This question focuses on the potential for, and costs of, mitigation both in the near and long term. The issue of the primary mitigation benefits (the avoided costs and damages of slowing climate change) is addressed in Questions 5 and 6, and that of ancillary mitigation benefits is addressed in this response and the one to Question 8. This response describes a variety of factors that contribute to significant differences and uncertainties in the quantitative estimates of the costs of mitigation options. The SAR described two categories of approaches to estimating costs: bottom-up approaches, which often assess near-term cost and potential, and are built up from assessments of specific technologies and sectors; and top-down approaches, which proceed from macro-economic relationships. These two approaches lead to differences in the estimates of costs, which have been narrowed since the SAR. The response below reports on cost estimates from both approaches for the near term, and from the top-down approach for the long term. Mitigation options and their potential to reduce greenhouse gas emissions and sequester carbon are discussed first. This is followed by a discussion of the costs for achieving emissions reductions to meet near-term emissions constraints, and long-term stabilization goals, and the timing of reductions to achieve such goals. This response concludes with a discussion of equity as it relates to climate change mitigation.

Significant technological and biological potential exists for near-term mitigation.

Significant technical progress relevant to greenhouse gas emissions reduction has been made since the SAR, and has been faster than anticipated. Advances are taking place in a wide range of technologies at different stages of evelopment -- for example, the market introduction of wind turbines; the rapid elimination of industrial by-product gases, such as N₂O from adipic acid production and perfluorocarbons from aluminum production; efficient hybrid engine cars; the advancement of fuel cell technology; and the demonstration of underground CO₂ storage. Technological options for emissions reduction include improved efficiency of end-use devices and energy conversion technologies, shift to zero- and low-carbon energy technologies, improved energy management, reduction of industrial by-product and process gas emissions, and carbon removal and storage. Table 7-1 summarizes the results from many sectoral studies, largely at the project, national, and regional level with some at the global level, providing estimates of potential greenhouse gas emissions reductions to the 2010 and 2020 time frame.

Forests, agricultural lands, and other terrestrial ecosystems offer significant carbon mitigation potential. Conservation and sequestration of carbon, although not necessarily permanent, may allow time for other options to be further developed and implemented (see Table 7-2). Biological mitigation can occur by three strategies: a) conservation of existing carbon pools, b) sequestration by increasing the size of carbon pools,¹³ and c) substitution of sustainably produced biological products (e.g., wood for energy-intensive construction products and biomass for fossil fuels). Conservation of threatened carbon pools may help to avoid emissions, if leakage can be prevented, and can only become sustainable if the socio-economic drivers for deforestation and other losses of carbon pools can be addressed. Sequestration reflects the biological dynamics of growth, often starting slowly, passing through a maximum, and then declining over decades to centuries. The potential of biological mitigation options is on the order of 100 Gt C (cumulative) by the year 2050, equivalent to about 10 to 20% of projected fossil-fuel emissions during that period, although there are substantial uncertainties associated with this estimate. Realization of this potential depends upon land and water availability as well as the rates of adoption of land management practices. The largest biological potential for atmospheric carbon mitigation is in subtropical and tropical regions.