Coral reef growth affected by sea level rise

Already weakened by local damages (overfishing, pollution, coastal development) and global damages (rising surface water temperatures and acidification of the marine environment), coral reefs face an additional challenge: the rise of sea levels.

It is now well known that rising greenhouse gas concentrations are causing the global average ocean temperature to rise. However, global warming is also causing water levels to rise due to melting ice and increased thermal expansion of seawater. According to a recent study, the annual rate of water rise, currently equivalent to 3 millimetres per year since 1993, could more than double by 2100 (to more than 6 millimetres per year). This observation is particularly well supported by the Representative Concentration Pathway (RCP) models forecasting future climate presented by the Intergovernmental Panel on Climate Change (IPCC). The latter predict a global rise in sea level from +0.40 m to over +0.82 m by 2100, according to the most pessimistic scenarios.

The extent of coastal change and coastal vulnerability to extreme events (floods, typhoons, etc.) will depend directly on maintaining reef elevation relative to water levels. Indeed, the reef structure serves as a natural barrier against erosion by reducing wave energy and regulating water circulation – a service it would no longer be able to provide by the end of the century.

In an article published in the Nature magazine in June, an international research team measured the growth rate of 200 coral reefs in the Atlantic and Indian Oceans. The researchers then compared these rates with projections from the most pessimistic CPR scenarios. The study concludes that even under a modest climate change scenario (2.6°C increase in atmospheric temperature), 3% and 6% of the coral reefs studied in the Indian and Atlantic oceans respectively would be able to compensate for projected sea-level rise. For the most pessimistic cases, most of the reefs studied would experience a rise in water depth of more than 0.5 m, exacerbating the destructive impact of waves on reef areas.

Ecological and social consequences are expected

A change in hydrodynamic regime at the level of the reef ridge could eventually threaten the ecosystems inhabiting lagoons such as seagrass beds – prime areas for the development of young organisms (juvenile species). The rise in the average water level will also cause greater coastal erosion, favouring flooding of the land during extreme phenomena, as well as a reduction in arable land and fresh water reserves due to salinisation of the soil and groundwater. To date, more than 275 million people live within 30 km of reefs, so the socio-economic impacts could be catastrophic, forcing a significant number of local communities to abandon their lands, on a scale of a few decades.