Photo of a white coral taken underwater.

Warmer ocean temperatures lead to coral bleaching

Coral reefs are among the most important ocean areas for marine biodiversity. Globally, coral reefs face a variety of threats. Pollution and overfishing are some of the most visible problems that threaten diversity and can lead to the destruction of coral colonies.

Protection measures in recent years have helped stem some of these threats in some places, but we may also need to prioritize protection to mitigate the effects of climate change as well. The tough challenge that coral reefs are already facing is rising temperatures in the world’s oceans which could lead to mass bleaching events.

Usually, in El Niño years like 2023, ocean temperatures rise significantly. In addition, for decades the oceans have absorbed much of the increasing temperatures we’re experiencing. While this may have delayed some of the worst effects of climate change, it also means that ocean temperatures are now experiencing unprecedented levels of heat that threaten many species.

What is coral bleaching?

Coral bleaching is a phenomenon that occurs when the symbiotic relationships between corals and their resident algae (zooxanthellae) are disrupted. Warmer ocean temperatures are one of the primary factors contributing to this disturbance.

Photo of coral reefs off Islamorada, Florida, where colonies of “fire coral” have “bleached,” or lost their symbiotic algae. Due to extremely warm ocean temperatures during the summer of 2014, both hard and soft corals lost their symbiotic algae – all over the Florida Keys. Photo: Kelsey Roberts, USGS, Public Domain.

High temperatures stress coral reefs, which leads to the expulsion or reduction of photosynthetic algae. Since these algae are responsible for producing a large portion of a coral’s food and contribute to the vibrant colors of corals, their loss results in a pale or “bleached” appearance. Furthermore, a decrease in algal populations harms coral reefs’ energy production, making them more vulnerable to disease and other environmental stresses.

This process has been widely observed in various coral reef systems around the world, and is of great concern to marine biologists, as it may lead to a loss of biodiversity and ecological balance within these vital ecosystems.

Rising ocean temperatures in 2015 killed coral reefs off the coast of Hawaii

In 2015, off the coast of Hawaii, in a previous year that saw an unprecedented 2.2°C increase in water temperature above normal, water temperatures reached 29.4°C. This has resulted in a quarter of coral reefs in the region losing 20% ​​or more of their coral cover. The most affected coral reefs lost more than half of their cover, while 18% of the reefs were not affected or even expanded.

Map showing sea surface temperature anomalies with a red band extending higher than normal across the Pacific Ocean.
In 2015, higher-than-normal temperatures in the Pacific Ocean led to coral bleaching around the Hawaiian Islands, Fiji, and other Pacific islands. a map: NASA.

The good news is that corals that were less affected by pollution, where the water quality was relatively good, were better able to withstand the increased water temperatures. Herbivorous skimmer fish, such as parrotfish, help keep coral healthy by eating algae. This has helped, at least in local settings, to improve coral performance and even expand as skimmer fish remove algae that enable corals to build.

However, it is clear that this may not be enough. In 2016-17 in the Great Barrier Reef off Australia, large-scale bleaching events led to the death of large numbers of corals.[1] This is explained by high water temperatures as well as pollution, which weakens coral reefs. in 2022, The sixth widely recorded coral bleaching event since 1988.

Map showing sea surface temperature anomalies with warming in red.
Map of sea surface temperature (SST) anomalies off the northeastern coast of Australia on March 14, 2022 – many areas were more than 2°C (3.6°F) warmer than normal. Rising temperatures have triggered mass bleaching in the Great Barrier Reef. Map: NASA, public domain.

Overfishing and ocean pollution make coral reefs vulnerable to bleaching events

The researchers believe that coral reefs will continue to be under threat and will experience large-scale bleaching events similar to what occurred in the Great Barrier Reef in 2016-2017. However, this can be slowed, and some coral reefs can thrive if pollution in the ocean is reduced and overfishing is prevented.

Pollution of urban sewage and runoff makes coral reefs vulnerable, especially when that pollution contains a toxic mix of various chemicals from agriculture or drugs. In addition, overfishing near coral reefs reduces herbivorous fish populations because many algae-eating fish live near or in coral reefs.

The wheel of an old submerged ship covered in corals.
Dry Tortugas National Park is the site of hundreds of shipwrecks. The wheel of this submerged ship has been turned into an artificial reef. Photo: Brett Seymour, IRC, NPS, public domain.

Using data from Hawaii, the researchers calculated that there is a three to six times greater chance that corals will have more reef building, using data from Hawaii, if healthy fish populations are maintained and pollution is minimal to non-existent. In fact, if there is any death, these corals are also better able to recover. While in general most coral reefs are likely to be affected by rising water temperatures, this loss can be mitigated by maintaining healthy fish populations and reducing land pollution. This highlights that runoff pollution also needs to be more strictly monitored.[2]

In addition, data from satellites, including NASA’s Aqua MODIS satellite, show a rise in sea surface temperature in recent years across the tropics and more temperate regions. This has led to particularly worrisome heat waves in the oceans, which have dramatically raised temperatures by several degrees Celsius during these events.

Poor circulation of ocean water leads to higher sea surface temperatures

Using recent work off Moorea, French Polynesia, scientists have shown how oceanic eddies that weaken internal waves have led to warming of oceans located at different levels due to poor circulation of cold deep waters. The scientists conclude that the asymmetry in eddy fields, which affects thermal slope depths or the transition between warmer surface waters and cooler lower waters, will lead to increased water temperatures.

Satellite image of Shades of Blue showing coral reefs in Fiji.
Coral reefs on the northern shore of Vanua Levu, the second largest island in Fiji. Satellite Image: NASA Landsat 8, May 10, 2015.

This will have an impact on the lengthening of the timing and intensity of marine heatwaves, as cold deep waters cannot easily compensate for higher surface water temperatures.[3] These impacts can be severe enough that not only affect the most vulnerable coral weakened by pollution and overfishing, but the healthiest coral may also be severely affected if eddies are severely disrupted.

Rising ocean temperatures will lead to more coral bleaching events

The oceans are extremely important to our global climate, and they have also helped us, to a large extent, to accommodate some of the increasing temperatures our planet is facing. However, as ocean water temperatures rise, coral reefs will begin to face severe challenges, especially if they are affected by pollution or overfishing.

We can mitigate this damage by better prioritizing reducing runoff pollution and fishing near coral reefs. However, scientists also warn that rising rates of water temperature are also likely to affect most of the world’s coral reefs in the next few decades.


[1] For more information on how coral reef health is affected by rising water temperatures and pollution, see: Nogrady B (2023) Controlling pollution and overfishing can help protect coral reefs — but it’s not enough. nature: d41586-023-02512-w. doi: 10.1038/d41586-023-02512-w.

[2] For more on the effects of pollution and depletion of fish populations on coral health see: Gove JM, Williams GJ, Lecky J, et al. (2023) Coral reefs benefit from reduced terrestrial and marine impacts as the oceans warm. nature. doi: 10.1038/s41586-023-06394-s.

[3] For more on marine heat waves and how they are affected by changes in marine eddies affecting heat lines, see: Wyatt ASJ, Leichter JJ, Washburn L, et al. (2023) Hidden heat waves and intense coral bleaching associated with mesoscale eddies and thermocline dynamics. Nature Communications 14(1): 25. DOI: 10.1038/s41467-022-35550-5.



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