The experts at the U.N.’s Intergovernmental Panel on Climate Change (IPCC) had a particularly pressing challenge as they prepared the newest assessment on global warming science, the first chapter of which was released in September. The problem was that the climate wasn’t acting the way they’d expected. In recent years, global greenhouse gas emissions had kept rising—hitting an all-time record in 2012. Yet even though the carbon concentration in the atmosphere gradually increased, passing the 400 parts per million threshold earlier this year, the planet’s average surface temperatures have remained pretty much the same over the past 15 years. The Earth hasn’t cooled—this past decade has still been the hottest on record—but temperatures haven’t risen as climate models predicted. Call it a “pause,” call it a “hiatus,” but the question is clear: where’s the heat?
Try the ocean. That’s one takeaway from a new paper published in Science today, one of a number of studies suggesting that the oceans depths seem to be soaking up the excess heat energy created by the accumulation of greenhouse gases. Researchers led by Yair Rosenthal at Rutgers University reconstructed temperatures in one part of the Pacific Ocean and found that its middle depths have been warming some 15 times faster over the past 60 years than at any other time over the past 10,000 years. It’s as if the oceans have been acting as a battery, absorbing the excess charge created by the greenhouse effect, which leaves less to warm the surface of the planet, where we’d notice it.
That means global warming is still happening, even if hasn’t necessarily been reflected in recent surface temperature changes. But there’s no guarantee that won’t change in the future. “We may have underestimated the efficiency of the oceans as a storehouse for heat and energy,” said Rosenthal in a statement. “It may buy us some time—how much time, I really don’t know. But it’s not going to stop climate change.”
The Science study isn’t the first to peg the oceans as a possible reservoir for the missing heat. An August study in Nature found that a cooler Pacific ocean seemed to be offsetting global warming, and other studies have indicated that the oceans began taking on significant heat around the same time that surface warming began to slow down in 1998. That shouldn’t be surprising—the vast oceans carry 93% of the stored energy from climate change, compared to just 1% for the atmosphere, with melting ice and landmasses making up the rest.
But the Science study goes back further, using sediment core samples taken from the seas around Indonesia, where the Indian and Pacific Oceans mingle. By measuring the levels of magnesium to calcium in the shells of the Hyalinea balthica, a uni-celled organism buried in those sediments, the researchers were able to estimate the temperature of the middle-depth waters where H. balthica lived, between 3,000 ft. and 1,500 ft. (914 m and 457 m) below the surface. Over the past 10,000 years—a period of time known as the Holocene, when the plant’s climate was relatively stable and human civilization arose—the Pacific generally cooled, with a few exceptions, until about 1600, when ocean temperatures began gradually warming.
Over the last 60 years, however, water column temperatures increased by 0.32º F (.185º C)—roughly 15 times faster than any other time over the past 10,000 years. That might not sound like much of a change—surface temperatures rose about 1.4º F (0.78º C) over the past century—but the sheer scale of the oceans underscores just how much energy you need to heat it up even that much. The study is also a reminder that climate change won’t unfold steadily. Surface temperatures could remain stable for a number of years, as they have recently, only to spike suddenly.
The ocean depths still remain somewhat of a mystery to scientists, and they remain woefully understudied given the outsized impact they have on the planet’s climate. Initiatives like the XPRIZE’s new ocean science contests may help produce needed data about the undersea world, though they’ll take place against further budget cuts in the underappreciated National Oceanic and Atmospheric Administration. It’s become very clear that if we’re going to understand climate change fully—and predict it more precisely—we’ll need to understand the oceans much better than we do now. We may live on land, but our planet is still a blue one.