equivalent level of atmospheric carbon dioxide, also known as the CO2-equivalent concentration.

APPROACH

The goal and implications of stabilizing climate change are most often discussed in terms of stabilizing atmospheric concentrations of CO2. This report takes a different approach by (1) using global temperature change as the frame of reference and (2) focusing in part on the relationship between accumulated carbon emissions and global mean temperature change.

The motivation for this approach is both practical and conceptual. Available data and modeling suggest that the magnitudes of many key impacts can be quantified for given amounts of global warming through scaling of local to global warming and through coupled linkages to warming (such as alterations in the water cycle that scale with warming). But although published analyses of future climate impacts can be tied to specific warming levels in particular studies, this information often cannot readily be linked to CO2-equivalent concentrations (because, for example, of lack of information on aerosol forcing used in many future climate impact studies based on emission scenarios).

Moreover, using warming as the frame of reference provides a picture of impacts and their associated uncertainties in a warming world—uncertainties that are distinct from the uncertainties in the relationship of CO2-equivalent concentrations to warming. Use of warming as a metric of change also permits coverage of the transient climate changes and impacts while concentrations increase, as well as the lock-in to further changes after stabilization. Further, the approach taken here facilitates cataloging ranges of impacts that should be expected for 1ºC, 2ºC, 3ºC, or other levels of warming. The reader can thus consider how much warming s/he considers to be an appropriate target. Information is also provided to translate warming into best estimates of associated CO2-equivalent target concentrations with these best estimates accompanied by estimated likely uncertainty ranges derived from uncertainty in climate sensitivity.

Furthermore, this report also describes the cumulative carbon framework, a perspective that has recently received considerable attention. Rather than CO2-equivalent concentration levels, this approach considers the amount of carbon emissions accumulated over time and the implications of different accumulated emissions targets. Models consistently suggest a persistent temperature response to a given level of cumulative carbon emissions. Accumulated carbon emission targets link to impacts through temperature (or



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