temperatures and Just as for the past, so for the future it is necessary to define the main timeframe of concern. This is, to some degree, predetermined by the approach used in the IPCC TAR (2001), the results of which provide points of reference and departure for much that follows in this and subsequent chapters. Most of the effort involved in that exercise has gone into establishing scenarios and projections through to 2100. A firm cut-off is, however, unrealistic, for what happens between now and then will have a huge influence on the subsequent course of events.

Future changes in global mean temperature

Dealing with uncertainties

Schneider (2002) points out that uncertainties, including both statistical and intrinsic uncertainties, cascade through multiplication into ever-widening ranges (Figure 13.1). Nevertheless, he strongly favours strenuous attempts to attach probabilities to alternative future scenarios. One of his key points counters the distinction made between biophysical and social systems by Grubler and Nakicenovic (2001). Schneider claims that in considerations of uncertainty there is no essential difference between ‘natural’ and ‘social’ systems in that they both include feedbacks and are to a greater or lesser extent path-dependent. He acknowledges, however, that social systems are harder to predict. Perhaps the best solution is to accept that there is a spectrum of predictability. At one extreme are systems characterised by such contingency-linked indeterminacy that probabilistic statements must remain entirely judgement-based and subjective.