Two new research papers together suggest that a dramatic decline in the overturning circulation of the North Atlantic looms closer than we thought.

Research hints at tipping point in the Atlantic’s currents

In "the Labrador Sea, between Canada and southern Greenland, and the Irminger Sea, between Greenland and Iceland" in the 2010/2011 Winter, "as much as 40 percent of the surface water was still in place when the spring arrived". Complete mixing is required. Two consecutive winters like this would significantly deprive the overturning circulation as those warm fresh top layers combine, stratifying those sections of the Atlantic.

Not only does our gradually warming climate increase the odds of this happening, a new mechanism to accelerate Greenland melt was just discovered.

Chain reaction of fast-draining lakes poses new risk for Greenland ...

Summer meltwater lakes transform the forces in the ice sheet in hours, from top to bottom and back to the top, speeding up flow in the affected region up to 4 times. The Greenland ice sheet is far more dynamic and responsive to temperature change than scientists imagined. This partly explains why the melt is accelerating.

"A growing network of lakes on the Greenland ice sheet has been found to drain in a chain reaction that speeds up the flow of the ice sheet, threatening its stability."

"These cascading events -- including one case where 124 lakes drained in just five days -- can temporarily accelerate ice flow by as much as 400%, which makes the ice sheet less stable, and increases the rate of associated sea level rise."

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A new paper in Nature, discussed here, claims that the Atlantic Meriodonal Overturning Circulation (AMOC), isn't getting slower, as previous findings suggested, but merely going through a 60-70 year cycle of faster and slower flow. Somewhere at AN, I mentioned their findings, but couldn't find that entry to correct. Why correct? That research was flawed!

See Does a slow AMOC increase the rate of global warming?

... a paper was published in Nature that claims the exact opposite: namely that a strong overturning circulation warms rather than cools the deep ocean.

But is this true, and what is the evidence for it?

The great Carl Sagan once said that extraordinary claims require extraordinary evidence,...

The only evidence presented is the putative coincidence of two phases of slow global surface warming (1942-1975 and 1998-2014) with strong AMOC (i.e. Atlantic Meridional Overturning Circulation), and a phase of more rapid warming (1975-1998) with weak AMOC (their Fig. 3). There is no statistical examination of this supposed correlation. A number of further graphs showing various spatial patterns do nothing to support the new hypothesis:...

...the distinction of phases of fast global warming up to 1998 and slow warming from 1998 is highly questionable.

... the supposed “warming slowdown” after 1998 is known to be largely an artifact of the HadCRUT temperature data, because these do not cover the Arctic which has warmed the most in this period.

On the mechanism for why a strong AMOC would heat rather than cool the deep ocean, Chen and Tung write: “Deep convections can now carry more heat downward.” any evidence for salinity-driven convection there which could carry heat downward? Yashayaev’s reply:

“In the polar and subpolar regions, under no circumstances we saw convection bringing warmer water down deep.”

Chen and Tung do not show any models simulations either to provide evidence that their mechanism can actually work, neither do they discuss the various published model results that have come to the opposite conclusion. [yellow emphasis mine]

In short, we can't dismiss AMOC shutdown due to climate change. But on the other hand, at least we don't have to fear an immanent cyclical boost to surface temperature from the Atlantic. 

In the Pacific, an El Nino is still 70% likely this fall.

thanks Ruth!


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