide policymakers with guidance to set long-term targets and to formulate short-term policies and measures. Some models take a long-term view to explore deep future impacts of climate change, but this must not be interpreted as suggesting optimal strategies for the next 50-100-200 years. Other models explore what are the most promising near-term policies and how to implement them. Similarly, many studies and models reviewed in this chapter consider the world as a whole or broken down into a few regions, at best. Others take a more detailed look at subnational and regional aspects. They shed light on the smaller scale implications of climate change and its management strategies, often in the context of other social concerns characterizing the country or region. Our assessment has found a healthy diversity of DAFs along both the long-term-short term and the global-local axes. Nevertheless, the analytical capacity and thus quotable results are still badly missing in most developing countries. This is probably the most severe problem to be solved by the time the world community will prepare its next climate change assessment report.

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