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1. Rising waters
The average sea level has risen by 20 cm in about 100 years. This increase stems from the thermal expansion of the ocean as it warms
up, and also from partial melting of the polar ice caps and continental glaciers. Scientists consider that the earth’s surface temperature
could heat up by 1.1 to 4.8°C and the sea level could increase by 26 to 98 cm in 2100.
Certain deltas, lagoons and coastal areas could then become submerged.
Nearly half the world’s population live less than 60 km from the coastline. The rise
in sea level is a particular threat for densely populated coastal zones like the great
deltas of Asia or the low-altitude islands such as the Maldives or the Marshall
Islands. Millions of people could be forced to leave their homes and lands.
The first climate refugees are people from Bangladesh. At the end of 2006, the
10,000 inhabitants of the island of Lohachara, in the Gulf of Bengal, had to flee their
island - which had been submerged by the upsurge of water – and take refuge on a
neighbouring island, also under threat in the long term.
2. A chain of impacts
The warming of the ocean surface waters results in a reduction in phytoplankton production, which
has a dual effect. This plankton is at the base of the marine food chain. When this diminishes, the whole
marine ecosystem is thrown out of balance. Moreover, phytoplankton uses carbon dioxide when it
develops. Less phytoplankton means that less CO2 is trapped in the ocean waters. The ocean then no
longer plays so strongly its role of atmospheric CO2 regulator.
The rise in sea surface temperatures favours a process called water stratification. This hinders the upsurge of
nutrient salts, necessary for the development of phytoplankton. Many plankton species then migrate towards more
temperate waters, attracting their predators (fish and sea birds) to follow in pursuit.
3. Fatal liaisons
The coral reefs, which are the oceanic ecosystems with the richest biodiversity,
are turning white and dying in warming seas (1.2°C in 100 years). Hundreds
of other marine species thus become deprived of their habitat and are also in
danger of extinction.
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4. An increasingly fragile biodiversity
The accumulation of CO2 in the ocean induces acidification of the
water and a reduction in the concentration of carbonate ions.
This process holds back the ability of many organisms to form their
structure or their hard shells. Many marine species are threatened,
290 °
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including corals, molluscs and crustaceans, as well as phytoplankton.