Why Some Scientists Are Worried About a Surprisingly Cold ‘Blob’ in the North Atlantic Ocean

In March several top climate scientists, including Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research and Michael Mann of Penn State, published a paper in Nature Climate Change suggesting that the gigantic ocean current known as the Atlantic Meridional Overturning Circulation, or AMOC, is weakening.  It’s sometimes confused with the “Gulf Stream,” but, in fact, that’s just a southern branch of it.

The current is driven by differences in the temperature and salinity of ocean water (for a more thorough explanation, see here).  In essence, cold salty water in the North Atlantic sinks because it is more dense, and warmer water from farther south moves northward to take its place, carrying tremendous heat energy along the way.  But a large injection of cold, fresh water can, theoretically, mess it all up — preventing the sinking that would otherwise occur and, thus, weakening the circulation.

In the Nature Climate Change paper, the researchers suggested that this source of freshwater is the melting of Greenland, which is now losing more than a hundred billion tons of ice each year.

I asked Mann and Rahmstorf to comment on the blue spot on the map above by e-mail. Here’s what Mann had to say:

I was formerly somewhat skeptical about the notion that the ocean “conveyor belt” circulation pattern could weaken abruptly in response to global warming. Yet this now appears to be underway, as we showed in a recent article, and as we now appear to be witnessing before our very eyes in the form of an anomalous blob of cold water in the sup-polar North Atlantic.

Rahmstorf also commented as follows:

The fact that a record-hot planet Earth coincides with a record-cold northern Atlantic is quite stunning.  There is strong evidence — not just from our study — that this is a consequence of the long-term decline of the Gulf Stream System, i.e. the Atlantic ocean’s overturning circulation AMOC, in response to global warming.

... 
So in sum, if Mann and Rahmstorf are right, a slowing of Atlantic Ocean circulation could be beginning, and even leaving a temperature signature for all to see.

This won’t lead to anything remotely like The Day After Tomorrow (which was indeed based — quite loosely — on precisely this climate scenario).  But if the trend continues, there could be many consequences, including rising seas for the U.S. East Coast and, possibly, a difference in temperature overall in the North Atlantic and Europe.

Read more at Why Some Scientists Are Worried About a Surprisingly Cold ‘Blob’ in the North Atlantic Ocean