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Policy

Motivation:

The vulnerability of extensive near-coastal habitation, infrastructure, and trade makes global sea-level rise a major global concern for society. The UK coastline, for example, has ~£150 billion of assets at risk from coastal flooding, of which with £75 billion in London alone. Consequently, most nations have developed/ implemented protection plans, which commonly use ranges of sea-level rise estimates from global warming scenarios such as those published by IPCC, supplemented by worst-case values from limited geological studies.

UKCP09 provides the most up-to-date guidance on UK sea-level rise scenarios and includes a low probability, high impact range for maximum UK sea level rise for use in contingency planning and in considerations regarding the limits to potential adaptation (the H++ scenario). UKCP09 emphasises that the H++ scenario is unlikely for the next century, but it does introduce significant concerns when planning for longer-term future sea-level rise. Currently, the range for H++ is set to 0.9-1.9 m of rise by the end of the 21st century. This range of uncertainty is large (with vast planning and financial implications), and – more critically – it has no robust statistical basis. It is important, therefore, to better understand the processes controlling the maximum sea-level rise estimate for the future on these time-scales.

Rationale:

The vulnerability of extensive near-coastal habitation, infrastructure, and trade makes global sea-level rise a major concern for society (e.g., Halcrow, 2001, Stern report, 2006). Most current methods for sea-level projection, based on either modelling (e.g., IPCC – Meehl et al., 2007) or semi-empirical statistical methods (e.g., Rahmstorf, 2007; Jevrejeva et al., 2008; Vermeer & Rahmstorf, 2009; Grinsted et al., 2010), reflect the rapid effects of thermal expansion and a worldwide reduction of glaciers, but largely omit the longer-term (multi-centennial) contribution of major ice-sheet volume reduction. This contribution dominates the uncertainty in future sea-level rise projections, and forms the study target of iGlass.

The UK coastline has ~£150 billion of assets at risk from coastal flooding, of which £75 billion in London alone (Halcrow, 2001). UKCP09 gives the most up-to-date guidance on UK sea-level rise scenarios and design limits of UK coastal flood defences (Lowe et al. 2009). UKCP09 provides a low-probability, high-impact range for UK sea-level rise for use in contingency planning and in considerations regarding the limits to potential adaptation, which they term the H++ scenario. Currently, H++ has a range of 0.9–1.9 m of rise by the end of the 21st century. UKCP09 emphasises that the H++ scenario is unlikely for the next century, but it does introduce significant  concerns when planning for longer-term future sea-level rise. The lower H++ limit derives from the maximum global mean sea-level rise value given by the IPCC Fourth Assessment Report, corrected for the likely non-uniform nature of this global rise. The upper H++ limit derives from one single reconstruction for one specific interglacial (Rohling et al., 2008a), after similar corrections. The range of uncertainty is large, with vast planning and financial implications, and especially the upper limit has no robust statistical basis. It is important, therefore, to better understand the processes of ice-sheet response to climate forcing, which control the maximum sea-level rise
estimate for the future towards the end of the coming century and beyond. This forms the overarching motivation for iGlass.

Taking into account that marine-terminating portions of the ice sheets will respond more rapidly than land-based ice, physically plausible scenarios based on studies of modern ice dynamics suggest that future rates of sea-level rise may reach 0.8 to 2.0 m per century on a global scale (Pfeffer et al., 2008), or 1.5 m per century from Antarctica alone (SCAR, 2009). Such values are similar to estimates obtained from geological archives of the last interglacial when sea level was above the present, which fall in a range of about 1-2 m per century (Rohling et al., 2008a; Kopp et al., 2009). This coincidence demonstrates the utility of past interglacials for improving the understanding of ice-volume response to different climate states, to improve future projections.

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