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Rising Seas Will Inundate Coral Reefs That Protect Vulnerable Coasts

Researchers studied more than 200 tropical reefs and found that few will grow fast enough to keep pace with higher water levels due to climate change. This means the reefs won’t be able to break destructive waves that can wreak havoc on coastal communities.

Written by Alastair Bland Published on Read time Approx. 6 minutes
Shallow-water reef zones, such as this one in the southern Maldives, absorb large amounts of wave energy, creating lower-energy conditions behind.Professor Chris Perry, University of Exeter


Around the world, shallow tropical coral reefs serve as wave breaks that protect islands and coastlines from destructive storms. That makes them suitable, and often very desirable, places to live.

But with rising sea levels, this will change. Coral reefs are living systems that grow upward toward sunlight. Now, new research shows that few reefs in the Caribbean Sea and the Atlantic Ocean will be able to keep pace with rising water levels as climate change accelerates. In a paper published Wednesday in the journal Nature, a team of more than two-dozen scientists predicted that a variety of ecological impacts – including polluted water and coral bleaching – will so seriously inhibit growth rates of corals that virtually all of the 202 reefs they studied will be at least 0.5m (1.6ft) deeper under water than they are today.

Coastal cities and habitats will likely take a battering as a result. Increased wave energy – previously absorbed by the top layer of reefs – will send storm surges inland, increase erosion and realign shorelines. In some places, according to other research, critical freshwater springs on low-lying atolls will be inundated by waves, forcing inhabitants to leave.

“When you’re looking at the impacts upon coral reefs and coastal communities, there are very big socioeconomic costs of not taking action against CO2 emissions,” said Chris Perry, a University of Exeter professor of physical geography and the lead author of the new study. “There will be management costs and relocation-type costs. These are big issues that will have big financial implications.”

The findings come as another study, also published Wednesday in Nature, reported that ice melt in Antarctica has tripled over the past 10 years, raising the specter of rapidly rising sea levels in the decades to come.

Perry and more than two dozen coauthors analyzed data collected between 2009 and 2017 on site-specific reef growth rates and coral health. They also calculated how coral bleaching – which is triggered by spikes in ocean temperatures and can kill corals unless the water cools quickly – is likely to impact growth rates. Then the scientists looked at different possible scenarios of global warming and rising sea levels. They found that for just a small handful of coral reefs, sea-level rise is not likely to outperform the coral’s capacity to grow – what the authors call the “reef accretion potential.” That is, these reefs will manage to grow quickly enough to maintain current mean water depths and protective benefits for coastlines.

A healthy and actively growing reef with high coral cover in Chagos in the Indian Ocean. The image was taken prior to the bleaching event of 2016. (Professor Chris Perry, University of Exeter)

However, most reefs, they concluded, will be unable to keep pace with predicted rates of sea-level rise “without sustained ecological recovery” efforts, such as improving water quality and protecting reef communities from overfishing. Under the most extreme sea-level rise scenarios, “most reefs are predicted to experience mean water depth increases of more than 0.5m by 2100,” the authors wrote.

Perry stressed that this will not immediately threaten corals.

“These reefs aren’t drowning – that’s not the issue,” he said, referring to the process by which increasing relative depth of the water over a reef deprives photosynthetic life forms of sunlight, killing the coral. “The effect of changing water depth is that it probably increases the potential for wave exposure.”

The scientists predicted the slowest coral growth rates for reefs along the coasts of Florida, Grand Cayman Island, Mexico and Belize, as well as in the Seychelles and the Maldives. Some of the fastest rates of growth – and perhaps the brightest prospects for a future in a swelling ocean – were predicted for reefs in Bonaire, the Southern Lesser Antilles and along the coasts of east Africa and western Australia. In these places, reef accretion will continue at relatively rapid rates – in some cases more than 6.1mm (0.24in) per year. Under the two different scenarios that the scientists considered, sea level is predicted to rise 6.7–9.7mm (0.26–0.38in) per year.

While Perry and his colleagues provided detailed new data on localized coral reef growth rates, the general conclusions of the study aren’t especially surprising. It has long been assumed that rising seas will leave many coral reefs in deeper water. For instance, research published in 2017 predicted that increasing wave energy in reef “shadow zones” would cause a flattening and realignment of tropical coastlines.

