A New Low for the Atlantic Meridional Overturning Circulation

In a recent paper published in Nature, a team of researchers from Canada, the United States and the United Kingdom collaborated across the Atlantic to break ground on a new study that the Atlantic Meridional Overturning Circulation (AMOC) is at its lowest in the past 150 years.

Instrumental in regulating global climate, the AMOC transports heat from the tropics and southern hemisphere toward the North Atlantic and the atmosphere. The strength of the AMOC varies periodically on a 60 to 80 year timescale, which is known as the Atlantic Meridional Oscillation. Currently in its negative phase, a weakening of the AMOC is often associated with the cooling of the Northern Hemisphere. However, it has other widespread impacts on American, African and Asian monsoons.

While other studies have shown that the AMOC has been weakening in recent years, the team demonstrated that the strength of the AMOC is at its lowest in 1,500 years, a finding that is cause for great concern. The researchers also suggested that the magnitude of decline has been severely underestimated due to the lack of sensitivity toward climate (freshwater) forcing. As meltwater is added into the oceans, changes in salinity and water density occur, which disrupt oceanic circulation patterns.

The Global Thermohaline Circulation System, with the AMOC boxed in red (Source: Authorship.me / Twitter).

One such example is the weakening of the circulation in the Labrador Sea as suggested by paleo-oceanographic evidence. This was determined by sorting the size of sediment grains deposited on the ocean bed, which showed a decrease in velocity of the Labrador Sea deep western boundary current.

The deep Labrador Sea density is always used as a predictor for changes in AMOC strength. As the Labrador Sea convection weakens, the velocity of the deep western boundary current decreases, which in turn decreases AMOC strength. This is attributed to freshwater inputs as Arctic and Nordic sea ice, ice sheets, and glaciers started to melt, which is usually thought to have been the outcome of the end of the Little Ice Age in the mid-nineteenth century.

Meanwhile, various methods and models were also used to reconstruct the surface temperatures of regions affected by the AMOC. In an interview following the publication of the study, lead author David Thornalley, a senior lecturer at University College London. stated that the “AMOC has weakened over the past 150 years by approximately 15 to 20 percent.”

Yet, the researchers results indicate the onset of AMOC weakening during the late Little Ice Age, coinciding with the industrial era. At that time, increased activity of exporting Arctic and Nordic sea ice occurred, coupled with melting circum-Arctic ice shelves.

In the same interview, Thornally had asserted that “warming and melting are predicted to continue in the future due to continued carbon dioxide emissions.” This comes as no surprise, with many studies already proving that global temperatures have been steadily increasing since the Industrial Revolution.

As the AMOC represents a dynamic coupled ocean-atmosphere system, it can be viewed as both a driver or a responder to climate change. In this case, the study believes that the AMOC has largely responded to the recent century and a half of climate change. Nonetheless, as stated in their report the AMOC may have “modified northward ocean heat transport, as well as atmospheric warming by altering ocean–atmosphere heat transfer, underscoring the need for continued investigation of the role of the AMOC in climate change.”

Thankfully, the AMOC is unlikely to completely shutdown. In an interview with GlacierHub, Wallace Smith Broecker, a well-known geoscience professor in Columbia University’s department of Earth and Environmental Sciences, said, “For what it’s worth, there are dozens of ‘water hosing’ experiments” that simulated freshwater input of higher magnitudes coming from Greenland. “Still they failed to shutdown the AMOC,” Broecker added.

North Atlantic Icebergs: Hubris, Disaster, and Safeguards

The view out Diane Davis’ kitchen window on June 23, 2017 (Source: Diane Davis/Newfoundland Iceberg Reports).

2017 marked the fourth consecutive year of “extreme” iceberg conditions in the North Atlantic Ocean. According to the U.S. Coast Guard International Ice Patrol, 1,008 icebergs entered shipping lanes in 2017, almost twice the number in a normal season.

Funded by a treaty of 13 nations, the International Ice Patrol is operated by a U.S. Coast Guard unit, which conducts aerial surveys of the Grand Banks, a region southeast of Newfoundland prone to rough seas and a density of icebergs. Institutions from both the U.S. and Canada comprise the North American Ice Service, which creates a daily iceberg analysis for mariners. The patrol was founded following the sinking of the R.M.S. Titanic in 1912, and, except for the two World Wars, has been in continuous operation since 1913.

