Coral bleaching is caused by global warming, which causes corals to expel essential algae that live in their tissues, resulting in the death of these species. Now, scientists have identified a much more lethal threat, marine heatwaves. These heat waves kill corals even faster than previously believed according to new research published in Current Biology. But what is a marine heatwave? What actually happens to a coral reef during a marine heatwave? And what does this mean for the future of coral reefs? In this latest blog piece, we guide you through marine heatwaves and how they impact coral reefs.

What Is A Marine Heatwave?

In addition to coral bleaching, corals are also threatened with an even bigger threat known as marine heatwaves. These are a distinct phenomenon from coral reef bleaching events. A marine heatwave is a brief period of at least 5 days of abnormally high temperatures. Marine heatwaves are caused by a combination of impacts associated with significant decline in marine biodiversity, increased prevalence of diseases (e.g., sea star wasting disease), toxic algal blooms, and mass mortality in coral reef ecosystems.

Due to a rapidly warming climate, marine heatwaves have recently become a more frequent threat to coral reefs, biologically distinct in its effects on corals. In fact, scientists have found that marine heatwave events are more deadly as they result in immediate heat-induced coral mortality. The most severe marine heatwave was recorded in 2016 across the Great Barrier Reef. Data suggests that the number of days per year that a part of the ocean experiences a marine heatwave has increased by a staggering 54% between 1925 and 2016. In addition, the data shows that heatwaves have become 34% more common and are lasting around 17% longer. 

What Happens To A Coral Reef During A Marine Heatwave?

Marine heatwaves result in instantaneous heat-induced coral death. In addition to coral death, coral reefs exhibit rapid coral skeletal breakdown, and loss of their delicate three-dimensional reef structure. During heatwave-induced death, coral skeletons exposed to tissue degradation quickly become enclosed by a complex biofilm of phototrophic microbes. These microbes increase the rate of calcium carbonate breakdown, to a rate that exceeds the accretion of healthy corals. This accelerated coral breakdown reduces the density and hardness of the coral skeleton, as well as increasing how porous the skeleton is. Essentially, this leads to rapid and extreme coral decay. 

This research into marine heatwaves indicates that significant heat-induced mortality events can be called biologically distinct events that cause much more severe harm than bleaching or mass bleaching of coral reefs. The phenomenon of marine heatwave mortality and rapid coral degradation will increase in frequency as marine heatwaves increase. Results also demonstrate that urgent action is required on a global scale to mitigate marine heatwave events and the damage they inflict.

What Does This Mean For Coral Reefs?

It’s no secret that rising sea surface temperatures have devastated many of the world’s coral reefs. Even iconic and relatively well-researched reefs are not immune. A new study into the 2016 marine heatwave in the Great Barrier Reef has found almost one-third of the coral reef died. Around the world, coral reefs are in increasingly warmer waters, and there is little sign of respite to allow coral regeneration from these dramatic events. While corals can rebound from bleaching if conditions change, this seems impossible considering that the warming events are increasingly becoming more extreme. There are concerns among scientists that these incidents could damage coral reefs beyond repair. If coral bleaching events and marine heatwaves were annual phenomena, they will undoubtedly lead to an ecological deterioration of coral reefs, as their habitats will be too degraded in their capacity to provide environmental resources such as food, coastal security, medicines and recreation.

Is There Hope For The Future?

New research is pointing to which coral species will be able to adapt, as an extreme form of natural selection. Scientists studying staghorn corals (Acropora millepora) have found that this species of corals are particularly vulnerable to marine heat waves. But in a surprising turn of events, staghorn corals may provide hope for the future of coral reefs due to their resilience. Crucially, staghorn coral is a branching, fast-growing coral key to reef building. A new analysis shows it is genetically diverse enough to survive for another 100 to 250 years. This is encouraging when other research is predicting that the vast majority of coral reefs will not survive the next 80 years.

Even more encouraging is the fact that it seems that staghorn corals are adapting as quickly as ocean temperatures are warming. That is, at least for now. To be continually adapted to warmer water, these corals will have to undergo quite extreme genetic changes through a mutation in their fundamental DNA. Mutations are random events and can take time.

Fundamentally, the global community needs to get serious about reducing the rate of global warming. There has been a widescale push to reduce the global temperature rise, including oceans. This is to reduce the well-documented impacts of global warming like rising sea-levels and loss of habitat. If the global average temperature continues to rise, it is certain that climate extremes such as heat waves will become more frequent, severe and widespread. This will have a knock on effect too, which leads to a downward spiral caused by ecological collapse, making recovery even more difficult. 

Furthermore, it is predicted an increase in average temperatures by 2 °C, which would lead to heat waves inducing coral bleaching and subsequent coral death. By the end of the century the  recent Emissions Gap Report, published by the UN Environment Program, predicts that global average temperatures are set to rise above 3 ^oC. 

Coral reefs are a source of inspiration and wonder, and are of vital significance to the societies that depend on them. Given that the loss of coral reefs would result in the destruction of ecosystem resources that feed more than half a billion people, action is desperately needed both internationally and locally to protect and conserve these truly wonderful ecosystems.