Call it the climate change conundrum: Even though humans are pumping more greenhouse gases than ever into the atmosphere, the world’s average air temperature isn’t rising as quickly as it once did. Some scientists have proposed that the missing heat is actually being trapped deep underwater by the Pacific Ocean. But a new modeling study concludes that the Pacific isn’t acting alone. Instead, it finds, several of the world’s oceans are playing a role in the warming slowdown by absorbing their share of the “missing” heat.
“There are a lot of details about exactly which ocean basin is taking up the energy,” says Andrew Dessler, an atmospheric scientist at Texas A&M University, College Station, who wasn’t involved in the study. But “I don't see anything in here that changes our expectations of long-term climate change.”
The world’s average air temperature has warmed 0.8°C since the late 1880s, but the warming has slowed precipitously in the last 15 years. Scientists have identified a number of factors—among them a temporary downturn in solar activity and more sun-blocking aerosol pollution—that at least partially explain why air temperatures have barely risen since the turn of the millennium. But recent research suggests that Earth is still taking in more energy from the sun than it’s letting out, to the tune of almost a 60-watt light bulb’s worth for every 100 square meters. This excess energy has to go somewhere. A potential answer? The tropical Pacific Ocean. Changing trade winds here may have helped lower sea surface temperatures by altering ocean circulation patterns and making it so heat that otherwise would be warming the air is now trapped deep underwater.
But Sybren Drijfhout, a physical oceanographer at the University of Southampton in the United Kingdom, questioned whether the Pacific can really account for all the ocean heat trapping and maintain the cool state at its surface on its own. He didn’t study this issue with a traditional climate model. Climate models usually struggle to simulate the warming slowdown on their own unless they incorporate the Pacific’s altered state from the start, he says. Plus, some studies have found heat trapped deep in oceans outside the Pacific, but Drijfhout says these studies don’t show where the heat actually first enters the ocean and how it may travel around once it’s underwater.
Armed with a unique model that simulates the world’s oceans using historical weather data on temperature, humidity, and wind, Drijfhout and his colleagues calculated how much heat is moving between the oceans and the atmosphere, as well as where it first enters the ocean. The model revealed that the oceans were trapping more than 80% of the missing heat. But the change in sea surface temperatures in the tropical Pacific could account for only 30% of the extra ocean heat uptake. The other 70% was split between the Atlantic Ocean, Indian Ocean, and waters just off Antarctica, Drijfhout and his team reported online this month in Geophysical Research Letters. That suggests that the Pacific isn’t single-handedly running the show after all.
The study has prompted disagreement from some other scientists, including Kevin Trenberth, a climate scientist at the National Center for Atmospheric Research in Boulder, Colorado. He maintains that the Pacific is still in charge of the heat uptake elsewhere because its surface cooling effect is strongly linked to how other ocean basins behave. In a modeling study reported online in August in Nature Climate Change, Trenberth and colleagues found that cooling the surface of the tropical Pacific altered atmospheric and ocean circulation patterns in ways that were felt around the globe, including in the Atlantic and polar regions, and resulted in an increase in the amount of heat deep in those oceans.
Despite the disagreement on which ocean basin is doing what in the air-warming slowdown, however, a broader picture of Earth’s climate is still emerging. “I think all of these things are sort of adding up to an increased picture that, yes, global warming is continuing, but it’s not just at the surface of the Earth,” Trenberth says.