The lead author of that study, Eduardo Siegle, an oceanographer at the Oceanographic Institute of the University of Sao Paulo, said his research, conducted on reefs in northeastern Brazil, shows that wave energy will increase by 50 percent, with a 0.5m increase in sea level relative to underlying reefs, and by 90 percent with a 2m rise.

“Two meters is very possible, because not only do some scenarios show two meters of rise but also because of reef loss,” he said.

These changes, Siegle said, will lead to increased sediment transport, more erosion and coastline retraction.

“One major problem we have is that some of the sand projections behind these reefs have populated areas and cities on them,” he said. Joao Pessoa and Recife, for instance, are two Brazilian cities that depend to some degree on reef protection.

Another paper, published in April, warned that many atoll communities will see their freshwater sources – especially low-lying springs – increasingly inundated during storms and periods of large oceans swells, representing a clear threat to future habitation. The paper predicted a “profound impact on low-lying coastal areas” from sea-level rise by the end of the 21st century. The research, led by United States Geological Survey research geologist Curt Storlazzi, warned that effects will be “amplified in the tropics, where sea level will be higher than the global average.” Storlazzi’s paper predicted “annual wave-driven overwash of most atoll islands by the mid-21st century.”

Waves breaking along the shoreline of a low-lying reef island in Chagos in the Indian Ocean. (Professor Chris Perry, University of Exeter)

“This annual flooding will result in the islands becoming uninhabitable because of frequent damage to infrastructure and the inability of their freshwater aquifers to recover between overwash events,” Storlazzi and his coauthors wrote.

Nancy Knowlton, the Smithsonian Institution’s Sant chair for marine science and a veteran of coral reef studies, said in an email that “any low-lying island in the hurricane typhoon belt is at existential threat” from rising sea levels and loss of coral wave protection.

In their new paper, Perry and his team describe a grim outlook for coastlines currently sheltered by coral reefs. Perhaps the most troubling prospect for these islands is the fact that “because of the delayed response of processes contributing to [sea-level rise],” including deep ocean warming and melting of glaciers, “submergence trends are projected to increase toward the end of the century.” This makes the higher-end projections for sea-level rise very feasible, they warned, “exacerbating the threat to coastal communities and to small island developing states.”

Among the fastest coral growth rates seen by Perry and his collaborators were those in the Chagos Archipelago. However, a major 2016 bleaching event – which occurred after the researchers collected their data – seriously damaged the coral reefs in this Indian Ocean island group.

“We just returned from a three-week expedition out there,” Perry said. “Bleaching has had a big impact there.”

Perry said the consequences on coral growth rates from bleaching events can last as long as a decade or more. If bleachings become routine events that occur every several years, they could essentially halt coral growth.

The researchers describe a “worrying end-point scenario” in which the increasing frequency of coral bleaching events causes reefs to “become locked into permanent low accretion rate states, leading to increasing rates of submergence under all [sea-level rise] scenarios.”

By an unlucky chance of coral physiology, it happens that the fastest-growing corals are also the most susceptible to bleaching, Perry said, making it especially likely that more frequent bleaching will cause the water depth above those reefs to increase. Ocean acidification, too, “may negatively impact reef calcification and increase bioerosion” of reefs, the paper states.

Perry said global-scale reductions of greenhouse gas emissions will be necessary to alter the future he and his team have predicted.

“But there is also obviously a need for improved local management and for actions that can increase the resilience of coral communities to better deal with or recover from disturbances,” he added.

Knowlton said that “reducing the local threats [to corals] buys incredibly valuable time.” These threats include overfishing of algae-eating fish that keep corals healthy and water pollution.

Siegle expects widescale problems for coastal areas as coral reefs essentially sink underwater. He noted that protective seawalls tend not to provide lasting protection, since seaward waves undercut their foundations.

“To protect shorelines will be a very difficult task, but something needs to be done,” he said.

Perry and his colleagues call for “urgent action” to mitigate the changes that will impact coral reefs, but Siegle believes there may be little hope of significantly altering the future.

“We will have to adapt,” he said. “Sea-level rise is happening – it’s inevitable.”

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