In this section a selected set of key policy-relevant scientific questions is examined in some detail. It surveys new developments and new results to foster our ability to make critical choices in climate policy, such as striking the right balance between mitigation and adaptation, the timing and location of actions, the costs of actions, and options to reduce and share them. After a brief discussion of the broad climate policy portfolio, the focus is on mitigation questions. The issues involved in these policy responses are structured as follows.

What should the response be? What are the most important factors to consider in crafting a short- to medium-term portfolio of mitigation and adaptation actions, and in acquiring information to resolve the large uncertainties? Drawing largely on IAMs, Section 10.4.2 takes a closer look at the first two components.

When should the response be made? The relationship between the timing of various types of mitigation responses, their costs, and their social, economic, technological, and environmental implications, raises a broad array of policy issues. The most important insights are summarized in Section 10.4.3.

Where should the response take place? Closely related to the timing issue, the location of mitigation responses is a multifacetted concern also. While the environmental value of a given amount of unreleased GHG is equal wherever its abatement takes place, there are far-reaching implications of whether and to what extent nations are allowed to use international flexibility instruments. The questions range from cost and efficiency concerns, to incentives for technological development, to implementation and verification problems. Section 10.4.4 summarizes some of the aspects.

Who should pay for the response? The location of the mitigation action can largely be separated from the question of who carries the costs. Numerous guidelines have been proposed for burden sharing. They range from historical responsibility, to various equity principles, to efficiency and international competitiveness concerns. Some fundamental points are reviewed in Section 10.4.5.

Towards what objective should the response be targetted? Current analyses of climate change impacts, adaptations, and mitigation normally cover the range between 450 and 850ppmv CO₂-equivalent concentration or an increase of between 1°C and 6°C in the global mean temperature. Completing the circle that started with the discussion of how the costs and benefits of balancing mitigation and adaptation activities influence the choice of the climate and/or GHG stabilization target, the issue of high versus low levels of stabilization is raised again in Section 10.4.6.

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