Present Day Sea Level Rise

The IPCC’s Fourth Assessment Report (2007) predicts that global mean sea level will have risen between 0.18 – 0.59m by 2090, relative to 1980 levels (although there has been much debate over the accuracy of these predictions). Sea level rise is in response to a warming climate, which is currently causing the melting of polar ice and glaciers and the thermal expansion of the oceans. Coral reef ecosystems are dependent upon being able to maintain a certain level within the water, in order to receive sufficient solar energy. Global sea level rise is likely to affect coral reefs by reducing the amount of available sunlight and causing 'drowned' reefs. This happens when the sea level rises at a faster rate than coral growth, meaning there is insufficient levels of sunlight for the maintenance of zooxanthellae within the coral (Hoegh-Guldberg, 1999). Coral reefs are able to adapt to gradually rising sea levels by growing vertically. However, it is predicted that present day sea level rise coupled with ocean acidification (which reduces the calcification rate), will limit the ability of coral reefs to adapt.

An early paper that attempted to assess the threat of sea level rise to coral reef ecosystems published in 1988 by Buddemeier and Smith. This paper estimated that sea levels will rise at an average rate of 15mm per year, which is five times the modal rate of vertical coral reef growth. The authors expect the vertical growth rates to increase in response to rapidly rising sea levels, however they still conclude that this accelerated rate will be insufficient. The paper attempts to predict the response of coral reefs to sea level rise by assessing the maximum growth capacity of coral organisms. This is approached through the analysis of coral reef response during the period of rapid post-glacial sea level rise at the start of the Holocene and the last interglacial period. Their study suggests that rapidly growing corals will be more abundant, as this will enable the reefs to adapt. However, the success of coral reef adaptation is dependent on the level of sea level rise; if it exceeds the maximum growth rate of corals then many reefs will be lost. It is interesting to note that this article does not mention ocean acidification, which is now recognised as a limiting factor for coral reef growth. 




Done (1999) suggests that coral reefs will easily be able to adapt to sea level rise as they are capable of vertical growth rates of up to 20cm per year (compared with current sea level rise which is given as less than 1cm per year). However, he admits that the response will vary between and within regions, reflecting differences in species and the events and processes that operate at that scale.




Figure 1: Photos taken 6 hours apart on the Pacific island of Tuvalu, indicating the potential impact of sea level rise on low lying islands (Source: Gary Braasch).




Aside from the impact on coral reefs, sea level rise will also detrimentally affect coastal populations, especially those in low lying tropical regions where coral reefs are common. One of the most well publicised cases is the island of Tuvalu, which has been featured in the press and academic papers (Lewis, 1989; Church et al, 2006). Tuvalu is a low lying coral attol, where the highest point is less than 5m above sea level and is therefore likely to be one of the first casualties of a rising sea level. Vafeidis et al (2008) developed a global coastal database to assess sea level rise, within the DINAS-COAST project. It aims to support impact and vulnerability analysis to sea-level rise at a range of scales, by collecting high resolution data and will therefore facilitate future modelling exercises. It is this form of monitoring and modelling projects that will enable the better understanding of sea level rise, and the potential impact this will have on coral reef ecosystems and coastal regions.