Oceans cover more than 70 percent of the Earth’s surface, and yet the vast majority have not been mapped or explored.

But there are two things we can say with certainty: Oceans are the hottest they have been in recorded history, by a wide margin. And man-made climate change is to blame.

From the North Atlantic to Florida to Antarctica, record water temperatures are forcing scientists to grapple with how climate change is warming the oceans, often in unpredictable and extreme ways, with implications for the entire planet. In today’s newsletter we’ll take a deep dive into what’s happening, with help from our colleagues.

‘Astonishing’ heat

The average temperature of the world’s oceans spiked to 70 degrees Fahrenheit in April, and it’s nearly back to that level once again. Those are the highest readings since records began, our colleague Elena Shao reports. (She also made all of the graphics in today’s newsletter.)

Take a look at daily average sea surface temperatures. The light gray lines are each a year’s worth of temperatures, the dotted dark gray line is the average from 1991-2020 and the blue line is this year:

Some of this year’s warmth can be attributed to El Niño, a natural climate pattern that began in July and is linked to marine heat waves. But the underlying cause of the heat is human-driven climate change.

It’s hot all over the world right now, with record heat waves searing three continents during July, which was probably Earth’s hottest month in recorded history. But it’s not just the current high temperatures on land that are making the oceans so hot.

Sea surface temperatures have been rising since at least the early 20th century, when humans began pumping many more greenhouse gases like carbon dioxide into the atmosphere.

Oceans have absorbed almost all of that heat, as you can see in the graphic below:

Water has a much higher heat capacity than land, meaning it can absorb large amounts of energy with only a slight increase in temperature. But after many decades of cumulative heat, we’re now starting to see a big shift.

Gregory Johnson, an oceanographer at the National Oceanic and Atmospheric Administration, called this year’s temperatures “astonishing.” The oceans have “been doing us a big service by delaying global warming considerably,” he told Elena, “but it comes at a cost.”

As oceans store more heat, they expand, contributing to sea level rise. Warmer ocean temperatures also provide more fodder for tropical cyclones and other extreme weather.

Much of the recent increase in ocean temperatures is shocking, but not unexpected, according to models developed by climate scientists. Take a look at how oceans could heat up if we continue to pump greenhouse gases into the atmosphere at roughly our current rate. July’s global average sea surface temperature falls within that expected range, though at the higher end:

However, the spike in the North Atlantic is beyond the expectations of most models, with temperatures consistently reaching more than 2 degrees Fahrenheit, or 1.1 Celsius, higher than what is typical for this time of year.

That suggests “something somewhat extraordinary may be happening there,” said Zeke Hausfather, a climate scientist at Berkeley Earth, a nonprofit research institute. (His newsletter, written with Andrew Dessler, a climate scientist at Texas A&M, is an excellent read.)

Last month, a buoy off the coast of Florida recorded a stunning reading of 101.1 degrees Fahrenheit, or just over 38 Celsius, a possible world record. And those temperatures are having an immediate and deadly effect on one of the world’s must crucial ecosystems.

Coral reefs are dying, and we need them alive

Few patches of the world’s oceans recorded more striking temperatures this July than the waters around Florida, which has been devastating for coral reefs.

Our colleague Catrin Einhorn wrote an extraordinary story about the people who have spent their careers trying to restore coral reefs. They are now racing to remove coral from the ocean and put it in tanks on land.

“They’re pushing through feelings of grief and fear over the future to save what genetic material and young corals they can,” Catrin wrote. “But in the background, an existential question looms: How can they restore reefs if the ocean is getting too hot for coral to live there?”

“It just felt like, Oh my God, we’re in the apocalypse,” said Bailey Thomasson, who works for the Coral Restoration Foundation, a nonprofit group based in the Florida Keys. The grief isn’t just in Florida; over a third of reef-building coral species around the world are threatened with extinction.

An increase in temperature between one or two degrees Celsius over several weeks can force corals to expel the algae living in their tissues. The algae are what makes corals colorful, so this process is called bleaching.

Algae are also the main source of energy for corals. If temperatures remain high and algae don’t return, the corals die. Between 2016 and 2017, an enormous bleaching event caused the Great Barrier Reef, the largest living structure on Earth, to lose half of its corals.

Losing more coral would be an ecological tragedy. “People were choking back tears or openly crying in interviews,” Catrin said. But it’s more than that.

A quarter of all marine life depends on corals. Many of these species are a major source of food for millions of people, especially in poor countries. Corals also protect coasts from sea level rise, support tourism industries around the world and serve as an early warning system for other marine life.

Researchers are studying how some species are migrating to cooler environments. They’re also trying to find corals that can withstand heat stress so they can one day create tougher reefs.

But the success of restoration efforts ultimately depends on our ability to stop emitting greenhouse gases. One specialist told Catrin that until that happens, all of the work by coral experts may just “buy time for just a few years.”

More unknowns

Scientists are investigating how quickly things are changing in the oceans.

It’s winter in Antarctica, but sea ice isn’t forming like it should: There are about a million fewer square miles of ice than expected. “This year is really different,” one expert told our colleagues Delger Erdenesanaa and Leanne Abraham. “It’s a very sudden change.”

Some researchers suspect that we are finally seeing the effects of the slowly but steadily warming oceans on Antarctica’s previously resilient sea ice. That ice also serves as a protective, frozen moat — shielding the continental ice sheet and its glaciers, which have already been destabilized by climate change, from the warmer ocean and the eroding force of wind and waves.

The amount of ice Antarctica loses to the ocean is one of the biggest factors in determining sea level rise.

A key system of ocean currents could collapse this century, a study found. The mighty network of ocean currents that shapes the climate around the North Atlantic is called the Atlantic Meridional Overturning Circulation, or AMOC. The last time it slowed down, 12,800 years ago, it seems to have plunged Europe into a deep cold for over a millennium, our colleague Ray Zhong explained.

Some scientists are skeptical about the methods that were used in the study, and more work is certainly needed to confirm these findings. But they’re still alarming.

“There is something happening, and it’s likely out of the ordinary,” one scientist told Ray. “Something that wouldn’t have happened if it weren’t for us humans.”

The big picture: Oceans have dampened the effects of climate change for decades by absorbing most of the heat we unleashed. But now they’re sounding the alarm, loudly.

Other climate news

Waking up to the alarm bells

Yes, the message the oceans are sending is scary. But in the last few months there have also been positive developments. I called Jack Daly, a marine scientist at Oceana Canada, an ocean conservation nonprofit, to ask what makes him hopeful these days.

Nations approved a treaty to protect biodiversity in the international waters that cover almost half of the world. And a few days ago, the international agency charged with regulating seabed mining effectively delayed efforts to extract metals from the deep seas. “These things are really great for the unregulated part of the ocean,” Daly said.

Countries are also moving to protect waters in their own jurisdictions, he said, as part of an agreement to protect 30 percent of the planet’s land and oceans, signed in December. “For example, Canada has been legislating in the past few years to legally rebuild fisheries,” he said.

That doesn’t make up for the fact that we have not stopped emitting greenhouse gases. “That’s the real long term solution,” he said. But “what we’re seeing now is a real concerted effort globally to look at the ocean as a global commons,” Daly said. “I do think that our world governments are waking up to the alarm bells.” — Manuela Andreoni