And predicting this threat to what is often referred to as the rainforests of the sea, the US government’s Coral Reef Watch added three new alert categories to represent the ever-increasing temperature extremes.

“We are entering a new world in terms of heat stress where the impacts are becoming so pervasive that we had to rethink how we were doing things,” the director of Coral Reef Watch Dr Derek Manzello told The Guardian.

What are coral reefs?

The National Oceanic and Atmospheric Administration describes the organisms thus: “Almost all corals are colonial organisms. This means that they are composed of hundreds to hundreds of thousands of individual animals, called polyps.”

Coral reefs, which comprise only about 0.5% of the ocean floor, are complex three-dimensional structures built up over thousands of years as a result of the deposition of calcium carbonate skeletons of the reef-building coral species.

It can take up to 10,000 years for a coral reef to form from a group of larvae. Depending on their size, barrier reefs and atolls can take from 100,000 to 30,000,000 years to fully form.

As they grow, these reefs provide structural habitats for hundreds to thousands of different marine species.

A complex life structure

Most reef-building corals contain photosynthetic cells, called zooxanthellae, living in their tissues. While the coral provides the zooxanthellae with a protected environment and the compounds they need for photosynthesis, the zooxanthellae produce oxygen and help the coral to remove wastes.

The relationship between the zooxanthellae and coral polyp facilitates tight recycling of nutrients in nutrient-poor tropical waters. Nearly 90% of the organic material produced by the zooxanthellae is transferred to the host coral tissue. This is the driving force behind the growth and productivity of coral reefs.

Zooxanthellae are also what lend the unique colourful hues to the stony corals often the poster child for island tourism.

The importance of corals

The first, and perhaps most basic role they play is the circulation of nutrients in the intricate food web and provide food at all levels of the food chain.

The more complex roles include: The provision of habitat for about 25% of all marine life, food and livelihood for nearly one billion people living along the coasts, and acting as natural buffers that absorb and dissipate wave energy thus reducing the potential damage from waves and storm surges.

According to estimates, there may be millions of unknown organisms living around the reefs, whose gradual discovery over the last few decades has been key to finding new medicines, including possible treatments for cancer.

This interdependent relationship in the marine ecosystem translates into most fisheries depending on coral reefs and related habitats for at least a part of their life cycles.

In the US, the National Marine Fisheries Service estimates the value of fisheries from coral reefs is over $100 million.

The reefs act as natural buffers protecting coastal regions from 97% of the energy from waves, storms and floods. Their absence, hence, translates to more violent storms and more devastating floods.

The threat to reefs

Reefs, complex creatures as it is, require a very specific environment to grow.

First, they can only thrive in seawater with temperatures above 18°C. Many reef-building corals grow optimally in temperatures between 23°C and 29°C, but some can tolerate temperatures as high as 40°C for short periods. Hence, the abundance of the ecosystem in the tropics.

Secondly, corals feed on sugars produced by the zooxanthellae which need light to photosynthesize and grow.

And the third condition, somewhat an extension of the second, is unpolluted water. Sediments in the water reduce the amount of light.

In the absence of any of these, corals become physically stressed, forcing the polyps to expel their zooxanthellae lending the colony a stark white appearance: a process more commonly known as coral bleaching.

If the polyps go for too long without zooxanthellae, coral bleaching can result in the coral starving to death without the energy that the algae provide.

At the local scale, many stressors such as tropical storms, disease, sedimentation, destructive fishing, over-exploitation of reefs, and salinity and temperature variations could cause coral bleaching.

One reason for mass coral bleaching is ocean acidification, or higher levels of CO2, which lower the pH of seawater, reducing the abundance of phytoplankton and decreasing calcification in corals, weakening their skeletons and impairing growth.

In warming water

But what has emerged as one of the key drivers of bleaching over the past few decades is higher ocean temperature. Mass bleaching events are strongly correlated with increased sea surface temperatures of 1°C or more above average.

The most recent example of this is the Florida coral bleaching event that was triggered after multiple heatwaves last year. In July 2023, ocean temperatures in Florida soared to 37.7°C.

The bleaching threshold for coral is typically around 30.5°C. When water temperatures cross that red line and stay that way for an extended period, a bleaching event is triggered.

At the time, coral bleaching along the Pacific coasts of Colombia, Costa Rica, El Salvador, Mexico and Panama, as well as along the Atlantic coasts of Belize, Cuba, Puerto Rico and the US Virgin Islands was also underway.

A similar event took place in the Great Barrier Reef, Australia, in 2015-16 led to the bleaching of nearly 56.6-80.6% bleaching in the region. The extent was much higher than the bleaching events of 2002 and 1998 — both El Nino years according to Copernicus records.

What’s in store?

According to NOAA projections from December 2023, bleaching hotspots (at SST >1°C) in the last quarter of the year were concentrated in the central and eastern equatorial Pacific. For 2024, the agency predicted heat stress remaining in the region during the winter, and expanding to Indonesia by January and the Great Barrier Reef by February 2024.

The ocean absorbs 90% of the heat caused by human-driven climate change. At the beginning of the year, the temperature has already spiked to 0.8°C above the pre-industrial average. And to current climate models, 99% of corals could succumb to marine heatwaves by the 2030s if the rise continues at the current rate.

While it’s time to redo our data visualisation methods with the temperature highs soon reaching the top of the axis, the world must also prepare for a breakdown of the marine ecosystem.

Cause and Effect is a weekly column in which Tannu Jain picks climate news from around the globe, and analyses its impact