The Atlantic Ocean Has a Hidden ‘Heat Valve’ That Controls Earth’s Temperature – And It Could Be Closing

Our relatively stable global climate is governed by an incredibly complex set of factors, all interacting with each other in dynamic and changeable ways – but there are a couple of really big, key players in that system.

The Atlantic Meridional Overturning Circulation (AMOC) is one of them. 

This system of ocean currents redistributes heat from tropical regions up to cooler places, and cools warmer waters down too (important for sensitive marine life like corals).

Nutrients that ocean species rely on also get pushed around on this giant, mostly invisible conveyor belt beneath the waves.

Right now, the AMOC is weakening due to human-caused climate change, which could have huge repercussions, changing temperature and rainfall patterns humans rely on to grow food. 

So it makes sense that researchers want to understand when and how, as lead author Christo Buizert writes in a research briefing, “a critical threshold beyond which AMOC weakening becomes self-reinforcing and irreversible.”

That was the focus of a truly international team’s study for Nature Geoscience that included Buizert.

(A fun note: Buizert was encouraged to submit this work to the journal after a chat over cheese fondue at a meeting in Switzerland. Proving that once again that giving scientists delicious free food is always a wise investment.)

Looking to the past might hold some answers: During the Pleistocene ice ages, the AMOC repeatedly switched between strong and weak states.

These shifts corresponded with abrupt climate events known as Dansgaard–Oeschger, or D–O, events, recorded particularly clearly in Greenland ice cores. 

The conventional explanation for these D–O events was what the researchers call a “thermal bipolar seesaw.” This climate seesaw was a model that seemed to explain how the AMOC-driven heat transport caused warming at one pole and cooling at the other pole. 

Taking a new perspective, the researchers came up with another explanation which included “synthesizing two decades of observational and theoretical advances in understanding the crucial parts played by ocean heat content (OHC), sea ice, and the planetary radiation balance (the difference between the incoming solar radiation and outgoing radiation emitted by the Earth),” according to Buizert’s statement. 

Instead of thinking of the AMOC primarily as a conveyor belt shuttling heat from one hemisphere to another, researchers propose thinking of it more like an “ocean heat valve”.

The researchers ran three climate models capable of producing spontaneous, self-sustaining D–O-like climate oscillations under glacial conditions.

They found that when the AMOC was strong, vigorous deep convection in the North Atlantic allowed the ocean to lose large amounts of heat to the atmosphere. The planet also lost energy to space.

When the AMOC weakened, deep convection shut down. The North Atlantic lost less heat than the rest of the ocean was absorbing, so heat accumulated in the ocean interior. The planet as a whole gained energy.

In other words, the AMOC was acting less like a simple heat conveyor that moved heat from one place to another, and more like a valve controlling whether heat was allowed to escape from the ocean.

When the valve was open, heat drained out. When it was closed, heat accumulated.

This heat-valve model also helps explain a long-standing puzzle in varying ice-core records, which showed rapid changes in Greenland and slower changes in the Antarctic over the same time periods.

This new heat-valve model shows that this is due to the rapid changes in North Atlantic heat loss and slower changes in the amount of heat stored throughout the ocean, as reflected by the Antarctic records.

Rather than the two poles simply playing opposite ends of a heat-transfer seesaw, the researchers argue, both were responding to different aspects of a globally connected ocean heat cycle.

“The AMOC acts as an ocean heat valve regulating planetary temperature through Earth’s radiative balance,” wrote David Bonan, a climate scientist at the University of Washington, who provided an expert assessment of the work.

So, what happens in this valve model when applied to today’s warming world?

As human-caused climate change alters the ocean, a weaker circulation could reduce deep convection and increase the amount of heat stored in the ocean. The researchers therefore conclude that future AMOC weakening could actually amplify planetary warming by “throttling down” the ocean heat valve.

The researchers point out that their model gets an important asterisk:

The ice-age climate system was very different from today’s. The D–O cycles occurred under glacial conditions, when enormous amounts of sea ice helped create the feedbacks that allowed the AMOC to flip between states. In a warmer future, the North Atlantic may be largely or entirely ice-free, removing a mechanism that was central to those ancient oscillations.

Related: A Major Ocean Current Is On The Verge of Collapse. Scientists Say The Effects May Reach California

So the past doesn’t provide a crystal ball for the AMOC’s future. But it does offer something perhaps nearly as useful: a natural experiment showing that changes in this circulation can affect not just where heat goes, but how much heat the planet as a whole retains.

Buizert is calling for significantly more research into this because, as he writes, “Resolving whether the future AMOC has more than one stable state is a crucial research objective, because it might be the difference between transient weakening and an irreversible collapse, with profound societal implications.”

The research has been published in Nature Geoscience.