The timescales of ocean warming are much longer than those of surface air temperature rise. As a result, sea-level rise due to thermal expansion is expected to continue at a significant rate for centuries, even if climate forcing is stabilised (Meehl et al., 2005; Wigley, 2005). Deglaciation of small land-based glaciers, and possibly the Greenland and the West Antarctic ice sheets, may contribute large additional rises, with irreversible melting of Greenland occurring for a sustained global temperature rise of 1.1 to 3.8°C above today's global average temperature: this is likely to happen by 2100 under the A1B scenario, for instance (Meehl et al., 2007). More than 10 m of sea-level rise is possible, albeit over very long time spans (centuries or longer), and this has been termed 'the commitment to sea-level rise'. The potential exposure to these changes, just based on today's socioeconomic conditions, is significant both regionally and globally (Table 6.12) and growing (Section 6.3.1). Thus there is a conflict between long-term sea-level rise and present-day human development patterns and migration to the coast (Nicholls et al., 2006).
The rate of sea-level rise is uncertain and a large rise (>0.6 m to 0.7 m/century) remains a low probability/high impact risk (Meehl et al., 2007). Some analyses suggest that protection would be an economically optimum response in most developed locations, even for an arbitrary 2 m/century scenario (Anthoff et al., 2006). However, sea-level rise will accumulate beyond 2100, increasing impact potential (Nicholls and Lowe, 2006). Further, there are several potential constraints to adaptation which are poorly understood (Section 6.4.3; Nicholls and Tol, 2006; Tol et al., 2006). This raises long-term questions about the implications of 'hold the line' versus 'retreat the line' adaptation policies and, more generally, how best to approach coastal spatial planning. While shoreline management is starting to address such issues for the 21st century (Eurosion, 2004; Defra, 2006), the long timescales of sea-level rise suggest that coastal management, including spatial planning, needs to take a long-term view on adaptation to sea-level rise and climate change, especially with long-life infrastructure such as nuclear power stations.
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