As the Earth began to thaw when the last Ice Age came to an end about 13,000 years ago, melting sea ice in the Arctic poured freshwater into the salty North Atlantic and drastically changed the world’s climate, a new study by researchers at UMass Amherst and the Woods Hole Oceanographic Institution suggests.
The study, published in the journal Geology, proposes a new mechanism for the influx of freshwater into the ocean that scientists believe altered currents like the Gulf Stream — which brings heat from the equator to North America and Europe — and cooled the Earth’s temperature just as the Earth was emerging from the Ice Age.
Previous researchers have suggested a number of mechanisms, including an “outburst flood” from Lake Agassiz, a giant prehistoric lake in North America, for the influx of freshwater.
“Although the lake was big by modern standards, it has been difficult in the climate modeling community to trigger a 1,000-year cold period with the water it contained, because the volume of water is not large enough to weaken the Atlantic circulation over a long period,” said Alan Condron, the study’s lead author and a research scientist at Woods Hole.
The study found that the amount of water stored as sea ice in the Arctic could have been much larger than the volume of water found in Lake Agassiz. Researchers said this pointed to sea ice as the main cause for the North Atlantic’s weakened currents.
“If we weaken this circulation pattern, then we shut off this pipeline of heat,” said Condron. “This idea is thought to have caused the Younger Dryas cooling 12,900 years ago, which plunged Europe and North America back into Ice Age conditions for 1,000 years.”
James Teller, a professor emeritus at the University of Manitoba in Winnipeg, Canada, and one of the researchers who worked on the original Lake Agassiz study that was published in 1989, said Condron and his team have made an “excellent contribution” to scientists’ understanding of the causes of the Younger Dryas.
“I think this paper will be a widely cited contribution that may become part of the accepted conclusions about where large volumes [and flows of water] came from that have been implicated in past large-scale global climate change,” Teller said.
“As the authors note, their calculations and conclusions are not intended to replace the role that Lake Agassiz played in changing past ocean circulation, only to supplement the volume of freshwater available from continental sources," he said.
The counterintuitive theory that a period of global cooling can result from global warming was the basis of the plot of the 2004 disaster movie “The Day After Tomorrow,” though, for dramatic purposes, the global cooling and the onset of a new Ice Age in the film is sudden and drastic (causing helicopters to drop out of the sky and people to freeze into statues as millions of Americans flee to Mexico).
The Younger Dryas cooling came at the end of the last Ice Age after the Earth had started to warm, and temporarily cooled the planet between 12,900 to 11,700 years ago. Condron said the melting sea ice from Greenland, Iceland, and Norway that flooded the oceans with freshwater acted as a “possible trigger” for the Earth’s return to colder temperatures during this period.
Raymond Bradley, director of UMass Amherst’s Climate Systems Research Center, said this freshwater sat on top of the salt water and prevented warm surface water from getting saltier, cooling, and sinking into the cold depths, thus slowing currents and the transportation of heat around the world thousands of years ago.
Melting sea ice "suggests an answer to a problem that has puzzled scientists for decades― why was there a period when the climate suddenly reverted to cold conditions as we were coming out of the last Ice Age?” Bradley said.
Researchers used a model created at MIT to determine how much, and how thick, Arctic ice used to be. They also used the model to study the Arctic Ocean, and referenced the journals of 19th and early 20th century Arctic expeditions to see if those explorers, who lived at the end of a small Ice Age that cooled North America and Europe between the 16th and 19th centuries, found any unusually thick sea ice.
While the study focuses on the past, researchers said it could have modern-day implications.
“Observations of the Arctic tell us that it is warming extremely fast, compared to the rest of the planet, and that rates of sea ice loss are accelerating,” Condron said. “The increased loss of Arctic sea ice we've seen in recent years is causing more fresh water to enter the North Atlantic.”
According to NASA, the amount of Arctic sea ice in September, the month when the expanse of sea ice in the region is at its smallest size each year, has been declining at a rate of 12.85 percent each decade since the late 1970s.
Bradley said some scientists believe they have already seen rapid melting sea ice affecting currents today, just as it did thousands of years ago.
Condron said the world should be wary if this is the case.
“Times of strong circulation (in the ocean’s currents) coincide with a warmer climate, and a weaker circulation seems to trigger a shift to a colder, Ice Age climate,” Condron said. “This is backed up by about 40 years of research.”
“While our main concern right now should be the consequences of global warming, a rapid shift to a much colder climate would also come with huge consequences,” Condron said.
A colder climate due to changing currents would make it harder for crops to grow, change species migration patterns, harm fish, and alter storm patterns and hurricanes, Condron said. It would also increase the human demand for heating fuel, he said.
“If we don’t limit fossil fuel emissions, further warming will result in an ice-free Arctic Ocean for much of the year, with many implications for weather patterns outside the Arctic, and big changes in ocean ecosystems,” Bradley said.
Condron said researchers still aren’t sure how much freshwater needs to enter the North Atlantic to weaken the ocean’s currents, or how much havoc a diminished Gulf Stream would wreak on the climate.
Here’s a NASA visualization of how the ocean currents work. Note the warmer water off the East Coast heading north and then over to Western Europe and the colder water returning. The concern is that the “conveyor belt” bringing warmth north will slow down.