New evidence suggests that the larger system the Gulf Stream is part of is approaching a tipping point that could cause dramatic shifts in global weather patterns.
BY KARA NORTON THURSDAY, FEBRUARY 17, 2022 NOVA NEXT
Around 15,000 years ago, the Earth began to transition from a state of heavy glacial coverage to a period of warming. As temperatures rose, large ice sheets that once blanketed much of North America and Europe thawed, and the oceans began to rise.
But after 2,000 years of warming, the Earth abruptly cooled to near-glacial conditions. And it stayed that way for more than a thousand years.
Scientists have long debated how this abrupt climatic event, which they call the Younger Dryas, occurred. This question has become an urgent one as some researchers think an abrupt climatic event could happen again and disrupt the Gulf Stream, the forceful ocean current that transports warm water from the Gulf of Mexico up North America’s Northeast coast.
An analysis published in the journal Nature Climate Change in August found that the larger system that the Gulf Stream is part of, called the Atlantic Meridional Overturning Circulation (AMOC), is approaching a tipping point. Over the last century, this ocean circulation system has “moved closer to a critical threshold, where it may abruptly shift from the current, strong circulation mode to a much weaker one,” says study author Niklas Boers, a climate researcher at the Potsdam Institute for Climate Impact Research in Germany. Should the AMOC weaken substantially, it could bring intense cold and stronger storms to Europe, raise sea levels across the northeast coast of North America, and disrupt the flow of vital nutrients that phytoplankton, marine algae that make up the foundation of the aquatic food web, need to grow in the North Atlantic.
“It’s important to acknowledge just how big of a system we are trying to measure,” says Nicholas Foukal, a physical oceanographer at Woods Hole Oceanographic Institute who was not involved in the study. “If you calculate the flow of all of the rivers in the world, the amount of water is only about 1% to 2% of the Gulf Stream.”
The AMOC is an Atlantic section of a global conveyor belt that drives surface and deepwater currents in every ocean, and influences the rate of sea ice formation at the poles. The AMOC has exhibited two states: a strong, stable state and a slow, weak one. Warming temperatures weaken the AMOC, the new analysis suggests, ultimately pushing it toward a tipping point where it could rapidly turn off.
Scientists hypothesize that such a shutdown may have occurred during the Younger Dryas. As one theory goes, as the planet warmed, glacial flood waters leaked into the Arctic and the North Atlantic, diluting the oceans’ saltiness, and weakening the AMOC. In addition, a massive ice sheet covering millions of miles—which helped contain an enormous glacial lake in what is now Canada—began to melt away. This triggered a freshwater flood, which traveled north up a river system and eventually emptied into the Arctic Ocean. This influx of fresh water ultimately decreased the salinity, and thus the density, of surface water in the northern Atlantic ocean to the point that it was no longer able to sink and make the return journey to the equator. So the AMOC shut down.
Now, tens of thousands of years later, scientists have cautioned that the AMOC is at its weakest point in over a millennium. But researchers have not been able to pinpoint how close the AMOC is to a total shutdown. Boer’s findings suggest a new insight: The AMOC’s decline may be indicative of an “almost complete loss of stability.”
The new study’s primary findings are based on eight independent sea-surface temperature and salinity datasets spanning 150 years. Climate change is destabilizing the AMOC, Boers concludes, noting that the “restoring forces” that keep the AMOC functioning, such as temperature and salinity, have become more erratic.
The stabilizing force of the AMOC is cold, salt-rich water, which sinks to the ocean floor and powers the circulation that keeps the AMOC in its “on” mode. Meanwhile, warming surface waters and fresh water from melting ice push the AMOC away from stability.
Boers analyzed how fast the sea surface temperature was returning to its state of equilibrium and how big the deviations from this state were becoming. Boer’s results indicate that the strength of the stabilizing current is declining and that an AMOC tipping point—an ecological point of no return that once crossed could take hundreds of years to stabilize—is much closer than previously understood.
