Why are corals turning white and what does it mean for us?

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Bleached Corals, Cenderawasih Bay, West Papua, Indonesia. Photo: Reinhard Dirscherl/ullstein bild/Getty Images

Why are corals turning white and what does it mean for us?

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Bleached Corals, Cenderawasih Bay, West Papua, Indonesia. Photo: Reinhard Dirscherl/ullstein bild/Getty Images

Why are corals turning white and what does it mean for us?

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Crown-of-thorns Starfish feeding on coral, Acanthaster planci, Komodo National Park, Indian Ocean, Indonesia. Photo: Reinhard Dirscherl/ullstein bild/Getty Images

Why are corals turning white and what does it mean for us?

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Bleached Branching Coral, Acropora, South Male Atoll, Maldives. Photo: Reinhard Dirscherl/ullstein bild/Getty Images

Why are corals turning white and what does it mean for us?

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Malaysia, South China Sea. Photo: Reinhard Dirscherl/ullstein bild/Getty Images

Why are corals turning white and what does it mean for us?

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Bleached Mushroom coral, Ctenactis echinata, Komodo National Park, Indian Ocean, Indonesia. Photo: Reinhard Dirscherl/ullstein bild/Getty Images

Why are corals turning white and what does it mean for us?

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Maldives Islands, Indian Ocean. Photo: Reinhard Dirscherl/ullstein bild/Getty Images

Why are corals turning white and what does it mean for us?

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Sun bleached coral from the Maldives. Photo: Reinhard Dirscherl/ullstein bild/Getty Images

CORAL BLEACHING

Why are corals turning white and what does it mean for us?

Nihar Gokhale @nihargokhale

Corals across the world's oceans are turning white, pointing to a rise in ocean temperature.

Twice in the past - in 1998 and in 2010 - these immobile underwater animals have turned white, but scientists expect the current phase to be the worst.

By 2016, the bleaching is likely to spread to most coral-bearing regions - a clear fallout of climate change.

Reefs support a vast amount of marine life and are important to humans too. Some like Australia's Great Barrier Reef are a huge draw for tourists. In India, the Lakshadweep islands stand on a coral reef system.

While scientists had begun noticing signs of bleaching last year, it became worse recently. On 8 October, the US-based National Oceanic and Atmospheric Administration declared a "global bleaching event". It has estimated that by 2016 a third of all coral reefs in the world will be affected.

Why do corals turn white?

Corals are covered in algae, which give them their distinctive colour. Whenever there is an extreme change in temperature, the intensity of sunlight or amount of nutrients in water, corals expel these algae, thus turning white.

Being the main source of nutrition, algae are crucial to the survival of corals. So, though corals are not dead while bleached, their lifespan is reduced and they become vulnerable to diseases. The 1998 bleaching led to the death of 70% corals in the Indian Ocean, including those in the Andaman and Nicobar Islands and Lakshadweep.

Corals across the world are turning white as they lose their algae cover; and this may push them to death

Corals have been under threat largely because of human activities. In the last four decades, 80% of the corals in the Caribbean, and over 50% around Indonesia and the Pacific ocean, have been affected by bleaching. In all, about 1% of corals are considered to be dying each year and bleaching is one of the biggest reasons for the loss of corals.

The 1998 event was the first such bleaching event in history. It was the first definitive indicator of how corals are getting affected by global warming.

What is causing the current bleaching?

The current bleaching has been caused by warming up of ocean water - a direct result of global warming and the El Nino effect, which occurs irregularly every few years. This year is one such and the effect is quite strong - ocean temperatures were 2 degrees Celsius above normal in August and September.

El Nino, combined with global warming, has led to 2015 becoming the hottest year on record - In July, ocean temperatures reached a record high and global temperatures were 0.81 degrees Celsius above the 20th century average, caused by ocean temperatures climbing to a record high.

Read more: Be Afraid: July was the hottest month known to mankind).

So hot has 2015 been that the bleaching seems par for the course. It highlights the increasing effect due to climate change and greenhouse gas. Higher dissolved carbon dioxide is turning ocean water acidic, which slows down the formation of coral reefs.

Why are corals important?

Corals shelter nearly a quarter of all marine species, playing a crucial role in the underwater food pyramid. Losing corals, thus, puts other marine beings at risk including fish. This in turn affects coastal communities and economies. The NOAA estimates that the livelihoods of 500 million persons worldwide depend on corals.

In terms of monetary losses to humans alone, losing corals is quite expensive. A 2007 study estimated that corals add the equivalent of $172 billion-$375 billion a year to the world economy. As a whole, corals are worth $9.9 trillion, according to a study in 2014. Of course, like other parts of nature, corals are not important to just humans but also the rest of the ecosystem.

How bad is the current bleaching event?

NOAA expects the current wave of bleaching to last until 2016. By then it is expected to affect a third of all reefs in the world over 12,000 square kilometres, including the Indian Ocean. About 95% coral reefs around the United States are at risk of bleaching.

Bleaching is reversible. Some amount of bleaching occurs naturally and corals can survive that. But scientists say that if the frequency of bleaching increases, it may not give enough time for the corals to recover before they are hit again.

Nihar Gokhale

Nihar Gokhale @nihargokhale