Introduction

Working Group III (WGIII) of the Intergovernmental Panel on Climate Change (IPCC) is charged with assessing scientific research related to the mitigation of climate change. ‘Mitigation’ is the effort to control the human sources of climate change and their cumulative impacts, notably the emission of greenhouse gases (GHGs) and other pollutants, such as black carbon particles, that also affect the planet's energy balance. Mitigation also includes efforts to enhance the processes that remove GHGs from the atmosphere, known as sinks (see glossary (Annex I) for definition). Because mitigation lowers the anticipated effects of climate change as well as the risks of extreme impacts, it is part of a broader policy strategy that includes adaptation to climate impacts—a topic addressed in more detail in WGII. There is a special role for international cooperation on mitigation policies because most GHGs have long atmospheric lifetimes and mix throughout the global atmosphere. The effects of mitigation policies on economic growth, innovation, and spread of technologies and other important social goals also implicate international concern because nations are increasingly inter-linked through global trade and economic competition. The economic effects of action by one nation depend, in part, on the action of others as well. Yet, while climate change is fundamentally a global issue, the institutions needed for mitigation exist at many different domains of government, including the local and national level.

This chapter introduces the major issues that arise in mitigation policy and also frames the rest of the WGIII Contribution to the AR5. First we focus on the main messages since the publication of AR4 in 2007 (Section 1.2). Then we look at the historical and future trends in emissions and driving forces, noting that the scale of the mitigation challenge has grown enormously since 2007 due to rapid growth of the world economy and the continued lack of much overt effort to control emissions.