Climate Change 2001:
Working Group III: Mitigation
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Control of greenhouse gas concentrations implies eventual limitations on energy-related emissions
Energy is the single largest source of GHG emissions. It is responsible for approximately 80% of net carbon emissions to the atmosphere. While net emissions of carbon are associated with fossil fuel combustion, the carbon-to-energy ratio varies between high-carbon fuels, such as coal, and low-carbon fuels, such as CH4, approximately by a factor of two. Technologies such as hydroelectric power, nuclear fission, wind power, and solar power are generally treated as if they have little or no direct carbon emissions, though this may not be the case. For example, CH4 may be released in the process of creating a hydroelectric facility and carbon may be released in the manufacture of cement used in nuclear power reactors. Technologies do exist that can biologically sequester or physically remove and store carbon. Thus, in principle, controlling energy-related carbon emissions is possible for several sources of carbon emissions without foregoing fossil fuel use. These technologies are discussed in Chapter 3.

Narrowly defined technological solutions are unavailable, but a broad development and deployment of technology is key to controlling the cost of emissions limitation
Emissions of GHGs are associated with an extraordinary array of human activities. CO2 emissions are associated with the combustion of fossil fuels and changes in land-use. They are thus affected by activities that range from, for example, household heating and cooling to commercial lighting and appliances, to the transportation of goods and provision of services, to the manufacture of materials, to the growth and harvest of crops, and to the generation of electric power. As a consequence, GHG emissions are greatly affected by other exogenous and non-climate-policy factors. Narrowly defined technological solutions, such as were available to address the problem of stratospheric ozone depletion, are impossible for the climate issue. While no single technology provides a complete solution to the problem of controlling emissions of GHGs, a significant set of existing, emerging, and potential technologies is available to mitigation climate change, as discussed in Chapters 2, 3 and 42.

Policy interactions will be significant
Future emissions depend to a large degree on the rate and direction of technological developments in a broad array of human endeavours. For example, China’s policies to stabilize its population size, taken for reasons unrelated to climate change, will have a profound effect on Chinese emissions of GHGs to the atmosphere. Policies to control non-GHG air pollutants can greatly affect GHG emissions. For example, measures to substitute natural gas and non-carbon-emitting energy forms, such as solar and nuclear power, for coal in electricity generation to control local and regional air pollution can affect GHG emissions as well. On the other hand, some policies that reduce local air pollution, such as scrubbing power plants for sulphur, can reduce power-plant efficiency and increase GHG emissions.

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