A sinister warming event is unfolding in the oceans

The dark red areas on the map above show that the Pacific waters off South America are currently very warm. This is an unusual “coastal El Niño” that is not related to the larger El Niño with global climate implications, says biological oceanographer Francisco Chavez of the Monterey Bay Aquarium Research Institute. A classic El Niño is a band of warm water that develops across the Pacific. That’s in contrast to the La Niña we’ve had in recent years, which is a stretch of cold water in the Pacific.

Models suggest there is a 62 percent chance of a classic El Niño developing in June or July, with a four in 10 chance of a strong El Niño. But it is not a sure thing because El Niño is a consequence of complex atmospheric dynamics, basically, the wind that blows hot water from Asia. “There is still a lot of uncertainty,” Chávez says. “Forecasting the true El Niño is difficult because the atmosphere is chaotic.”

Whenever El Niño arrives, it will have consequences. On the plus side, there tends to be less hurricane activity in the Atlantic when El Niño is active in the Pacific. But the rainfall results are mixed: For Peru, El Niño tends to bring more rainfall, but east of the Amazon rainforest, it can lead to a devastating drought. And all that extra heat in the Pacific could raise global temperatures significantly. “There is a chance that 2023 will be the warmest record year,” says Rohde. “If El Niño develops, as we now believe is likely, 2024 will likely be warmer than 2023.”

In the ocean itself, warmer waters, due to El Niño or simply long-term general warming, can become less biologically productive. Some organisms that reach their thermal limit can migrate to colder waters, transforming both the ecosystems they leave behind and the new ones where they take refuge. But others, like corals, are stuck in place. these animals are particularly sensitive to heatand they blanch in response, releasing their symbiotic algae that provide them with energy.

The ocean food chain also depends on the natural circulation of water, which is influenced in part by temperature. When cold water from the depths rises to the surface, it brings nutrients that fertilize the phytoplankton. These microscopic plants grow in the sunlight and become an essential food source for tiny animals called zooplankton. But when the water warms up at the surface, it stratifies and becomes a kind of layer that sits on top of the cooler waters below. “The bigger the cap, the harder it is to break. By warming the ocean, you’re basically going to decrease the amount of nutrients that come out,” says Chávez. “A longer-term concern is: How much will this general warming change natural fertilization processes, such as upwelling? Will the ocean turn into a desert over time?