Global sea levels are not constant, during the Pliocene sea level was at least 25m higher than present levels with temperatures only 2-3°C higher than today (Dowsett et al., 1994). At the last glacial maximum sea levels were 120m lower than present and average temperatures were 4-7°C less (Waelbroeck et al., 2002). This data shows that it only takes a temperature change of a few degrees to dramatically influence sea levels. Recent accelerations in climate change are believed to have a major impact on sea level rise (IPCC, 2007). Since 1961 sea level has risen at an average of 1.8±0.5mm per year (IPCC, 2007), 57% of which was due to thermo expansion of water and 28% due to the melting of glaciers and icecaps, with polar ice sheet melting contributing the remaining 15% (IPCC, 2007).
Light is an essential requirement for photosynthesis of the symbiotic algae (zooxanthellae). The amount and quality of the light decreases with depth hence corals have to remain high in the water column to receive the useful wavelengths of light. With sea levels predicted to rise between 0.18 and 0.59m by the end of the century (IPCC, 2007) corals will have to grow at rate of 1.8 to 6.3mm per year (depending on the sea level rise predictions) to remain within the suitable photosynthesis zone. The growth rates of coral vary by species and location on the reef. Porites asteriodes, a massive coral in the Gulf of Mexico, has an average growth rate of 3.67±0.65 mm per year (Elizalde-Rendón et al., 2010). The Great Barrier Reef, Australia, grows vertically at an average of 3.5-5.1 mm per year (Kennedy and Woodroffe, 2002). Granus and Macintyre (1998) used a computer model to predict how coral reef growth will keep up with sea level rise; they found that reefs will not be able to keep up with any of the scenarios of sea level rise past 2060. This shows us that coral reefs are under a serious threat due to sea level rise. More recent simulations are required with the latest climate data and coral growth rates in order to fully understand the effects.