WASHINGTON — We’re now seeing a slowdown of the great ocean circulation that, among other planetary roles, helps to partly drive the Gulf Stream off the U.S. East Coast, according to a study just out in Nature Climate Change, by Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research and a group of co-authors. The consequences could be dire — including significant extra sea level rise for coastal cities like New York and Boston.
A vast, powerful and warm current, the Gulf Stream transports more water than “all the world’s rivers combined,” according to the National Oceanic and Atmospheric Administration. But it’s just one part of a larger regional ocean conveyor system — scientists technically call it the “Atlantic meridional overturning circulation” — which, in turn, is just one part of the larger global “thermohaline” circulation (“thermohaline” joins terms meaning “temperature” and “salty”).
For the whole system, a key driver occurs in the North Atlantic Ocean. Here, the warm Gulf Stream flows northward into cooler waters and splits into what is called the North Atlantic Current. This stream flows still further toward northern latitudes — until it reaches points where colder, salty water sinks due to its greater density, and then travels back southward at depth.
This “overturning circulation” plays a major role in the climate because it brings warm water northward, thereby helping to warm Europe’s climate, and also sends cold water back towards the tropics.
The day after fiction
The system above has a key vulnerability. What keeps everything churning in the North Atlantic is the fact that cold salt water is more dense than warm water — so it sinks. However, if too much ice melts in the region — from, say Greenland — a freshening of the cold salt water could occur. If the water is less salty it will also be less dense, reducing its tendency to sink below the surface.
This could slow or even eventually shut down the circulation. In the scientifically panned 2004 blockbuster film “The Day After Tomorrow,” it is precisely such a shutdown that triggers a New Ice Age, and utter global disaster and chaos.
That’s not going to happen, say scientists. Not remotely.
Nonetheless, the new research finds that global warming does indeed seem to be slowing down the circulation. And while hardly catastrophic, that can’t be good news. Among the very real effects, notes the Potsdam Institute’s Rahmstorf, could be a possible increase in U.S. sea level if the whole circulation were to break down — which would be seriously bad news for cities like New York and Boston.
The study uses a reconstruction of sea surface temperatures in the North Atlantic to find that starting in around 1970 or 1975, the overturning circulation started to weaken — an event likely triggered by an unusual amount of sea ice traveling out of the Arctic ocean, melting, and causing freshening. The circulation then started to recover in the 1990s, but “it seems this was only a temporary recovery, and now it’s actually further weakened,” says Rahmstorf.
The hypothesized reason for further declines presented by the paper is that the massive Greenland ice sheet may now be losing enough freshwater due to melting to weaken the circulation. And indeed, it appears that a particular ocean region of the North Atlantic south of Greenland and between Canada and Britain is becoming colder — an indicator of less northward heat transport.
Rahmstorf points to a recent release by the National Climatic Data Center of the National Oceanic and Atmospheric Administration, finding that the winter of December 2014 through February 2015 was the warmest on record for the globe as a whole. However, there were several anomalies — not just a cold winter for the eastern U.S., but also record cold temperatures in the middle of the North Atlantic.
“These new NOAA data got me quite worried because they indicate that this partial recovery that we describe in the paper was only temporary, and the circulation is on the way down again,” says Rahmstorf.
(Optional add end)
So far, the study finds, we’re looking at a circulation that’s about 15 to 20 percent weaker. That may not sound like much, but the paper suggests a weakening this strong has not happened at any time since the year 900. Moreover, this is already more weakening than scientifically expected — and could be the beginning of a further slowdown that could have great consequences.
So in sum: It appears that we’ve just seen yet another surprise from the climate system — and yet another process, like the melting of Antarctica, that seems to be happening faster than previously expected. And indeed, much like with that melting, the upshot if the trend continues is an especially bad sea level rise for the United States — the country more responsible than any other on Earth for the global warming that we’re currently experiencing.
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Mooney reports on science and the environment.
This link has a NASA video that visualizes the thermohaline circulation for the entire globe. Rahmstorf also has a blog post up at RealClimate.org explaining his research.
https://pmm.nasa.gov/education/videos/thermohaline-circulation-great-ocean-conveyor-belt
