2.4.1 Why and how do we characterise future conditions?
Evaluations of future climate change impacts, adaptation, and vulnerability require assumptions, whether explicit or implicit, about how future socio-economic and biophysical conditions will develop. The literature on methods of characterising the future has grown in tandem with the literature on CCIAV, but these methods have not been defined consistently across different research communities. Box 2.1 presents a consistent typology of characterisations that expands on the definitions presented in the TAR (Carter et al., 2001), for the purpose of clarifying the use of this terminology in this chapter. Although they may overlap, different types of characterisations of the future can be usefully distinguished in terms of their plausibility and ascription of likelihood, on the one hand, and the comprehensiveness of their representation, on the other (see Box 2.1 for definitions). Since the TAR, comprehensiveness has increased and ascriptions of likelihood have become more common. The following sections make use of the typology in Box 2.1 to address notable advances in methods of characterising the future.
The most significant advance in artificial experiments since the TAR is the development of a new set of commitment runs by AOGCMs. These are climate change projections that assume that the radiative forcing at a particular point in time (often the current forcing) is held constant into the future (Meehl et al., 2007). The projections demonstrate the time-lags in the climate response to changes in radiative forcing (due to the delayed penetration of heat into the oceans), and of sea level to warming. Recent experiments estimate a global mean warming commitment
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