The heating across the world’s oceans is literally off the charts. Last week, a buoy in Florida’s Manatee Bay recorded a water temperature of 101 degrees Fahrenheit. And since March, global average sea surface temperatures have kept smashing record highs.
You can see the global average for 2023 represented as the solid black line in the chart below. (The other squiggles are previous years.)
Courtesy of University of Maine
Meanwhile, temperatures in the North Atlantic have been climbing higher and higher above the highs in previous years. Last week, that ocean notched its highest temperature since records began in the early 1980s.
Scarier still, the North Atlantic doesn’t typically see its highest annual temperature until early September, so it’s likely to keep breaking records in the coming weeks. (In the chart below, the North Atlantic’s record-setting temperatures are the solid black line.)
Courtesy of University of Maine
What we’ve been seeing this summer is the product of the natural variability inherent in Earth’s climate combined with humanity’s rapid warming of the planet. El Niño, the band of warm water in the Pacific, has also formed and strengthened, raising global temperatures. When variability meets a long-term trend of rising temperatures, “the warms just get warmer,” says Michael Jacox, an oceanographer at the National Oceanic and Atmospheric Administration. “And when things would have been sort of extreme already, they’re that much higher.”
Media attention has focused on corals—in Florida’s hot-tubbish waters and elsewhere—and rightfully so. When stressed by high temperatures, they bleach, 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 their economic importance,” says Peter Roopnarine, a paleoecologist and the curator of invertebrate zoology and geology at the California Academy of Sciences. “But this is really affecting everything, in every aspect of the ocean and ocean life, and it goes well beyond the corals.”
Consider the plankton—literally “wandering” from the Greek. This galaxy of organisms makes up the base of the oceanic food web. Phytoplankton are microscopic floating plants that feed on sunlight. These are in turn eaten by animals called zooplankton, including small crustaceans and fish larvae. Zooplankton are consumed by larger critters, like adult fish. “Phytoplankton will drive zooplankton, which will drive fish and will feed other things,” says Francisco Chavez, a biological oceanographer and senior scientist at the Monterey Bay Aquarium Research Institute. “The whole ecosystem has to be impacted in some form or another under warmer sea surface temperatures.”
Warmer temperatures will themselves stress out any number of the species in the planktonic community. Like corals do in reef ecosystems, the organisms of the open ocean have certain tolerances for heat. “A big part of the problem is we don’t know the optimal temperature ranges for probably 99 percent of the organisms out there,” Roopnarine says. “We know they have them, but it’s a very difficult thing to measure.”
The sea has absorbed around 90 percent of the excess heat humanity has pumped into the atmosphere—and it shows. By 2014, half of the world’s ocean surface was logging temperatures once considered extreme, which rose to 57 percent by 2019. In other words, extreme heat has become the new normal.
“Twenty years ago, we were talking about how 2050 would be when we could really point to dramatic things beginning to happen, and we would be in trouble by 2080, 2100,” says Roopnarine. “Literally—I would say every year for the past 15 years—things are happening that tell us our models have been a bit too slow. The speed at which this has been happening, I think, is quite surprising.”
Heat on its own isn’t the only concern. When oceans warm, a few things happen physically and chemically to the surface waters that these organisms call home. The warmer seawater gets, the less oxygen it can hold. As the planet rapidly warms, scientists have found that ocean oxygen levels have been steadily dropping, in some cases precipitously: The loss is up to 40 percent in tropical regions. That, of course, deprives organisms of the oxygen they need to survive.
Secondly, the warmer water gets, the less dense it becomes. At the surface, you end up with a band of hot water, with cooler waters in the depths, a layering 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 cold pretty fast,” says Michael Behrenfeld, an ocean ecologist at Oregon State University. “That’s the stratification layer that you’re going through.”
In the ocean, this warm water acts like a cap that interrupts critical ecological processes. Normally, nutrients well up from the depths, providing food for the phytoplankton floating at the surface. Stratification prevents that. In addition, winds typically blow across the surface and mix that water down deeper, also bringing up nutrients. But with stratification, the contrast between the surface layer of warm water and the underlying cold water is so strong that it’s very difficult for wind energy to mix the two.
Together, all of these mean that phytoplankton in a warmer ocean are deprived of the nutrients they need. In response, they produce fewer of the pigments they use to turn sunlight into energy. “Phytoplankton will decrease their photosynthetic pigments because they’re becoming more nutrient-stressed,” says Behrenfeld. “They don’t need to harvest as much light because they don’t have enough nutrients to do as much photosynthesis as they did before.” (Behrenfeld can actually see transformation in satellite imagery.)
They also reduce their pigment production because of their increased exposure to light. Without the wind mixing the water, they’re stuck in that cap of hot water at the surface for longer. With access to more light, they need less pigment in order to do the same amount of photosynthesis.
“The nutrient stress part is what we’re really worried about,” says Behrenfeld. “If it’s more stressed, there’s less photosynthesis, which means less production of organic material for the food chain, which feeds fish.”
The warming of the world’s waters is creating winners and losers in the phytoplankton community. As temperatures go up, smaller species of phytoplankton tend to proliferate, which feed smaller species of zooplankton, which start to dominate the ecosystem. The larger species of zooplankton then have to spend more energy to gather enough of the tiniest phytoplankton to fill up. (Imagine surviving on a steady diet of cheeseburgers and then having to switch 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 best adapt to the warmer waters and changes in the food supply have an advantage. The zooplanktonic copepod species Calanus finmarchicus, for instance, typically lives at subarctic latitudes. “But it’s penetrating farther and farther north,” says Meyer-Kaiser, “and becoming more and more common, and coming to dominate the community up there as you have temperatures rising and warm water influx.”
But the heat threatens other varieties of zooplankton, like the larvae of seafloor-dwelling creatures. These are cold-blooded animals, whose metabolisms speed up as temperatures rise. So the larvae of crustaceans can carry yolk from their mothers as they wander the open ocean, but they burn through that food faster. “They may not be able to disperse quite as far before they become desperate and absolutely have to settle down to the seafloor,” says Meyer-Kaiser. “Early life history stages, such as larvae, tend to be more sensitive to environmental changes than adults of the same species. So they could potentially not be able to survive in a heat wave.” That, of course, could impact fisheries, threatening the livelihood of subsistence fishers.
These ripple effects could well extend into Earth’s climate system. When phytoplankton grow, they sequester carbon, like plants do on land. When zooplankton consume them, their poop sinks to the seafloor, locking that carbon in the depths. In addition to the ocean waters themselves sequestering carbon from the atmosphere (carbon dioxide dissolves in water, which raises its acidity, which is how you get ocean acidification), this is a hugely important way that the planet removes some of humanity’s emissions.
As plankton attempt to adapt to a hotter habitat, “we are going to see dramatic changes at both species and community levels. This is a revamping of the entire Earth system,” says Meyer-Kaiser. “My prediction is the ecosystem will figure out ways to adapt. Sure, we’ll lose biodiversity. Sure, we’re going to lose important functions. But animals will continue to persist.”