Icebergs are created when glaciers calve, releasing pieces of ice to the sea that can be as tall as skyscrapers. Most icebergs in the North Atlantic originate in Greenland, which is rimmed by glaciers that flow to the coast. According to the International Ice Patrol, the elevated count in 2017 was caused by severe storms and higher than normal calving rates of Greenland’s glaciers, which many scientists consider a response to climate change.

However, Mark Carey, an environmental historian at the University of Oregon, says it is overly simplistic to equate iceberg production and climate change, as even growing glaciers calve.

“The classic iconic representation of global climate change is a glacier calving into the ocean, creating icebergs,” he said. “When reports of high numbers of icebergs in the North Atlantic appear, like in the last few years, people might simply think that this is because glaciers in Greenland are shrinking fast and shedding ice.”

An iceberg and oil rig in Bay Bulls on May 1, 2017 (Source: Diane Davis/Newfoundland Iceberg Reports).

In fact, he says the journey an iceberg takes from a Greenland glacier to “Iceberg Alley,” a famously dense area of icebergs on the Grand Banks, is long and complex, and involves more than just glacial calving.

First, a newly-birthed iceberg may never actually leave the fjord in which it was formed. If it does reach the open ocean, it will follow the Labrador Current, which flows north up the west coast of Greenland and south along the east coast of Canada, for as long as two years. During this time, the iceberg may become trapped in sea ice or run aground in shallows. The vast majority of icebergs never reach Iceberg Alley, where the International Ice Patrol counts the icebergs that drift into shipping lanes below 48 degrees north latitude.

“Winter sea ice conditions also affect whether a berg survives and where it goes, so regional weather and not just global climate influence the iceberg journey,” Carey said.

Nevertheless, icebergs can have dangerous outcomes for ships traveling through the North Atlantic region, as the world saw during the sinking of the Titanic and the Danish ship Hans Hedtoft in 1959.

History and global politics makes the North Atlantic especially sensitive to the movements of icebergs. “The North Atlantic has been an integral part of the international political, economic and security system of the day for up to a millennium,” said Rasmus Bertlesen, professor of Northern Studies at the University of Tromsø.

“These shipping lanes are very important, since the U.S., the Canadian East Coast, and Western Europe are power houses of the world economy,” he added.

A life ring that washed ashore in Iceland was the only trace of the Hans Hedtoft recovered (Source: Rasmus Bertelsen).

No ship has collided with an iceberg in the region monitored since the M.S. Hans Hedtoft sank on its maiden voyage. To keep up with fast-moving ice, the Danish Meteorological Institute has recently launched a project that uses artificial intelligence to analyze ice distribution. Though Bertelsen agrees more frequent maps are necessary, he fears history will repeat itself.

“North Atlantic shipping has been the story of technological hubris, human disaster and then technological safeguards,” he said. “Hopefully, these artificial intelligence ice maps will not be the Titanic or Hans Hedtoft of our time leading to disaster and reckoning.”

Carey believes that the portrayal of icebergs as threats to shipping also adds allure to the subject, spurring tourism in places like Newfoundland and Alaska.

Diane Davis, a retired schoolteacher from Newfoundland who runs the Facebook page “Newfoundland Iceberg Reports” agreed.

“Icebergs are a huge tourist draw to Newfoundland and Labrador,” she said.

Davis created the Facebook page to facilitate iceberg sightings in the region. Currently, the page has 7,139 members, who monitor the photographs of icebergs and their locations.

Davis personally witnessed the higher density of icebergs in the North Atlantic over the last four years, and added that many of the icebergs drifted near coastal communities, where people were able to photograph them. The shipping industry is well-practiced at dealing with these icebergs, she said. More concerning to her is the interaction between icebergs and the offshore oil industry.

Diane Davis inspired a character in the Broadway musical “Come from Away,” and met Prime Minister Trudeau when the show toured in Newfoundland (Source: Justin Trudeau/Flickr).

Carey concurs with Davis’ concern. “Icebergs only pose a risk when people get close to the bergs, or when an iceberg drifts close to human populations, infrastructure like docks or drilling platforms, or boats,” he said.

In March 2017, for example, Husky Energy’s SeaRose floating platform came within 463 meters of a large iceberg, threatening 84 crew members and 340,000 barrels of crude oil aboard. The board that monitors industry in the oilfields off Labrador suspended operations for SeaRose, the first such suspension in over a decade.

“Iceberg risk is not just about iceberg production or numbers of bergs in the shipping lanes,” Carey said. “It is also influenced by how often and how many people live, work, travel, and vacation near icebergs–and these numbers are on the rise all the time.”