Sea level was relatively stable in the sixteenth to eighteenth centuries; it started to rise in the nineteenth century and rose about 20 cm by the end of the twentieth century, with a global rise of 17 ± 5 cm rise in that century [9,41]. Although this change may seem small, it has had many significant effects, most particularly in terms of the return periods of extreme water levels [35,42]. Worldwide there are many coasts that have been observed to be eroding . However, attributing particular impacts such as erosion to sea-level rise is difficult as erosion can be promoted by processes other than sea-level rise (Table 1). As already discussed, many of these non-climate drivers of change operated over the twentieth century. While sea-level rise is often inferred as an underlying cause of widespread retreat of sandy shorelines , negative sediment budgets also lead to erosion . Human reduction in sediment supply to the coast has contributed to observed changes through activities such as construction of levees, dikes and dams on rivers that drain to the coast [6,45]. Equally, changes in flooding and flood risk are difficult to attribute to global sea-level rise. For instance, flood defences have often been upgraded substantially through the twentieth century, especially in those (wealthy) places where there are sea-level measurements. Most of this defence upgrade reflects expanding populations on the coastal plains and changing attitudes to risk. In many places, relative sea-level rise has rarely even been considered in the design of past coastal infrastructure.
The accelerated rate of sea-level rise observed since the late 1800s has been accompanied by coastal erosion and rapid wetland losses in many low-lying coastal regions. On the US east coast, relative sea levels have risen at rates of between 2 and 4 mm-a 1 over the twentieth century due to varying patterns of subsidence caused by glacial isostatic adjustment. Both rates of sea-level rise and coastal retreat have been measured, providing the opportunity to explore shoreline response to sea-level rise. Away from inlets and engineered shores, the shoreline retreat rate is 50 100 times the rate of sea-level rise, as might be anticipated using the concept of the Bruun Rule . Near inlets, the indirect effects of sea-level rise which cause the associated estuary/ lagoon to trap beach sediment can have much larger erosional effects on the neighbouring open coasts than predicted by the Bruun Rule . So, whereas a simple heuristic like the Bruun rule describes the relationship for some shores, more general relationships are required to fully understand coastal change, taking account of sea-level change, sediment supply and coastal morphology .
In coastal Maryland and Louisiana, for example, wetland losses and shoreline retreat have led to a rapid restructuring of coastal ecosystems [33,48,49]. In Florida, a decline in coastal cabbage palm forests since the 1970s has been attributed to salt water intrusion associated with sea-level rise [50,51]. Due to extensive human development along these coastlines, it is not possible to quantitatively isolate climate change effects versus changes due to other human development activities.
Human responses to sea-level rise are even more difficult to document. A rare example is human abandonment of low-lying islands in Chesapeake Bay, USA, during the late nineteenth/early twentieth century which seems to have been triggered by the acceleration of sea-level rise and resulting land loss .
There have certainly been impacts from relative sea-level rise resulting from large rates of subsidence, such as the Mississippi delta where relative
sea-level rise approaches 1 cma 1! (see Grand Isle, Fig. 3). Between 1978 and 2000, 1565 km2 of intertidal coastal marshes and adjacent lands were converted to open water, due to sediment starvation and increases in the salinity and water levels of coastal marshes as a result of human development activities coupled with high rates of relative sea-level rise . The flooding in New Orleans during Hurricane Katrina was significantly exacerbated by subsidence compared to earlier flood events such as Hurricane Betsy in 1965
. Coastal retreat has occurred due to subsidence, such as south of Bangkok where shoreline retreat has been more than 1 km (Fig. 5). However, all the major cities that were impacted by relative sea-level rise have been defended, even when the change in relative sea-level rise was several metres.
Hence, while global sea-level rise has been a pervasive process, it is difficult to unambiguously link it to impacts, except in some special cases; most coastal change in the twentieth century was a response to multiple drivers of change. However, changes in two contrasting environments, polar coasts and tropical reefs, do appear to be directly exacerbated by warmer temperatures.
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