CORVALLIS, Ore. – A current system in the Atlantic Ocean that shapes regional climates and distributes ocean nutrients also serves as a control knob to help regulate temperatures across the planet, new research has found.
An analysis of past natural oscillations in the strength of the Atlantic Meridional Overturning Circulation system, or AMOC, shows that during periods with a strong AMOC, the global ocean and planet lost heat. When the AMOC was weak, the global ocean and planet gained additional heat.
"The AMOC works like a heat valve that controls the energy budget of the planet," said Christo Buizert , a paleoclimatologist at Oregon State University and lead author of the study, which was just published in Nature Geoscience.
Researchers have long known that the AMOC plays an important role in the circulation of heat by global ocean currents. While the AMOC has been strong over the last 11,700 years, since the end of the last Ice Age, many climate models project future weakening of the AMOC due to human-driven climate change.
Such AMOC weakening would exacerbate warming across the planet, said Buizert, an associate professor in OSU's College of Earth, Ocean, and Atmospheric Sciences. The most direct impact of AMOC weakening is cooling in the North Atlantic and the regions surrounding it, including Greenland.
"However, when we zoom out and look at the entire planet, the total amount of heat actually increases," Buizert said.
Previous research has shown that during each of the Earth's Ice Ages, which happened repeatedly between 11,700 years ago and 2.7 million years ago, the AMOC underwent a series of abrupt changes, known as Dansgaard-Oeschger events. These abrupt changes are probably the best example of climate "tipping points" – the critical thresholds that, when crossed, lead to sudden and potentially irreversible climate change.
During weak AMOC periods, abrupt cooling in the North Atlantic plunged areas such as present-day Europe, Greenland and New York into much colder temperatures. The prevailing scientific theory suggested that heat was transferred to the southern hemisphere in a theory known as the "thermal bipolar seesaw."
The new research shows that instead of a simple redistribution of heat, there is actually a net increase in the heat stored by the global ocean.
"To put this into perspective, events of AMOC weakening during the last Ice Age caused the same amount of warming as 25 ppm of carbon dioxide would today. That is the equivalent of about 10 years of human emissions."
To reach these conclusions, the researchers created a new framework, using simulations of abrupt AMOC change from three different climate models. In these three models, they tracked how heat moves around in the oceans and how much was lost or gained by the planet as a whole.
The ocean is continually taking up heat from sunlight, mostly in the tropics. When the AMOC is strong, that heat is circulated by ocean currents to the North Atlantic, where it is lost to the atmosphere in a process called deep ocean convection.
The new study found that when the AMOC weakens, this heat instead builds up in the interior of the ocean, including the North Atlantic. Only a thin surface layer of the North Atlantic cools down, while the rest of the ocean warms up.
"It's as if the whole ocean acts as a giant bucket of heat," Buizert said. The AMOC acts as a spigot that controls how much of that heat can flow out.
There is a silver lining, though.
"Our research also shows that in a warmer world, the AMOC tends to be more stable, which would suggest that these 'tipping point' events might not occur in the future," Buizert said. "There would be a future weakening of the AMOC with climate change, but it could recover. An irreversible collapse of the AMOC might not occur. But more research is needed to better understand the future stability of the AMOC."
Studies of the naturally occurring past changes to the AMOC are not a perfect comparison for modern-day climate shifts, but the more researchers can learn about the past, the better it helps understand what the future could look like, Buizert said.
Additional research is needed to further understand the role a changing AMOC plays in weather and climate patterns around the globe.
Coauthors of the study were: Ayako Abe-Ouchi, Yuta Kuniyoshi and Sune Olander Rasmussen of the University of Tokyo; Guido Vettoretti of the University of Copenhagen; Xu Zhang of the British Antarctic Survey and Xi'an Jiaotong University of China; Sarah Shackleton of Woods Hole Oceanographic Institution; Joel B. Pedro of the Australian Antarctic Division and the University of Tasmania; Eric D. Galbraith of the Universitat Autonoma de Barcelona and McGill University; and Thomas F. Stocker of the University of Bern, Switzerland.
