Climate change and coastlines

Climate change is adding to coastal stresses in many ways. The rise in sea level during the 20th century is estimated to have been near 0.2 m (about 8 inches) (Douglas, 1991, 1997; Peltier, 2001; Church and White, 2006). Projections are that, as a result of thermal expansion and glacier and ice sheet melting, human-induced changes in the climate could result in an additional rise in global sea level of as much as 0.5 m (about 20 inches) and possibly much more by 2100 (IPCC, 2007). Satellite altimeter data for the past 15 years indicate that there has been a 50 per cent increase in the rate of sea level rise over this time (Church and White, 2006) as compared to the historic rate estimated from older tide gauge data. This rate of sea level rise will accelerate the loss of coastal wetlands and erosion if it is maintained.

Sea level rise has been shown to be a significant driver of beach erosion (Leatherman et al, 2000; Zhang et al, 2004); certainly beach erosion has been ubiquitous over the last century, a period during which sea level has risen at a faster rate than any time over the last several millennia (Woodworth, 1999). Along a low-lying sandy beach coastline, the rate of erosion can be up to two orders of magnitude greater than the rate of sea level rise, so that even small changes in sea level are likely to result in significant beach loss. If present trends continue without extensive coastal engineering projects, it is estimated that one in four buildings located within about 150 m (about 500 ft) of the US shoreline will be destroyed as a direct or indirect result of coastal erosion during the next 50—75 years (Heinz Center, 2000).

Many low-lying mainland areas are protected by barrier islands from large oceanic waves along the US East and Gulf Coasts; coastal inundation and saltwater intrusion are the primary problems in these extremely low-sloping areas. Even a 1 m rise has the potential to inundate many kilometers inland (i.e. the ratio of the height of sea level rise to the distance of inland advance can be up to four orders of magnitude) — with storms serving to tip the ocean onto the land. These low-lying mainland areas are being densely developed because beachfront property is now so expensive. Clearly, growing coastal populations and development in low-lying coastal regions in the face of rising sea levels, shoreline recession and coastal storms have set the US on a coastal collision course.

In areas where the slope of the coastal plain is very gentle (for example, 1:10,000) with little sediment input, inundation destroys wetlands vegetation — the plants simply cannot keep up with the rapid change of sea level and drown. A preview of rapid wetlands loss can be seen in the Mississippi delta region (see also Chapter 8), where relative sea level rises by up to 1 cm per year (Douglas, 2005), largely because of subsidence. The resulting loss of wetlands is about 65 km2 per year (Boesch et al, 1994). Since Louisiana coastal wetlands amount to 40 per cent of the national wetland area, the loss there is responsible for about 80 per cent of the total US annual wetland loss.

Even more moderate rates of sea level rise, however, will cause wetlands loss. The rate of sea level rise in Maryland is nearly 4 mm per year, about twice the average global rate over the last century. This high rate is a combination of the modern global rate of about 1.8 mm per year and land subsidence due to tectonic adjustments associated with past deglaciation (Peltier, 2001). At this elevated rate, which has existed only since the mid-19th century when global sea level rise began to increase to the modern rate, the plants in the coastal wetlands drown. Figure 10.2 shows the wetland loss at Blackwater Wildlife Refuge in Cambridge, Maryland between 1938 and 1988. That even a modest increase in the rate of sea level rise can have severe consequences for wetlands emphasizes the challenge being faced by many coastal regions.

In addition to problems of erosion and inundation, hurricanes may become more intense as a result of climate change (IPCC, 2007). Greater losses of life and property would also be expected from more intense winter storms. In Alaska, the retreat of sea ice that previously buffered the coast against winter storms is further amplifying coastal damage by allowing waves to pound the frozen barrier islands that have served as the home of Inuit and other indigenous groups for many millennia; so much erosion is occurring that villages are literally falling into the sea and have to be relocated inland at great expense (Arctic Climate Impact Assessment, 2004; Robertson, 2004).

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