Isla Maragarita, Venezuela.

1Coastal zones are in the front line as regards physicochemical changes in the oceans. The rise in sea level is pushing back coastlines. The warming and acidification of seawater also disturb ecosystems that are under marine influence. However it is very difficult for researchers to separate climate effects from those—more numerous—that result directly from human activities. Coastal areas pay a high price for their attractiveness, with increasing urbanisation and exploitation of resources. But one thing is certain: these environments have often become fragile and future population growth and changes in ways of life will mean that these direct anthropic pressures will continue to combine with the increasing effects of climate change.

The erosion and submersion of coasts

2Marine submersion models are now available for the simulation and forecasting of the movement of the sea into the land according to the rise in ocean water levels. For example, it is estimated in these models that 12 % of the islands in the world will be threatened with disappearance. But although projections based on the rise in sea level are sufficient for studies at the global scale, they are not necessarily sufficient for a predictive map of the future submerged areas at the scale of an ocean basin.

Small island in the Maldives.

3First of all, the rise in water level is very unevenly distributed. In the Pacific zone, differences in the rise of the level of the sea between New Caledonia and the islands of Micronesia in the past 50 years display a ratio of 1 to 10. More locally, the rise in sea level is also affected by climate disturbances and tectonics. For example, very significant differences in sea level are observed according to the intensity of the El Niño phenomenon.

A Lataw villager

Erosion depends on the local dynamics of environments

4The rise in sea level and greater frequency of storms increase the frequency of submersion events and hence coastal erosion. However, erosion also depends on the dynamics of sediment systems. After a hurricane for example, beaches may form again naturally from the stock of eroded sand deposited in the forward part of the beach. In contrast, if the ‘stock’ of sediment has been reduced by the removal of sand, the beaches exposed to the force of waves and swell will retreat.

5Coastal ecosystems also cushion erosion phenomena to a greater or lesser degree. Totalling some 600,000 km² along tropical coasts, coral reefs are an effective natural barrier to marine erosion. They cause waves to break, dissipating three-quarters of their power. Islands bordered by a reef thus have excellent natural protection. In addition, growth of the reef and coral sediment can also partially compensate a rise in sea level. For example, the Marshall Islands and Tuvalu still have the same surface area in spite of a rise in sea level of 2 mm per year during the second half of the 20th century.

Coastal erosion in Senegal.

Homes destroyed by the earthquake and tsunami of 27 February 2010 in the River Mataquito estuary in Chile

The village of Cabrousse in southern Casamance (Senegal).

Soil salinisation: a consequence of rising sea levels?

6Seawater intrusion has serious consequences for coastal land ecosystems. In particular, soil salinisation makes previously fertile soils non-productive. The salinisation of ground water also causes difficulties when potable water is required. Drought also aggravates the problem: more marked low water periods in watercourses contribute to a slow invasion of sea water in water courses and the salinisation of farmland.

7Nonetheless, the salinisation phenomena observed today are less linked to climate than to human activities. Population growth in coastal areas is accompanied by high water consumption. Urbanisation (concrete and tarmac) makes land impervious and this limits the infiltration of rainwater which is then drained by river systems and no longer recharges ground water. Increased use of water and increased runoff thus result in greater salinity of ground water. Coastal works are also strongly involved in sea water/fresh water exchanges and the consequences are sometimes unexpected (Box 21).

8Even in the absence of any human presence, the balance between fresh and salt water in coastal aquifers is both complex and unstable (Box 22).

Wooded coral islet in the lagoon at Nouméa, New Caledonia.

Amazonian mangrove swamp in French Guiana.

Mangrove swamp, a vulnerable ecosystem between land and sea

9Mangroves cover three-quarters of the coastlines of the tropics and form a specific ecosystem. However, they are currently disappearing at a rate of 1 % to 2 % per year. The reasons are population growth, the intensification of urbanisation and the exploitation of natural resources. The expansion of shrimp farming in South-East Asia, Central America and East Africa has had a particularly devastating effect. Climate change exerts further pressure on ecosystems that are already fragile. The disappearance of mangroves leads to the loss of certain essential ecological functions. Mangroves are home to rich biodiversity and form a key element in the balance of coastal ecosystems by allowing the return of nutrients, which, without these trees, would have been definitively lost in deep sediment.

10The increase in the number and intensity of cyclones could be fatal for these ecosystems. They have a destructive effect on mangroves, which, because of this, rarely colonise the most exposed stretches of coast, preferring quieter areas where sedimentation occurs. According to recent scientific work, if cyclones were to occur too often, mangrove swamps would be incapable of surviving.

