It’s barely the middle of summer in the northern hemisphere, and heatwaves are once again breaking temperature records.
In the United States, dozens of cities sweltered through their hottest Fourth of July celebrations, with temperatures over 100 degrees Fahrenheit. In New Jersey alone, dozens died from the heat over the weekend. European authorities have linked thousands of deaths during the end of June to heat-related causes. And on Thursday, the European Union’s Copernicus Climate Change Service announced that Western Europe had experienced the hottest June ever recorded in the region.
The seas are running a fever, too. It was also the hottest June on record for the world’s oceans, according to Copernicus. Nearly 40 percent of ocean area worldwide is undergoing a marine heatwave, with intense hot patches in the Mediterranean Sea and the Pacific Ocean more than 10 degrees Fahrenheit hotter than usual. It’s the latest in a wave of ocean warming that began in 2023, fueling devastating cyclones and damaging the majority of the planet’s coral reefs.
As humans burn fossil fuels, the oceans absorb more than 90 percent of the excess heat trapped by the resulting greenhouse gases, causing them to warm. Waters at the surface also exchange heat and moisture with the atmosphere, helping to drive hotter temperatures and more extreme weather. A recent study found that at least a fifth of heatwaves on land begin in the ocean.
“Heat is one of the features of climate change that has already arrived,” said Ruth Engel, a data scientist for environmental health and extreme heat with the World Resources Institute. “It’s not something we need to prepare for 25 years from now. It’s already a deadly health issue now.”
In mild climates and historically hot ones alike, extreme heat has become an expected part of summer, Engel said. Last year, more people died from extreme heat than from road crashes in Europe, she added.
Because weather, climate, and oceans are so intertwined, it can take months for scientists to pinpoint the cause of any one heat event. But there are some early clues that can help researchers connect weather events to ocean heatwaves, said Zachary Labe, a scientist at Climate Central, a research nonprofit.
For example, scientists suspect that the recent heatwaves across Europe are tied to extra-warm temperatures in the Mediterranean Sea, he said. And the high temperatures and humidity causing Floridians to crank up their air-conditioners are probably related to a marine heatwave with water temperatures near 90 degrees F off the state’s Gulf coast. Unusually warm temperatures in the Pacific Ocean may have helped set up a weather pattern driving a potentially record-breaking heatwave forecast for the central and northern U.S. in the coming week, he said.
“The current patterns that are creating these heat domes are similar to those we’ve seen before,” Labe said. “But climate change is just acting to boost everything.” Essentially, warmer global temperatures mean worse consequences when extreme weather does hit, he said.
A thermometer attached to a bus stop in the city center of Madrid reads 53 degrees Celsius, or 127 degrees Fahrenheit.
Luis Soto/SOPA Images via Getty Images
Hotter air also makes the atmosphere spongier and capable of holding more water. For every 1 degree Celsius of warming, the atmosphere can hold about 7 percent more moisture. It also holds onto the water for longer. That means more time between rainfalls, and heavier, more dangerous deluges when rain does fall.
Scientists have linked climate change to heavy rainfall events such as the one that flooded Central Texas last summer. In recent days, Super Typhoon Bavi bore down on the Mariana Islands in the Western Pacific. It’s the second super typhoon with the “fingerprint of climate change” to form in the region in a handful of months.
Ocean warming is also a direct cause of sea level rise thanks to a phenomenon known as thermal expansion. As the seas warm, they expand and push further up coastlines, setting the stage for storms like Bavi to cause greater damage.
And recently, a recurring pattern of warm water known as El Niño began cycling through the Pacific, adding even more extreme weather to the equation. That’s because El Niño churns up some of the heat that the ocean absorbs, bringing it toward the surface where it interacts with the atmosphere and reshapes weather around the world. This year’s emerging event, considered a “super El Niño,” has a 81 percent chance of becoming one of the strongest in history, according to a recent forecast by the National Oceanic and Atmospheric Administration.
That means global average temperatures are likely to remain elevated well into early next year, said Matthew England, a professor of physical oceanography and climate science at the University of New South Wales in Australia. “The first year of an El Nino cycle tends to be on the warm side, but the records are generally broken in the second year,” he said.
Every El Niño is different, but each one comes with predictable effects that can help forecasters figure out what to expect, said Alex Sen Gupta, a professor at the Climate Change Research Center, also at the University of New South Wales.
In general, El Niño years are associated with heavier rainfall in places like California and South America and drier conditions across Australia and Southeast Asia. And although El Niño tends to suppress hurricanes near the United States, other regions tend to see stronger cyclones with more rainfall.
There isn’t evidence yet that climate change is making El Niños or La Niñas (the inverse pattern of cool water that forms in intervening years) stronger. But a hotter planet means more intense effects from both of them, Sen Gupta said. And while scientists need more years of data to say for sure, some have hypothesized that the swings between strong El Niño and La Niña years are becoming more frequent.
This story was originally published by Grist with the headline The oceans are full of heat, and it’s coming ashore on Jul 10, 2026.
