JUANA SUMMERS, HOST:
Global warming can have sudden impacts - what scientists call tipping points. One is the collapse of a vital ocean current in the Atlantic, which controls weather around the globe. New research has found that collapse could happen a lot sooner than people think. Lauren Sommer from NPR's Climate Desk reports.
LAUREN SOMMER, BYLINE: Alaska has some pretty cold winters, but head over to Europe at the exact same latitude, and it's not quite so bad.
PETER DITLEVSEN: In Scandinavia, we have a sort of pleasant, mild climate.
SOMMER: Peter Ditlevsen is a professor of climate physics at the University of Copenhagen in Denmark. He says it's thanks to the Atlantic Ocean and a huge ocean current there. It's called the Atlantic Meridional Overturning Circulation. Warm water from the tropics goes up the east coast of the U.S., flows to Europe, releases its heat and sinks.
DITLEVSEN: It's called the conveyor belt, which is sort of a nice way of describing it.
SOMMER: But that conveyor belt in the ocean can turn off, which it did around the last ice age. Europe got colder. The tropics got even hotter. And rainfall patterns shifted. This collapse can happen fast, not over centuries but decades. It's a climate tipping point.
DITLEVSEN: The tipping point is a strong response to a small change. It's when you're pushed over the cliff. When you reach the cliff, you drop.
SOMMER: In a study in the journal Nature Communications, Ditlevsen analyzed ocean temperatures for the last 150 years. He found that if humans keep emitting greenhouse gases at the levels they are now, the collapse could happen by mid-century.
DITLEVSEN: But it's a worrisome result. It calls for quite immediate actions.
SOMMER: But that timeline for collapse is much sooner than other researchers have found.
NICHOLAS FOUKAL: Whether it will collapse is still a question.
SOMMER: Nicholas Foukal is a scientist at the Woods Hole Oceanographic Institution. He says while the study is interesting, it leaves out an important thing - the cause of the collapse. The leading theory is that melting ice from the Arctic and Greenland could add huge amounts of cold, fresh water to the Atlantic. That would interfere with the conveyor belt. But this study doesn't simulate how that works, and it's hard to actually measure it in real life.
FOUKAL: We're trying to measure a massive system, and we're doing this in the middle of hostile or treacherous locations that are difficult to measure.
SOMMER: What's more likely, he says, is that the Atlantic current will weaken this century, which could still have devastating effects.
FOUKAL: It's going to affect agriculture. It's going to affect disease, especially in the equatorial regions - patterns of disease. It's going to affect mass migration.
SOMMER: The Atlantic circulation is a bedrock of the climate system, he says. And the way humans are changing the planet, drastic shifts could happen. It's really a question of when. Lauren Sommer, NPR News.
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