Qatar World Cup 2022Goncalo Ramos hat-trick guides Portugal to quarterfinals

The phenomenon of coral bleaching 
Premium

How important are coral reefs to the marine ecosystem? Can this process be reversed and the reefs rejuvenated?

March 29, 2022 10:30 am | Updated April 12, 2022 04:21 pm IST

Perils underwater: A diver swimming among the corals on the Great Barrier Reef, off the coast of Queensland, Australia

Perils underwater: A diver swimming among the corals on the Great Barrier Reef, off the coast of Queensland, Australia | Photo Credit: AFP

The story so far: The management authority of the world’s largest coral reef system, Australia’s Great Barrier Reef, confirmed on March 25 that the reef is experiencing a mass coral bleaching event. This is the sixth time that the coral reef system is being hit by a widespread and damaging bleaching event and the fourth time in six years that such an event has occurred. The bleaching event coincides with a 10-day UNESCO (United Nations Educational, Scientific and Cultural Organization) scientific mission currently underway in Australia.

What are coral reefs?

Corals are marine invertebrates or animals not possessing a spine. Each coral is called a polyp and thousands of such polyps live together to form a colony, which grows when polyps multiply to make copies of themselves.

Corals are of two types — hard coral and soft coral. Hard corals, also called hermatypic or ‘reef building’ corals extract calcium carbonate (also found in limestone) from the seawater to build hard, white coral exoskeletons. Soft coral polyps, however, borrow their appearance from plants, attach themselves to such skeletons and older skeletons built by their ancestors. Soft corals also add their own skeletons to the hard structure over the years and these growing multiplying structures gradually form coral reefs. They are the largest living structures on the planet.

Corals share a symbiotic relationship with single-celled algae called zooxanthellae. The algae provides the coral with food and nutrients, which they make through photosynthesis, using the sun’s light. In turn, the corals give the algae a home and key nutrients. The zooxanthellae also give corals their bright colour.

Australia’s Great Barrier Reef is the world’s largest reef system stretching across 2,300 km. It hosts 400 different types of coral, gives shelter to 1,500 species of fish and 4,000 types of mollusc.

What is coral bleaching? 

Bleaching happens when corals experience stress in their environment due to changes in temperature, pollution or high levels of ocean acidity. Under stressed conditions, the zooxanthellae or food-producing algae living inside coral polyps start producing reactive oxygen species, which are not beneficial to the corals. So, the corals expel the colour-giving zooxanthellae from their polyps, which exposes their pale white exoskeleton, giving the corals a bleached appearance. This also ends the symbiotic relationship that helps the corals to survive and grow.

Bleached corals can survive depending on the levels of bleaching and the recovery of sea temperatures to normal levels. If heat-pollutions subside in time, over a few weeks, the zooxanthellae can come back to the corals and restart the partnership but severe bleaching and prolonged stress in the external environment can lead to coral death. Over the last couple of decades, climate change and increased global warming owing to rising carbon emissions and other greenhouse gases have made seas warmer than usual. Under all positive outlooks and projections in terms of cutting greenhouse gases, sea temperatures are predicted to increase by 1.5°C to 2°C by the time the century nears its end.

The first mass bleaching event had occurred in 1998 when the El Niño weather pattern caused sea surfaces in the pacific ocean to heat up; this event caused 8% of the world’s coral to die. The second event took place in 2002. In the past decade, however, mass bleaching occurrences have become more closely spaced in time, with the longest and most damaging bleaching event taking place from 2014 to 2017. This started with reefs in Guam in the Western Pacific region getting affected, to then affecting the North, South-Pacific, and the Indian Ocean. Global temperature in 2017, was the third-highest to ever be recorded. In the 2014-17 event, more than three times as many reefs were exposed to bleaching-level heat stress as compared to 1998.

​​

A 2021 study by the Global Coral Reef Monitoring Network (GCRMN), which is supported by the United Nations, showed that 14% of the world's coral on reefs had been lost between 2009 and 2018, with most of the loss attributed to coral bleaching.

Why does it matter? 

Coral reefs support over 25% of marine biodiversity, including fish, turtles and lobsters; even as they only take up 1% of the seafloor. The marine life supported by reefs further fuels global fishing industries. Even giant clams and whales depend on the reefs to live. Besides, coral reef systems generate $2.7 trillion in annual economic value through goods and service trade and tourism. In Australia, the Barrier Reef, in pre-COVID times, generated $4.6 billion annually through tourism and employed over 60,000 people including divers and guides. Aside from adding economic value and being a support system for aquatic life, coral reefs also provide protection from storm waves.

Dead reefs can revive over time if there are enough fish species that can graze off the weeds that settle on dead corals, but it takes almost a decade for the reef to start setting up again. The reefs which were severely damaged in 1998 did recover over time.

What is the current condition of the Great Barrier Reef? 

The United Nations Intergovernmental Panel on Climate Change (IPCC) released its report this month, which warned that the life of the Great Barrier is in grave danger. The report said that if temperatures continue to rise, bleaching events may occur more often and a large proportion of the remaining reef cover in Australia could be lost.

Just a couple of weeks after this warning, the Barrier Reef Authority confirmed a mass bleaching phenomenon affecting all pockets of the reef system.

The Authority said that its a first that the current bleaching event has occurred during a La Niña weather pattern, when warm areas in the pacific ocean shift, giving more cloud cover, rain and creating cooler weather conditions over the reef. The Guardian quoted ​​Dr. David Wachenfeld, the chief scientist of the reef managing authority, as saying, “The climate is changing and the planet and the reef is about 1.5 degrees centigrade warmer than it was 150 years ago. Because of that, the weather is changing. Unexpected events are now to be expected. Nothing surprises me anymore.” The Authority further stated that bleached reefs are affected at different levels from mild to severe, and while stressed, they are alive and could recover if temperatures moderate.

The 10-day UNESCO mission which is currently in Australia is supposed to assess how much the country’s administration is doing to pull the reef out of danger. Climate activists have said that it is important that the scientists are shown the extent of the coral bleaching to give a true picture of the reef’s condition, rather than just being shown the vividly coloured unaffected reef patches.

In July last year, Australian Prime Minister Scott Morrison had managed to convince UNESCO to not list the Great Barrier Reef as being in danger, even as the Heritage agency had recommended such a listing earlier that year. In 2015, when UNESCO had threatened to downgrade the Barrier Reef’s World Heritage Listing, Australia created a "Reef 2050" plan, putting billions of dollars into protecting it.

THE GIST
Corals are marine invertebrates or animals not possessing a spine. They share a symbiotic relationship with single-celled algae called zooxanthellae.
Bleaching happens when corals experience stress in their environment due to changes in temperature, pollution or high levels of ocean acidity. Over the last couple of decades, climate change and increased global warming owing to rising carbon emissions and other greenhouse gases have made seas warmer than usual.
Coral reefs support over 25% of marine biodiversity. Also, coral reef systems generate $2.7 trillion in annual economic value through goods and service trade and tourism.

Top News Today

Comments

Comments have to be in English, and in full sentences. They cannot be abusive or personal. Please abide by our community guidelines for posting your comments.

We have migrated to a new commenting platform. If you are already a registered user of The Hindu and logged in, you may continue to engage with our articles. If you do not have an account please register and login to post comments. Users can access their older comments by logging into their accounts on Vuukle.