Atlantic Ocean's 'Heat Engine' Chills Down

New research suggests something's happening to the so-called "conveyer belt" in the Atlantic Ocean — the huge circulation system that takes warm waters from the south Atlantic and dumps them off the coast of Europe, keeping northern Europe relatively warm. Some believe global warming may be affecting this heat engine, but others are skeptical, given how complex the ocean can be. The debate is part of a larger effort by scientists to detect climate changes with sketchy data.

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Many scientists say a warming climate is already having noticeable effects. Glaciers are retreating, for example, and oceans are slowly rising. But there's still a lot of debate about what else might be in store from a warner Earth. The Atlantic Ocean is one place scientists are watching. Changes there could be subtle, but they carry enormous consequences. A new study out today suggests something is happening in the Atlantic, and as NPR's Christopher Joyce reports, the evidence is sketchy, but the stakes are high.

CHRISTOPHER JOYCE reporting:

The Atlantic Ocean is hard to study. It's vast, and it's fickle. Lots of things like wind and sunshine influence its behavior. But it's important to know if climate change is altering the Atlantic. One reason is that the ocean warms Northern Europe. It works like a conveyor belt for heat. Warm surface water from the tropical South moves north, mostly carried up by the Gulf Stream, a sort of river in the ocean. When it gets near Greenland and Iceland, that water dumps its heat into the air over Northern Europe. Now the water is colder and denser, so it sinks and crawls back southward at great depths. Oceanographer Harry Bryden has been measuring this conveyor and says something's different now.

Mr. HARRY BRYDEN (Oceanographer, National Oceanography Center): What we found, looking at the measurements that we've made, is that the Gulf Stream hasn't really changed in size over the last 50 years. But what has changed is more of the Gulf Stream water is recirculating southward, and less is flowing northward into the far northern latitudes.

JOYCE: Bryden and colleagues at the National Oceanography Center in Great Britain say more of that warm water is getting caught up in a circular gyre that spins around the Atlantic. And they've also noticed that the southward flow of the cold water from the polar seas--their return part of the cycle, if you will--has slowed down. Is that serious? Well, computer simulations predict that if this conveyor ever stops, Northern Europe would get several degrees colder within decades. Europeans, understandably, worry about this.

Bryden, who's written a paper about his findings in the journal Nature, says he can't be sure this slowing will continue. He only has five sets of measurements, the first made in 1957; the last in 2004. Carl Wunsch, a well-known oceanographer at the Massachusetts Institute of Technology, says that's not nearly enough information to claim something's afoot in the Atlantic.

Mr. CARL WUNSCH (Oceanographer, Massachusetts Institute of Technology): It's a bit like taking a snapshot around the world of the whole atmosphere and doing that once every 10 or 15 years and then proclaiming that the system is changing.

JOYCE: Some scientists say the changes Bryden measured in the Atlantic are so slight, they could simply be an error in measurement. Wunsch says getting hard information about the ocean's behavior is hard.

Mr. WUNSCH: It's so variable from day to day, week to week, even year to year that the pulling out of trends takes decades. And even then there are fierce arguments about their significance, and that's what's going on here in this paper.

JOYCE: Another oceanographer, Ruth Curry at the Woods Hole Oceanographic Institution in Massachusetts, is less skeptical. Yes, the latest measurements are sketchy, but other scientists have seen changes in temperature and salinity in parts of the Atlantic, changes that theoretically could slow the north-south circulation.

Ms. RUTH CURRY (Oceanographer, Woods Hole Oceanographic Institution): What this study is revealing is that the circulation in between those two sources--you could think of it as a cog in the ocean conveyor belt--is beginning to wobble. So I would say that it's entirely possible that we are right now witnessing the prelude to a slowing of the Atlantic's heat circulation engine.

JOYCE: The rub here is it could take decades to know for sure. Christopher Joyce, NPR News.

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