Sea level rise poses a major threat to coastal ecosystems and the biota they support. With an anticipated rise of between 0.18 and 0.59m (and possibly greater) by the end of the century, species that depend on low-lying coastal habitats—especially small oceanic islands—are particularly at risk.
As a result of climate change, the world’s oceanic surface-waters are warming. Higher temperatures reduce the density of the water causing it to expand. This process of “thermal expansion”, exacerbated by an influx of melt water from glaciers and polar ice fields, is causing sea levels to rise. During the 20th century, global (eustatic) sea level rose by c.0.17m at an average annual rate of 0.002m per year. This rate was ten times faster than the average during the previous 3,000 years (IPCC 2007). The rate of sea level rise continues to accelerate and is currently believed to be about 0.003m per year (Church and White 2006). It is estimated that sea level will rise by a further 0.18 to 0.59m by the century’s end (IPCC 2007). However, some research suggests the magnitude may be far greater than previously predicted due to recent rapid ice loss from Greenland and Antarctica (Overpeck, et al. 2006, Rignot and Kanagaratnam 2006). Accounting for this accelerated melting, sea level could rise by between 0.5 and 1.4m by 2100 (Rahmstorf et al. 2007).
Sea level rise is likely to have a dramatic impact on low-lying coastal and intertidal habitats—causing widespread flooding and accelerated coastal erosion. Ultimately, many coastal ecosystems may be lost or irreversibly altered. Rising sea level pushes the high-water mark landward; however, many coastal habitats are prevented from migrating inland due to natural or man-made barriers. This “coastal squeeze” could result in the loss of habitats, such as mudflats and marshes, which are critical for wildfowl and wader species (Galbraith et al. 2002, Hughes 2004, Le V. dit Durell et al. 2006). Rising sea levels, in combination with more frequent and intense storm surges, will prove particularly catastrophic to shore-nesting birds, such as terns (Bennett et al. 2007). The Australian Department of the Environment and Water Resources (DEW) has carried out an analysis of the impact of sea level rise on the country’s nationally threatened birds. It identified a number of species, such as the Critically Endangered Orange-bellied Parrot Neophema chrysogaster, for which more than 40% of their breeding range is in coastal areas beneath 10m elevation (Bennett et al. 2007).
Small islands, reefs and atolls are particularly vulnerable to sea level rise. Islands also tend to be important hotspots for biodiversity and endemism. For instance, a disproportionately high number of threatened birds, almost half, occur on islands, especially remote oceanic archipelagos (BirdLife International 2008). Research in the Northwestern Hawaiian Islands suggests that sea level rise could cause significant habitat loss with severe consequences for the region’s unique biota, including globally threatened species such as Laysan Finch Telespiza cantans (Baker et al. 2006). Populations of some species, already regarded as Critically Endangered, are entirely restricted to low-lying islands. For example, both Tuamotu Kingfisher Todiramphus gambieri and Cozumel Thrasher Toxostoma guttatum are confined to islands with maximum altitudes of less than five metres (BirdLife International 2008).
The loss of coastal ecosystems would have profound implications for neighbouring human communities. Coastal habitats, such as mangroves, provide many vital services. For instance, they act as nurseries for pelagic fish, are a source of food and fuel, and are barriers against tidal surges and flooding. Given that 10% of the world’s population (634 million people) live in coastal regions less than 10m above present sea level (McGranahan et al. 2007), it is imperative that steps are taken to safeguard low-lying coastal ecosystems against the worst impacts of sea level rise, not just for wildlife, but also for human societies.
BirdLife International (2009) Sea level rise poses a major threat to coastal ecosystems and the biota they support. Presented as part of the BirdLife State of the world's birds website. Available from: . Checked: