A worldwide inventory by Bird (1985) indicates that approximately 70% of the world’s sandy coastlines have shown retreat over a period of decades; less than 10% have shown net progradation, while the remaining 20% to 25% have remained approximately stable. Recently, an analysis of satellite derived shoreline data for 1984–2016 showed that 24% of the world’s sandy beaches–sandy beaches making up 31% of the world’s ice-free shorelines–are eroding at rates exceeding 0.5 0.5 \(m/yr\), while 28 % are accreting and 48% are stable (Luijendijk et al., 2018).
While shore-line retreat on a geological timescale is undoubtedly connected with eustatic sea-level rise, it is more or less generally assumed that even the relatively small sea-level changes of the last century is driving this worldwide tendency of shore retreat (cf. Vellinga & Leatherman, 1989).
The rate of sea-level rise is likely to increase during the 21st century, although considerable controversy exists about the likely size of the increase. The latest3 International Panel on Climate Change (IPCC) projections indicate a sea-level rise range from 0.26 m to 0.82 m for the period 2081-2100 relative to 1986-2005 (IPCC, 2014). Any future rise in the mean sea level will result in the retreat of unprotected coastlines. Low-lying countries without resources for extensive coastal defence are the most vulnerable to this danger. Figure 2.25 shows Bangladesh which lies in the Ganges-Brahmaputra Delta on the Bay of Bengal. About 25000 \(km^2\) (18% of the total land area) will be inundated with a sea-level rise of 1 m. A sea-level rise of 0.5 m by the year 2100, which is within the IPCC estimated range of ‘global’ sea-level rise, would inundate more than 10000 \(km^2\).
It must be borne in mind though that there generally exists a complicated interaction of agents affecting shore retreat (as illustrated in Fig. 2.26). For instance, both the sea-level rise relative to the land and its effects are rather site-specific. Besides the eustatic sea-level rise, local contributions can for instance be due to glacial rebound, subsidence, compaction, and changes in ocean circulation. Additional (and often dominating) causes of erosion can be due to alongshore or cross-shore losses, which in their turn can be due to a variety of causes such as the physical geometry (e.g. head-lands, submarine canyons), hydraulic boundary conditions (e.g. related to waves, tides, wind) or human interference (e.g. harbours, erosion mitigating structures). As seen in Ch. 1, human interference at the land side can also influence the sediment budgets of the coastal zone, for instance by the construction of dams as has been done in many rivers, especially in Asia.
3. 5th assessment report, 2014