Mangrove colonisation zone in French Guiana.

Artisanal oyster fishing in a mangrove swamp in Siné Saloum, Senegal.

Protection against erosion

11In the face of long-term changes, it is important to understand how mangroves have managed to adapt to environmental constraints up to now. Work in French Guiana shows the exceptional regeneration of mangroves in the face of strong environmental constraints and their contribution to the stabilisation of sediments (Box 23).

12This distinctive ecosystem forms a buffer zone between sea and land and may play a role in the protection of muddy, particularly unstable coasts against erosion. For example, IRD scientists and their partners have shown how a decrease in mangrove swamp would cause large-scale erosion of 370 km of coast in French Guiana. In this South American country, coastal marsh zones have been developed as ‘polders’ for the development of aquaculture and rice growing. Embankments have been constructed, reducing the 1 km of mangrove fringe to a strip only a few tens of metres wide. But the embankments would not withstand the force of waves and an increase in sea level if the mangroves were to disappear. Furthermore, embankments and rockfill prevent the sedimentation of mud from the Amazon on which mangrove areas are regenerated.

13Mangroves also play an important role in the carbon cycle because of their strong capacity for turning atmospheric carbon dioxide into organic matter. Together with primary tropical forest, mangrove forms part of the terrestrial ecosystems that produce most biomass. The quantities of carbon stored in these forests is still a subject for debate among scientists but their sequestration potential means that they are already the subject of protection and reforestation policies within the framework of fighting climate change (Box 24).

The biodiversity of coral barrier reefs threatened

14Coral reefs form another tropical coastal ecosystem that is threatened today. Several long-term quantitative studies confirm the degradation or loss of coral communities in numerous reefs. Here again, the causes are to be sought in human activities. Excessive fishing, biological invasions, pollution from the shore, development operations and mechanical damage to reefs are some of the numerous anthropic pressures involved. In some regions and especially in the Caribbean the occurrence of diseases affecting coral in recent years has been attributed to urban development.

15Climate change thus has effects in ecosystems that have often been seriously damaged by humans in the past. Coral reefs are sensitive to ocean warming and acidification and have now been made fragile by heat stress and bleaching phenomena (see page 92). Waves caused by cyclones and tropical storms also destroy fragile coral communities. It can take 10 to 20 years for a damaged reef to be reconstituted. However, if the frequency and intensity of climate events and anthropic stress increase, the return to normal will be much slower. In addition, acidification reduces available calcium carbonate in seawater and may also slow the calcification of coral polyps and hence the growth of reefs. However, there are still too many gaps in knowledge of the physiology of these organisms for us to know whether corals are capable of adapting to rapid changes of the environment.

White patch syndrome on a Porites colony in Mayotte.

Towards new underwater landscapes

16Scientists are addressing evaluation of how the increase in climatic and anthropic pressures will affect coral reefs in the future. Much work carried out in the 2000s on the future of coral reefs was very alarmist. But recent research shows that although numerous coral species have clearly been declining for more than 30 years, others are holding their ground or even becoming more plentiful. A vast international study in which IRD participates has observed the evolution of 7 coral reefs around the world (In the Caribbean and the Indo-Pacific Ocean) for some 15 years. Scientists have shown the spread of certain genera such as Porites stony corals that withstand rises in temperature. They also put these recent changes into perspective with regard to past events recorded in fossil reefs; it was seen that the abundance and structure of coral populations had already varied strongly in past millennia. These new data made it possible to revise the projections for the coming decades. As water temperatures continue to rise, a subset of ‘winning’ species will succeed—those with the greatest tolerance to heat, the best population growth rates or the greatest longevity.

A quarter of known marine fish species

17The ecological consequences of the ongoing changes go beyond corals alone as these ecosystems house a quarter of known marine fishes. In collaboration with international teams, IRD studied the impact of coral bleaching on the fish communities housed in this coral. For this, scientists compared coral and fish populations at some 60 coral sites in 7 countries (the Maldives, the Chagos Archipelago, Kenya, the Seychelles, Tanzania and the islands Mauritius and Réunion) before and after massive coral bleaching following an El Niño event in 1998. This scientific work shows that the decline of coral communities is followed by reduction in diversity, a reduction of size and loss of structure in fish populations.

18The changes in coral reefs are also preoccupying for food security in numerous southern countries as they provide the protein requirements of neighbouring populations.

Shoal of fish in Acropora branchus coral (New Caledonia).