Impacts of global warming on corals

Coral reefs bleaching to death

Coral reefs around the world have been severely damaged by unusually warm ocean temperatures. A WWF report shows that less than 5% of the Great Barrier Reef will remain by 2050 if the world fails to reduce carbon dioxide emissions. Destructive fishing practices, pollution, coastal development and climate change are all taking their toll.

Coral reefs are extremely important for biodiversity, providing a home to over 25% of all marine life. They are also vital for people and business. They provide nurseries for many species of commercially important fish, protection of coastal areas from storm waves, and are a significant attraction for the tourism industry. However, coral reefs are very fragile sensitive ecosystems that can only tolerate a narrow temperature range.

One of the most visually dramatic effects of climate change on corals has been bleaching. When the ocean warms, the oxygen content reduces, and corals become ‘bleached’.

The heat affects the tiny algae which live symbiotically inside the corals and supply them with food. The heat stress damages the algae and in consequence leads to coral death.

In autumn 2005, in the biggest bleaching event in the Caribbean, corals died that were already alive when Columbus shipped these waters.

Global warming could now mean a death sentence for many coral reefs. If the present rate of destruction continues, most of the world's coral reefs could be killed within our lifetime.



Climate Change threatens the Seychelles Atoll and American Samoa

The Seychelles Islands are justly famous for their coral reefs and the remote Aldabra Atoll is the largest raised atoll in the world. It supports a huge coral diversity and rare land species like the giant tortoise (Aldabrachelys elephantina), vast seabird colonies and important sea turtle breeding beaches. In recognition of its status it is listed as a natural World Heritage site by UNESCO.

Apart from a handful of scientists at a research station, the atoll is visited only by occasional passing yachts: the post boat takes 12 days from the mainland. Yet despite its remoteness and protected status, the Seychelles suffered a severe coral bleaching event in the late 1990s and a recent assessment by the Seychelles Foundation judged climate change to be the most significant threat facing the atoll.

In March 2003, WWF reported that coral bleaching was occurring at all its 7 research sites in American Samoa, including within the National Park of American Samoa, Fagatele Bay National Marine Sanctuary and Maloata Bay Community Reserve. The climate change pressure on coral reefs is not limited to increases in sea temperature.


Other global warming effects could also threaten coral reefs

• More frequent tropical storms caused by global warming could break up the coral.
  
  
• Unusually warm water (by up to 5°C) caused by more frequent El Niño years, would also be an additional stress.
  
  
• More frequent heavy rains means more flooding, more river runoff, and therefore more sediment deposit in the seas.
  
  
• Finally, climate change could also reduce the ability of corals to form their limestone skeletons.

Bleached stony coral at a depth of 24m. Phoenix Islands, Kiribati.
© WWF-Canon / Cat HOLLOWAY


Local conservation actions alone will not be sufficient to protect coral reefs from the effects of climate change. Our only real hope of rescuing the coral reefs from the damaging effects of climate change depends on limiting the temperature increase by the end of this century to less than 2°C. If we do something now, we may be able to help ensure that there are coral reefs left for future generations to appreciate.