Most AR4 studies of future climate change are based on a small number of studies using SRES scenarios, especially the A2 and B2 families [2.3.1]. This has allowed some limited, but incomplete, characterisation of the potential range of futures and their impacts [see Section 4 on key future impacts in all core chapters].
Scenarios are required:
• to describe the future evolution of the world under different and wide-ranging assumptions about how societies, governance, technology, economies will develop in future;
• at the regional and local scales appropriate for impacts analysis;
• which allow adaptation to be incorporated into climate-change impact estimates;
• for abrupt climate change such as the collapse of the North Atlantic Meridional Overturning Circulation, and large sea-level rises due to ice sheet melting [6.8];
• for beyond 2100 (especially for sea-level rise) [6.8,11.8.1].
Increasingly, climate modellers run model ensembles which allow characterisation of the uncertainty range for each development pathway. Thus, the impacts analyst is faced with very large quantities of data to capture even a small part of the potential range of futures. Tools and techniques to manage these large quantities of data are urgently required [2.3, 2.4].
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