Australia’s Great Barrier Reef sees largest coral die-off ever

• Warm seas around Australia's Great Barrier Reef have killed two-thirds of a 700-km (435 miles) stretch of coral in the past nine months.
• This is the worst die-off ever recorded on the World Heritage site.
• Bleaching occurs when the water is too warm, forcing coral to expel living algae and causing it to calcify and turn white. Mildly bleached coral can recover if the temperature drops and the survey found this occurred in southern parts of the reef, where coral mortality was much lower.
• While bleaching occurs naturally, scientists are concerned that rising sea temperatures caused by global warming magnifies the damage, leaving sensitive underwater ecosystems unable to recover.
• Climate scientists argue that increased carbon dioxide in the atmosphere traps heat radiating from earth, creating global warming. Australia is one of the largest carbon emitters per capita because of its reliance on coal-fired power plants for electricity.

About coral reefs:

• Coral reefs are often dubbed the rain forests of the ocean. Anchored by millions of coral polyps — tiny, soft-bodied animals that create elaborate calcium carbonate skeletons that shelter fish — these reefs cover just 0.1 percent of the sea floor but are home to 25 percent of marine fish species. They're popular spots for divers and tourists. They protect coasts from storms. They sustain food for half a billion people.

What is coral bleaching

• Many types of coral have a special symbiotic relationship with a tiny marine algae (zooxanthellae) that live inside corals' tissue and are very efficient food producers that provide up to 90 per cent of the energy corals require to grow and reproduce.
• Coral bleaching occurs when the relationship between the coral host and zooxanthellae, which give coral much of their colour, breaks down. Without the zooxanthellae, the tissue of the coral animal appears transparent and the coral's bright white skeleton is revealed.
• Corals begin to starve once they bleach. While some corals are able to feed themselves, most corals struggle to survive without their zooxanthellae.
• If conditions return to normal, corals can regain their zooxanthellae, return to their normal colour and survive. However, this stress is likely to cause decreased coral growth and reproduction, and increased susceptibility to disease.
• Bleached corals often die if the stress persists. Coral reefs that have high rates of coral death following bleaching can take many years or decades to recover.

What causes coral bleaching

• The main cause of coral bleaching is heat stress resulting from high sea temperatures. Temperature increases of only one degree celsius for only four weeks can trigger bleaching events.
• If these temperatures persist for longer periods (eight weeks or more) corals begin to die. High water temperatures can affect reefs at regional and global scale.
• Other stressors can also cause bleaching, including freshwater inundation (low salinity) and poor water quality from sediment or pollutant run-off.

Coral bleaching and the Great Barrier Reef

• The Great Barrier Reef has experienced mass coral bleaching events in the past.
• In 1998, there was a global mass bleaching event where 50 per cent of the reefs on the Great Barrier Reef suffered bleaching. During this time, sea temperatures on the Great Barrier Reef were the highest ever recorded.
• Mass bleaching also occurred in 2002, with 60 per cent of reefs were affected. This was the largest coral bleaching event on record. Two periods of hot weather resulted in sea surface temperatures a few degrees centigrade higher than long-term summer maxima.In both events, about five per cent of the Great Barrier Reef's coral reefs were severely damaged

By: Vishal