SOURCE: Nature

DATE: July 18, 2018

SNIP: In a paper in Nature, Chen and Tung report that the system of ocean currents known as the Atlantic Meridional Overturning Circulation (AMOC) can explain changes in rates of global surface warming. Rather than the conventional picture of a vigorous AMOC associated with elevated surface temperatures in the Atlantic Ocean, the authors emphasize the role of the AMOC in taking heat from the surface and storing it in the deep ocean.

Atmospheric concentrations of greenhouse gases are currently being increased at a rate that is unprecedented in millennia and most likely millions of years. As a result, the role that climate mechanisms might have had in the past might not be a good guide to their current or future role. The authors contend that half of the heat arising from ever-increasing greenhouse-gas concentrations is stored in the deep waters of the North Atlantic when the AMOC is increasing, thereby reducing overall global surface warming.

The authors show that a cycle of increasing and then decreasing AMOC from the 1940s to the mid-1970s coincided with a period of global-warming slowdown; a quiescent period of weak AMOC from the mid-1970s to the late 1990s coincided with rapid global warming; and an increase in AMOC strength from the late 1990s to 2005 and a decrease thereafter coincided with the ‘hiatus’ in global warming.

The AMOC is deemed “very likely” to weaken in the coming decades. Indeed, the Atlantic has seen muted rises in surface temperature relative to the global ocean over the past few decades. This relative lack of warming has been interpreted as a fingerprint of AMOC decline, potentially linked to anthropogenic climate change. Whether the AMOC observatories will document the predicted decline remains to be seen, but they have already observed that the AMOC is in a weakened state. Chen and Tung predict that such a weak AMOC will result in a period of rapid global surface warming that could last for more than two decades.