“The signs of destabilization being visible already is something that I wouldn’t have expected and that I find scary,” Boers told The Guardian. “It’s something you just can’t [allow to] happen.”
But a result of this magnitude needs to be tested by other researchers before it is widely accepted, Foukal says.
As carbon dioxide accumulates in Earth’s atmosphere, the atmosphere and ocean trap more heat. As global temperatures rise, they trigger more glacial ice to melt. An influx of freshwater into the North Atlantic from the Greenland ice sheet will reduce the North Atlantic’s salinity and density, making the water lighter and less able to sink. In addition, warmer temperatures cause water molecules to spread farther apart, making the ocean waters even less dense. Similar to what happened during the Younger Dryas, if the water in the North Atlantic is not heavy enough to sink, the entire AMOC will shut down, Boers explains.
In its 2019 “Special Report on the Ocean and Cryosphere in a Changing Climate,” the United Nations Intergovernmental Panel on Climate Change (IPCC) stated that the AMOC is “very likely” to weaken due to climate change. But a total shutdown within the next 300 years, the report states, is only likely under a worst-case carbon dioxide emissions scenario with global average temperatures rising by 4.3 degrees Celsius by the end of the century.
Still, Boers’ findings suggest that “the critical threshold is most likely much closer than we would have expected,” he says.
The implications of a weakening AMOC
Should the AMOC weaken substantially, it could “drastically” affect rainfall patterns in tropical monsoon systems, Boers told NOVA.
Monsoons supply water to many parts of the world. Yackar Mauzole, an oceanographer whose research focuses on the Indian Ocean, points out that if the amount of precipitation and location of monsoon systems were to change, it would have major implications for the Maldives, India, Thailand, and other populous countries that rely on monsoons for agriculture.
Weakening of the AMOC could also bring less summer rainfall to South Asia and the Sahel, the semiarid African region stretching from Senegal to Sudan that receives much of its rainfall from the West African monsoon, the IPCC projects.
“A change in monsoon systems would have severe consequences for the populations living in these regions,” Mauzole says. “Beyond oceanography and climate science, food insecurity should be something to consider if the AMOC were to transition in the future.”
To better assess what may happen in the future, some scientists are looking to the past. The AMOC shutdown 13,000 years ago led to an abrupt cooling of North America and Europe that lasted 1,000 years, they believe. Archaeological analyses of skeletal remains and plant fossils from modern-day Syria and the Nile Valley suggest that the climatic shock of the Younger Dryas upended early human settlements, causing conflict, starvation, drought, and malnutrition, and led to the collapse of several agricultural civilizations.
Today, as climate change destabilizes the delicate balance of temperature and salinity the AMOC relies on, some scientists worry that the AMOC is already showing symptoms of decline. “It’s as though the AMOC is a patient newly arrived in the emergency room, and Boers has provided scientists with an assessment of its vital signs,” Woods Hole Oceanographic Institution President Peter de Menocal told The Washington Post in August. “All the signs are consistent with the patient having a real mortal problem.”
But the size and complexity of the AMOC system offer some hope: It will take time, close monitoring, and meticulous data collection before researchers are able to confirm an AMOC slowdown through direct observations, Foukal notes. Many realize the risk of waiting for evidence of a collapse.
“The possibility that the AMOC tipping point is close should motivate us to try to stop it from happening. If we surpass a tipping point it could affect us for hundreds of years,” says Levke Caesar, a climate physicist at Maynooth University in Ireland who was not involved in the study. Research by Caesar conducted in 2021 shows the AMOC is at its weakest point in over 1,000 years.
Climate scientists are still unsure what threshold of carbon dioxide in the atmosphere would cause the AMOC to shut down. “The only thing to do is keep emissions as low as possible,” Boers told The Guardian. “The likelihood of this extremely high-impact event happening increases with every gram of CO2 that we put into the atmosphere.”
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