Brutal heat waves have baked the world this summer and they haven’t been contained to land. Earth’s oceans are the hottest they have been in modern history, by an unusually wide margin.
The planet’s average sea surface temperature spiked to a record high in April, and the ocean has remained exceptionally warm ever since. In July, widespread marine heat waves drove temperatures back up to near-record highs, with some hot spots nearing 100 degrees Fahrenheit (nearly 38 Celsius).
“I find it kind of astonishing,” said Gregory Johnson, an oceanographer at the National Oceanic and Atmospheric Administration, referring to this year’s trend. “This is a pretty big step up.”
The North Atlantic has seen some of the most exceptional warmth, with recent temperatures consistently reaching more than 2 degrees Fahrenheit (1.1 Celsius) higher than what is typical for this time of year.
Taking a dip in the waters off the coast of the Florida Keys could, at times, feel like stepping into a hot tub. Last week, one reading from a buoy recorded a stunning 101.1 degrees Fahrenheit (just over 38 Celsius), possibly a world record for sea surface temperatures.
The extreme heat is devastating Florida’s coral reefs, but high ocean temperatures can have more widespread impacts, too, disrupting other marine ecosystems and the communities that depend on them.
El Niño, a recurring global climate pattern that is typically linked to warmer conditions in many regions, arrived in June, and is one contributor to the spike in global sea surface temperatures, said Michelle L’Heureux, a climate scientist with NOAA’s Climate Prediction Center.
But the underlying influence of human-driven climate change is undeniable, she added.
Global sea surface temperatures have been increasing since at least the early 20th century when humans began sharply increasing the number of greenhouse gases pumped into the atmosphere.
This year’s spike in global sea surface temperatures is concerning, but it is not exactly unexpected in a warming world, said Zeke Hausfather, a climate scientist at Berkeley Earth, a nonprofit research institute.
Global climate models have projected how oceans could heat up if we continue to pump greenhouse gases into the atmosphere at roughly our current rate, and July’s high sea surface temperatures falls within the expected range, though at the higher end.
In the North Atlantic, however, temperatures have been warmer than climate models projected. That suggests “something somewhat extraordinary may be happening there,” Hausfather said.
Experts have speculated that other factors, in addition to human-caused climate change and the arrival of El Niño, could be contributing to this year’s exceptional ocean heat.
Some have suggested that international rules aimed at reducing air pollution from maritime shipping could have inadvertently increased ocean warming. Others point to the unusual absence of Saharan dust over the North Atlantic this year, which can also have a cooling effect by blocking sunlight.
The eruption of an underwater volcano in the Pacific Ocean near Tonga last year, which spewed tens of millions of tons of water vapor into the stratosphere, may have also influenced this year’s ocean temperatures. Water vapor, like carbon dioxide, is a greenhouse gas that traps heat near Earth’s surface.
But early analyses have so far suggested that those factors cannot account for all of this year’s extra warming.
“The level of warmth we are seeing today is only possible because of the warming over the past 150 years due to human activity,” Hausfather said.
Scientists expect warm ocean conditions to continue into the fall, with El Niño intensifying in the months ahead
While sea surface readings take the temperature of the top layer of the ocean, up to a few hundred feet deep, climate scientists have also been charting how much heat is being absorbed by the ocean as a whole. In short, it is a lot.
The ocean, which covers about 70% of the world’s surface, has absorbed more than 90% of the heat unleashed by the burning of fossil fuels, deforestation, and other human activity.
Put another way, the majority of the human-driven warming that has happened on Earth over the past six decades has accumulated in the ocean. But water has a much higher capacity than land to absorb and store all that heat.
The ocean has “been doing us a big service by delaying global warming considerably,” Johnson said, “but it comes at a cost.” As the ocean stores more heat, its water expands, contributing to sea level rise. Warmer ocean temperatures also provide more fodder for tropical cyclones and atmospheric river storms.
By the end of the century, ocean warming could contribute to a weakening, or even a shutdown, of the Atlantic Ocean currents that help regulate the climate for a swath of the planet, a new analysis found.
“In many ways,” Hausfather said, the ocean is “the most accurate thermometer we have for the actual effect of climate change because it’s where most of the heat ends up.”
This article originally appeared in The New York Times.