Technical Summary

�Mitigation�, in the context of climate change, is a human intervention to reduce the sources or enhance the sinks of greenhouse gases (GHGs). One of the central messages from Working Groups I and II of the Intergovernmental Panel on Climate Change (IPCC) is that the consequences of unchecked climate change for humans and natural ecosystems are already apparent and increasing. The most vulnerable systems are already experiencing adverse effects. Past GHG emissions have already put the planet on a track for substantial further changes in climate, and while there are many uncertainties in factors such as the sensitivity of the climate system many scenarios lead to substantial climate impacts, including direct harms to human and ecological well-being that exceed the ability of those systems to adapt fully. Because mitigation is intended to reduce the harmful effects of climate change, it is part of a broader policy framework that also includes adaptation to climate impacts. Mitigation, together with adaptation to climate change, contributes to the objective expressed in Article 2 of the United Nations Framework Convention on Climate Change (UNFCCC) to stabilize �greenhouse gas concentrations in the atmosphere at a level to prevent dangerous anthropogenic interference with the climate system [�] within a time frame sufficient to allow ecosystems to adapt [�] to ensure that food production is not threatened and to enable economic development to proceed in a sustainable manner�. However, Article 2 is hard to interpret, as concepts such as �dangerous� and �sustainable� have different meanings in different decision contexts (see Box TS.1). Moreover, natural science is unable to predict precisely the response of the climate system to rising GHG concentrations nor fully understand the harm it will impose on individuals, societies, and ecosystems. Article 2 requires that societies balance a variety of considerations�some rooted in the impacts of climate change itself and others in the potential costs of mitigation and adaptation