Mass bleaching of coral reefs caused by climate change and warming oceans, scientists say

Coral reefs around the world are experiencing global bleaching as a result of warming oceans and human-caused climate change, top scientists have said.

From February 2023 until now, coral reef bleaching has been confirmed across at least 53 countries, territories or local economies triggering the fourth global bleaching event, the National Oceanic and Atmospheric Administration (NOAA) and International Coral Reef Initiative said.

Bleaching occurs when stressed coral expels the algae that give them their colour when eaten.

It was described by ecologist David Obura, director of Coastal Oceans Research and Development, as like a “fever in humans” and if it occurs long enough can cause coral to die.

The latest mass bleaching event has been caused by a sustained heating of the world’s oceans due to the effects of both climate change and an El Nino climate pattern.

In March, the global average sea surface temperature reached a record monthly high of 21.07C (69.93F), according to the EU Copernicus Climate Change Service.

“As the world’s oceans continue to warm, coral bleaching is becoming more frequent and severe,” Derek Manzello, NOAA Coral Reef Watch coordinator, said in a statement.

Scientists have found that for bleaching to begin, coral reefs have to be in waters that are at least 1C warmer than what they are used to for about four or more weeks.

This is important because coral reefs are a crucial part of the ecosystem that sustain underwater life, protect biodiversity and slow erosion. They also support local economies through tourism.

The last mass bleaching event ended in March 2017. It lasted three years – with two other bleaching events occurring in 2010 and 1998.

Although bleaching is nothing new, for it to be declared on a global scale, significant levels of bleaching had to be documented within each of the major ocean basins, including the Atlantic, Pacific and Indian oceans in both the northern and southern hemispheres.

Bleaching has already affected 90% of the coral assessed in 2022 in Australia’s Great Barrier Reef. The Florida Coral Reef also experienced significant bleaching last year.

The announcement comes as 2023 was declared the hottest year on record with the global average air surface temperature reaching 14.98C – beating the previous record set in 2016 by a “large margin” of 0.17C.

What can be done to help coral reefs?

Put simply, cutting greenhouse gas emissions and limiting climate change is the best chance for coral survival.

But some scientists and conservationists think the world has already heated more than a key threshold for coral reef survival, meaning between 70% and 90% of the world’s coral could be lost.

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Scientists are therefore trying to plan for the future by bringing coral larvae into cryopreservation banks, and breeding corals with more resilient traits.

“[It is] critical the world works to reduce carbon emissions,” Selina Stead, a marine biologist and chief executive of the Australian Institute of Marine Science, said.

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