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It’s even hot underwater

The Caribbean’s marine heat wave will have big impacts on tiny islands like Dominica.

Snorkeler among yellowish coral.
Simon Walsh points out diseased coral off the coast of Dominica.
UN Foundation/Kreig Harris
Umair Irfan is a correspondent at Vox writing about climate change, Covid-19, and energy policy. Irfan is also a regular contributor to the radio program Science Friday. Prior to Vox, he was a reporter for ClimateWire at E&E News.

SOUFRIÈRE, Dominica — The clear waters here tend to be pleasantly warm year-round, a key part of the appeal for snorkelers and divers eager to see the elkhorn, brain, and flower corals just a few feet off the rocky beach.

The 72,000 residents of Dominica, a tiny volcanic island between Guadeloupe and Martinique, pride themselves on their relatively pristine coastline and bill their country as the Caribbean’s nature island.

But this year, the ongoing record-breaking heat wave on the surface of the Caribbean Sea is threatening the coral, the fish that live among them, and the economy that they sustain. “We know that extreme heat can be destructive and deadly for marine systems,” said Lauren Gaches, director of public affairs for NOAA Fisheries, in an email. It’s yet another blow to a delicate ecosystem already reeling from invasive species and disease.

Like heat waves on land, marine heat waves are usually defined as temperatures above the 90th percentile of the historical range in a given region. The world’s oceans have been unusually warm this year, and the Atlantic Ocean in particular has reached record high temperatures for this time of summer.

Map of sea surface temperatures in the Caribbean Sea on July 20, 2023
The Caribbean Sea is facing a record heat wave.
Ocean Prediction Center/NOAA

Connected bodies of water, like the Gulf of Mexico and the Caribbean Sea, are also experiencing extreme heat. Fueled by rising average temperatures, colliding weather cycles, and a strong El Niño, Caribbean waters have reached 90 degrees Fahrenheit, more than two degrees Fahrenheit above the average for this time of year. These are the hottest temperatures in at least a century.

Graph of June sea surface temperature anomalies in the Caribbean since 1910.
The Caribbean Sea just had its hottest June on record.
National Centers for Environmental Information/NOAA

It might seem like a small bit of warming, but it may have global consequences.

Scientists pay close attention to sea surface temperatures because that’s where the water meets the sky. It governs evaporation, which in turn shapes air currents and rainfall. These factors can then alter storms and drought, sometimes as far as the other side of the planet.

The warm waters also have an outsize effect on small island countries like Dominica, which are already front lines of the planet’s biggest environmental threat. The scale of the damage from the ongoing oceanic heat wave isn’t yet clear, but events like this in the past have proven devastating for coastal economies and marine wildlife.

The Caribbean and its ecosystems have been in hot water for a while

Simon Walsh, the proprietor of Nature Island Dive, floats from rock to rock, pointing out tiny discolored splotches on the golden and ochre corals anchored below.

The lesions are symptoms of stony coral tissue loss disease, an epidemic spreading through the waters of the Caribbean. According to NOAA, it can cause “complete colony mortality in a matter of weeks.”

Left unchecked, the fear is that the disease could wipe out huge swaths of precious coral species. “We are definitely seeing the most aggressive coral disease in the history of coral diseases,” said Walsh.

That, in turn, could ripple into one of Dominica’s most critical economic sectors. Tourism makes up 25 percent of Dominica’s economy. Coral loss would also disrupt fishing, a key source of food and revenue for many on the island.

So any threat to the coral is a threat to the way of life on the island. For his part, Walsh and his fellow divers have been treating infected corals as they spot them, applying antibiotics with syringes to halt the spread of the infection.

But stony coral tissue loss disease isn’t the only problem. Invasive species like lionfish have devoured smaller fish that eat algae on coral, allowing the algae to grow unchecked.

A sign reading “save the reef; eat a lionfish” Umair Irfan/Vox
A fried lionfish sandwich
Invasive lionfish have become part of the local cuisine in Dominica.
Umair Irfan/Vox

Now, this year’s record-breaking heat wave on the surface of the Caribbean Sea stands to deliver another blow.

Hotter surface water can slow upwelling, a phenomenon that brings nutrients from deep in the water like nitrogen and phosphorus compounds toward the surface, feeding the plankton that form the foundation of the food pyramid. Roughly half of the fish in the world are caught in upwelling zones.

Hotter water also holds onto less oxygen, which can suffocate sea life. Earlier this year, thousands of dead menhaden fish washed up on the shores of Texas, due in large part to high water temperatures. The water itself becomes more acidic as it heats up, which can disrupt coral formation. And when the temperature rises too high, coral will expel the symbiotic algae that live in their tissues, turning completely white. This is called coral bleaching. All these effects of hotter water temperatures can then make coral more vulnerable to disease.

Sargassum, a type of algae, has seen massive and growing blooms in the Caribbean Sea and Atlantic Ocean in recent years, but there’s been a record quantity this summer, enough to be seen from the sky, washing up on beaches from South America to Florida. As it rots, it emits smelly, toxic hydrogen sulfide gas. Higher water temperatures are likely a factor here as well.

It’s not clear yet exactly how the alarmingly hot waters of the Caribbean will affect seafood catches, but scientists warn that extreme marine heat has proven devastating to fisheries in the past.

Fishermen hauling in their daily catch in Soufriere, Dominica
Warmer sea surface temperatures threaten fishing, a vital source of food in Dominica and a key part of its economy.
Umair Irfan/Vox

A massive heat wave in the northeastern Pacific Ocean from 2013 to 2016, dubbed the blob, stretched from Alaska to California. “This led to an ecological cascade, causing fishery collapses and fishery disaster determinations,” said NOAA’s Gaches. The salmon population crashed while toxic algae bloomed, leading to thousands of dead birds and starving sea lions.

As with heat waves on land, climate change is poised to make marine heat waves more frequent and intense. The ocean, after all, has absorbed 90 percent of the heat due to humans burning fossil fuels. And that’s likely to affect everyone in the world, even if they don’t like seafood.

According to the UN Food and Agriculture Organization, 3.3 billion people get at least 20 percent of their animal protein from the water. And according to the World Bank, 600 million people’s livelihoods depend on fisheries.

So while small island countries like Dominica are facing the most direct and immediate effects of the warming oceans, the whole world will soon feel the heat.

This story was supported by a grant from the United Nations Foundation.

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