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The Thames River, painted by Abraham Hondius in 1677, used to freeze regularly during the little ice age.

Heritage Image Partnership Ltd/Alamy Stock Photo

The deep Pacific is a climate time capsule from the ‘little ice age,’ 19th century ship records show

NEW ORLEANS, LOUISIANA—A global cooling trend known as the “little ice age” ended centuries ago, but it lives on in the deepest parts of the Pacific Ocean, researchers reported here last week at a meeting of the American Geophysical Union. What’s more, this oceanographic time capsule could be helping blunt some of today’s human-driven warming, at least for now.

The oceans are a massive heat reservoir, absorbing some 90% of the warming from human-caused climate change. But this modern heat doesn’t penetrate evenly—or quickly—into their vast depths. As part of a network of global ocean currents called the thermohaline circulation, chilled surface waters in the North Atlantic Ocean dive into the deep and, over the course of many centuries, wind their way to the deep North Pacific, which is in many ways Earth’s cold storage locker.

That means the deep waters of the Pacific, unlike the relatively young Atlantic depths, should reflect surface temperature trends that are hundreds of years old. “From 1350 to the present day [those depths are] expected to be cooling,” says Jake Gebbie, a physical oceanographer at the Woods Hole Oceanographic Institution in Massachusetts, who presented the work. “Cooling—despite the fact that the surface is warming.”

A host of models of reconstructed global surface temperatures show that centuries ago, the world was unusually cold—as paintings of the frozen Thames River attest. After the “medieval warm period” ended in the 1400s, a cooling trend of a few tenths of a degree set in, ending only when human-driven warming began in the 1800s. By priming an ocean model with these historical surface temperatures, Gebbie and his co-author, Peter Huybers, a climate scientist at Harvard University, were able to predict where in the depths these trends would reveal themselves.

To test their model, they needed evidence of long-term temperature change from the deep ocean. But records below 2000 meters are sparse, produced only every decade or so during research cruises. And they’re seemingly nonexistent prior to the 20th century. But not entirely.

In the 1870s, a British research ship, the HMS Challenger, spent half a decade recording ocean temperatures during a grand scientific tour across the globe, making 760 readings below 2000 meters with thermometers lowered by rope. The duo compared the Challenger’s readings to measures taken from the 1970s onward, and tried to account for potential biases from that era, such as the stretchiness of hemp ropes and the behavior of early mercury thermometers under extreme pressure. After calibrating the old and modern data, “We see exactly what we see in the simulation,” Gebbie said, “deep Pacific cooling and deep Atlantic warming.”

In effect, the deep ocean acts as a filter, one that wipes out short-term temperature fluctuations and keeps the long-term trend. And this signal seems to persist despite large-scale phenomena, such as eddies, that can mix up the oceans. Assuming Gebbie’s model is correct, the deep Pacific will continue to cool for decades as the little ice age water arrives.

“This is fantastic,” says Yair Rosenthal, a paleoceanographer at Rutgers University in New Brunswick, New Jersey, who is impressed that Gebbie could trace the cooling flow of little ice age water to the deep Pacific. “If you caught a fish today in the deep Pacific and asked it what it thought about global warming,” Rosenthal says, “it’d think that we are talking about the medieval climate.”

But Greg Johnson, an oceanographer at the National Oceanic and Atmospheric Administration’s Pacific Marine Environmental Laboratory in Seattle, Washington, cautions that the researchers used a coarse model of the ocean, which may not adequately simulate real conditions. They also did not consider how decadeslong variations in North Atlantic currents could influence the trends they see. “It is an interesting result, but I am skeptical,” says Johnson, who is developing a program to regularly sample deep ocean temperatures with deep-diving robotic floats.

If real, this slow drop in deep ocean temperatures is a boon to a warming planet. If the little ice age hadn’t cooled the oceans, they’d likely be absorbing less heat from the atmosphere today, and surface warming would be much worse than it already is. “It’s buying us time,” Rosenthal says. “It’s buying us time.”