Coral Bleaching in a Warming World

At last there is some good news for coral reefs.  The worst ever coral bleaching event in history, which lasted over three years and had destroyed coral reefs all over the world appears to be coming to an end.

In Australia, the Great Barrier Reef was devastated by the worst bleaching event in 2016 and then again in 2017, killing half of the largest living thing on earth. In the Indian Ocean, the Maldives had almost all of its reefs affected, with 60% to 90% of coral colonies bleached.

The Australian territory of Christmas Island was devastated with 85% of coral reefs dead and 100% affected. The situation is equally grim in the Lakshadweep archipelago, but it has not received as much focus as the bleaching events down under.

Corals are the ocean’s richest biome and are vulnerable to changes in temperature, light and nutrients. According to reports, up to three-quarters of the existing reefs are likely to disappear by 2050 due to climate change impacts. In the Indian ocean nearly seventy percent of coral reefs appear to have died.

Inhabiting the world’s tropical seas, coral reefs are habitat to millions of marine species. Corals have a symbiotic relationship with microscopic algae or zooxanthellae that live in their tissue, and provide much of the energy requirement of the corals.

The brown and green colors of most corals come from the zooxanthellae; in shallow waters brighter pigments of blue, pink and yellow mask these. Even minor temperature rises of 1 to 2°C above normal leads to stress and breakdown of the relationship. With the loss of algae, the coral tissues become transparent and the bright white of the underlying coral skeleton gives it the appearance of bleaching.

Without the energy supplied by the zooxanthellae, many corals starve to death and become vulnerable to disease. In many locations around the world, high temperatures have caused severe mass bleaching events leading to widespread death of corals and long lasting reef damage.

Additionally, acidification of the oceans reduces the amount of calcium carbonate available for corals and other calcifying marine organisms to build their skeletons and shells.  There is little that can be done to reverse the process and the only way to reduce ocean acidification globally is to reduce CO2 concentrations in the atmosphere.

A recent European research has indicated that limiting warming to 1.5°C would provide a window of opportunity for some tropical coral reefs to adapt to climate change. In contrast, a 2°C temperature increase by 2100 would put virtually all of these ecosystems at risk of severe degradation due to coral bleaching. Hopefully, world leaders will soon realize that we are continuously shrinking the window of opportunity we have to avert catastrophic warming impacts on the planet.

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