The ocean absorbs carbon dioxide from the atmosphere making it more acidic. In 2005 there was 379ppm of CO2 in the atmosphere, which is increasing by 1.9ppm a year (IPCC, 2007). The ocean absorbs approximately a quarter of CO2 released into the atmosphere (Canadell et al., 2007). Once in the water column the CO2 reacts with water to form carbonic acid, turning the water more acidic. The carbonic acid then dissociates to form bicarbonate ions and protons. The protons react with carbonates forming more bicarbonate ions. This reduces the availability of carbonate ions for calcifying organisms such as coral (Kleypas et al., 1999).
Experimental data shows that if CO2 concentrations were to double from pre-industrial levels, coral reef growth would decrease by 40% because the carbonate ions decrease thus aragonite formation (the calcium carbonate crystal that makes up the coral’s skeleton) is inhibited (Kleypas and Langdon, 2006). Corals have several responses to the change in the availability of carbonate ions (Table 1). These methods have implications for the coral survival. Reducing the skeletal density means that the coral will become frail hence more likely to break in storms and other disturbances (Szmant and Gassman, 1990). Decreasing the growth rate means that the coral stands less of a chance of keeping up with sea level rise (See page on sea level rise), and increasing the energy input results in less energy being spent on other factors such as reproduction, resulting in lower larval output thus the coral has less chance of recolonising areas after disturbances (Szmant and Gassman, 1990).