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