Land and ocean temperature percentiles for January-August 2015. Graphic: NOAA

By Chris Mooney
25 September 2015

(Washington Post) – It is, for our home planet, an extremely warm year.

Indeed, last week we learned from the National Oceanic and Atmospheric Administration that the first eight months of 2015 were the hottest such stretch yet recorded for the globe’s surface land and oceans, based on temperature records going to 1880. It’s just the latest evidence that we are, indeed, on course for a record-breaking warm year in 2015.

Yet, if you look closely, there’s one part of the planet that is bucking the trend. In the North Atlantic Ocean south of Greenland and Iceland, the ocean surface has seen very cold temperatures for the past eight months. What’s up with that? […]

At this point, it’s time to ask what the heck is going on here. And while there may not yet be any scientific consensus on the matter, at least some scientists suspect that the cooling seen in these maps is no fluke but, rather, part of a process that has been long feared by climate researchers — the slowing of Atlantic Ocean circulation.

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. [more]

Why some scientists are worried about a surprisingly cold ‘blob’ in the North Atlantic Ocean



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