THE HEATING IN THE WORLD’S OCEANS IS LITERALLY OUT OF THIS WORLD. A buoy in Florida’s Manatee Bay registered a water temperature of 101 degrees Fahrenheit last week. Since March, global average sea surface temperatures have continued to break records.

In the graphic below, the global average for 2023 is depicted by a solid black line. (The remaining squiggles are from prior years.)

Meanwhile, temperatures in the North Atlantic have been steadily rising past previous years’ highs. That ocean reached its highest temperature since records began in the early 1980s last week.

Worryingly, the North Atlantic normally does not reach its highest annual temperature until early September, so it is certain to continue breaking records in the following weeks. (The solid black line in the chart below represents the North Atlantic’s record-breaking temperatures.)

What we’ve been seeing this summer is the result of natural climatic variability mixed with humanity’s fast warming of the globe. El Nio, the Pacific belt of warm water, has also formed and strengthened, rising world temperatures. “The warms just get warmer,” says Michael Jacox, an oceanographer at the National Oceanic and Atmospheric Administration, when unpredictability meets a long-term trend of rising temperatures. “And when things were already kind of extreme, they’re that much higher.”

Corals have received a lot of attention recently, both in Florida and elsewhere, and with reason. They bleach when stressed by high temperatures, releasing the symbiotic algae that harvest energy for them. “Of course, there’s a lot of concern around coral reef systems because of their biodiversity and economic importance,” says Peter Roopnarine, paleoecologist and curator of invertebrate zoology and geology at the California Academy of Sciences. “However, this is affecting everything, in every aspect of the ocean and ocean life, and it extends far beyond corals.”

Consider plankton, which means “wandering” in Greek. This galaxy of species serves as the foundation of the oceanic food web. Phytoplankton are small floating plants that get their energy from sunshine. These are then devoured by zooplankton, which includes small crustaceans and fish larvae. Zooplankton is eaten by larger animals such as adult fish. “Phytoplankton will drive zooplankton, which will drive fish and feed other things,” says Francisco Chavez, a senior scientist at the Monterey Bay Aquarium Research Institute and a biological oceanographer. “Warmer sea surface temperatures must have an effect on the entire ecosystem in some way.”

Warmer temperatures will affect a variety of organisms in the planktonic ecosystem. The species in the open ocean, such corals in reef environments, have heat tolerances. “A big part of the problem is that we don’t know the optimal temperature ranges for probably 99 percent of the organisms out there,” says Roopnarine. “We know they have them, but it’s a very difficult thing to measure.”

The sea has absorbed around 90% of the excess heat that humans have blasted into the atmosphere—and it shows. By 2014, half of the world’s ocean surface was recording temperatures considered high, a figure that will rise to 57 percent by 2019. To put it another way, excessive heat has become the new normal.

“Twenty years ago, we were talking about how 2050 would be the year when we could really point to dramatic things beginning to happen, and we’d be in trouble by 2080, 2100,” Roopnarine adds. “Every year for the past 15 years, things have happened that tell us our models have been a little too slow.” The rate at which this has occurred, I believe, has been fairly startling.”

Heat isn’t the only source of concern. When the oceans warm, a few physical and chemical changes occur in the surface waters that these species inhabit. The less oxygen that ocean can hold, the warmer it becomes. As the earth quickly warms, scientists have discovered that ocean oxygen levels have been continuously declining, in some cases precipitously: the loss in tropical parts is up to 40%. Of course, this deprives creatures of the oxygen they require to exist.

Second, when the temperature rises, the density of the water decreases. The result is a band of hot water on the surface and colder waters in the depths, a layering process known as stratification. “If you’ve ever gone swimming in a lake in the summer, if you’re at the surface, it’s nice and warm, and then you dive down and it gets pretty cold pretty fast,” says Michael Behrenfeld, an ocean biologist at Oregon State University. “That’s the stratification layer you’re going through.”

This heated water acts as a cap on the ocean, disrupting vital ecological processes. Normally, nutrients rise from the depths, feeding the phytoplankton that floats at the surface. Stratification stops this from happening. Furthermore, winds often blow across the surface, mixing the water down deeper and bringing up nutrients. However, with stratification, the contrast between the surface layer of warm water and the underlying cold water is so stark that wind energy has a difficult time mixing the two.

All of this means that phytoplankton in a warmer water are being deprived of the nutrients they require. As a result, they create less pigments that they utilize to convert sunlight into energy. “Phytoplankton will reduce their photosynthetic pigments as they become more nutrient-stressed,” Behrenfeld explains. “They don’t need to harvest as much light because they don’t have enough nutrients to perform as much photosynthesis as they did before.” (In satellite imagery, Behrenfeld can witness transformation.)

They also inhibit pigment formation as a result of increased light exposure. They’re caught in that cap of hot water at the surface for a longer period of time without the breeze mixing the water. With more light, they require less pigment to do the same amount of photosynthesis.

“We’re most concerned about the nutrient stress,” Behrenfeld says. “If it’s more stressed, there’s less photosynthesis, which means less production of organic material for the food chain, which feeds fish.”

The phytoplankton community is seeing winners and losers as the world’s oceans warm. As temperatures rise, smaller phytoplankton species thrive, feeding smaller zooplankton species, which begin to dominate the ecosystem. The larger zooplankton species must then expend more energy to acquire enough of the little phytoplankton to fill up. (Imagine living on a continuous diet of cheeseburgers only to be forced to convert to sliders.)

“In a lot of cases, plankton can be quite resilient, but you get changes in the community composition,” says Kirstin Meyer-Kaiser, a marine biologist at the Woods Hole Oceanographic Institution. The species that can adapt the best to warmer seas and changes in food supply will have an advantage. Calanus finmarchicus, a zooplanktonic copepod, lives at subarctic latitudes, for example. “But it’s penetrating farther and farther north,” Meyer-Kaiser says, “and becoming more common, and coming to dominate the community up there as temperatures rise and warm water influx.”

However, the heat endangers other types of zooplankton, such as seafloor-dwelling invertebrate larvae. These are cold-blooded creatures whose metabolisms accelerate as temperatures rise. Crustacean larvae can take yolk from their moms as they travel the open ocean, but they consume it faster. “They may not be able to disperse quite as far before becoming desperate and having to settle down to the seafloor,” Meyer-Kaiser speculates. “Larvae, for example, tend to be more sensitive to environmental changes than adults of the same species.” As a result, they may not be able to endure a heat wave.” Of course, this might have an impact on fisheries, jeopardizing the livelihoods of subsistence fishermen.

These repercussions may well extend into the Earth’s climate system. Phytoplankton, like plants on land, sequester carbon as they grow. When zooplankton ingest them, their feces sinks to the seafloor, trapping the carbon. In addition to the ocean waters sequestering carbon from the atmosphere (carbon dioxide dissolves in water, raising its acidity, which is how ocean acidification occurs), this is a critical mechanism for the earth to remove some of humanity’s emissions.

“We are going to see dramatic changes at both the species and community levels” as plankton struggle to adapt to a hotter habitat. “This is a complete overhaul of the Earth system,” Meyer-Kaiser explains. “My prediction is that the ecosystem will find ways to adapt.” Yes, we will lose biodiversity. Sure, we’ll lose some critical functions. However, animals will continue to exist.”


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