The Earth’s atmosphere is changing due to anthropogenic increases of a range of gases and aerosols that influence the planetary energy budget. Policy has long been challenged to ensure that instruments such as the Kyoto Protocol or carbon trading deal with the wide range of lifetimes of these radiative forcing agents. Recent research has sharpened scientific understanding of the differences between various metrics used to compare emissions of different gases; as a result, there has been an improved understanding of how climate system time scales interact with the time scales of the forcing agents themselves. This has led to consideration of new metrics such as cumulative carbon, and recognition that short-lived forcing agents can ‘trim the peak’ of coming climate change, while long-lived agents, especially carbon dioxide, will be responsible for at least a millennium of elevated temperatures and altered climate, even if emissions were to cease. We suggest that these vastly differing characteristics imply that a single basket for trading among forcing agents is incompatible with current scientific understanding.
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