Climate Running AMOC

Thermohaline circulation (n.): deep ocean current driven by variations in salinity and temperature.

I used to love watching the weather forecast when I was a kid, because I thought it was a cartoon. There were lots of colorful arrows going every which way and a man waving his arms like a snake charmer. I had no idea what any of it meant.

Suddenly, I know more than I ever wanted to. A major study released in the journal Nature Climate Change warned that those arrows, and the ocean currents they represent, might be changing rapidly—with the potential to alter regional climates faster than anyone expected.

The story begins with a stream of water called the Atlantic Meridional Overturning Circulation (AMOC), which runs from the tropics south of the equator to the North Atlantic. Oceanographers describe it as a conveyor belt because of the looping pattern in which the water flows. Warm tropical water stays near the surface as it heads north because warm water is, generally speaking, less dense than cold water. When it reaches the North Atlantic, the water gets cold and some of it freezes. Very little of the salt winds up in the sea ice, causing the surrounding water to become more saline. Both processes—the cooling and intensified saltiness—increase the water's density. The cold, salty water then sinks toward the bottom and eventually moves southward along the lower rung of the conveyor belt.

Since variations in temperature and salinity are responsible for the difference in density that powers the conveyor belt, scientists refer to the AMOC and similar currents as thermo- (heat) haline (salt).

Oceanographers have always known that the speed of the AMOC’s conveyor belt changes, but they hadn’t detected a trend—until now. In this week’s bombshell study, Stefan Rahmstorf of the Potsdam Institute for Climate Research and his colleagues charted the strength of the AMOC going back more than a thousand years, by measuring climate changes known to be related to the power of the current. At 15 percent to 20 percent slower, the AMOC, they concluded, is weaker today than at any time in the past millennium.

That weakening is probably both a cause and an effect of climate change. The Greenland ice sheet is shedding hundreds of gigatons every year. The melting process adds freshwater into the North Atlantic, which offsets the salination process. Less salt makes the water sink more slowly, which, in turn, slows the conveyor belt.

A snail-like AMOC is a big problem. Remember, the AMOC doesn’t just transport water—it moves heat around the planet. This current keeps the water around New York and Boston cool, which slows the rise of sea levels in those cities (because cold water takes up less space than warm water). If the current slows, that could change. Rising seas on the East Coast is just one of many possible consequences, which could also include impacts on fisheries.

Rahmstorf’s study is the first of its kind, so his conclusions are up for debate (hopefully among scientists rather than senators). For now, keep your eyes on those arrows in the weather forecast.