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).

 

Midway Atoll is a low lying atoll that makes up part of the North-western Hawaiian Islands that holds nearly 75% of the global breeding population of Laysan Albatrosses Phoebastria immutabilis and 36% of black-footed Albatrosses Phoebastria nigripes nesting within the islands (Reynolds et al. 2015). The loss of important nesting sites as a result of sea level rise could therefore have large impacts on the global conservation status of these breeding birds. The potential habitat predicted to be lost through various sea level projections ranged from less than 10% at 1m rise, to 19% at 2m passive rise, and up to 55% when including storm waves. A sea level rise of 2m is predicted to flood 10-100% of Laysan Albatross nests and 16-100% of black-footed Albatross nests (Reynolds et al. 2015).

 

Populations of some species, already regarded as Critically Endangered, are entirely restricted to low-lying islands. Across IBAs in Oceania, seven species are restricted to islands with a maximum elevation of 10m. Great Frigatebird Fregata minor breeding colonies for example are largely restricted to islands with a maximum elevation of 8m above sea level, whilst the critically endangered Polynesian Ground-dove Alopecoenas erythropterus is restricted to islands with a maximum elevation of 5m, making them especially vulnerable to coastal surges (Birdlife International data 2015).

 

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.