Scientists say the withdrawal of layers of ice from North America in the last years of the last glacial era may have begun with catastrophic "ice" losses in the North Pacific Ocean along the coast of present-day British Columbia and Alaska, scientists say.
In a new study published Oct. 1 in Science, researchers found that these rapid ice loss pulses of what is known as the Western Cordillera ice sheet contributed and perhaps triggered the massive delivery of the Laurentian ice sheet in the North Atlantic Ocean thousands for years. That collapse of the Laurentide ice sheet, which at one point covered large strips of Canada and parts of the United States, eventually caused major disruptions in the global climate (SN: 11/5/12).
The new findings call into question the hypothesis that changes on a hemispheric scale in the Earth's climate originate in the North Atlantic (SN: 31/01/19). The study suggests that the melting of Alaska’s remaining glaciers in the North Pacific, while less extreme than past purges, could have far-reaching effects on global ocean circulation and climate in the coming centuries.
“People usually think the Atlantic is where all the action is, and everything else goes on,” says Alan Mix, a paleoclimatologist at Oregon State University in Corvallis. "We're saying it's the other way around." The Cordilleran ice sheet fails earlier in the reaction chain, "and then that signal is transmitted (from the Pacific) around the world like the fall of the domino."
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In 2013, Mix and his colleagues pulled sediment cores from the bottom of the Gulf of Alaska in hopes of finding out exactly how the Cordillera ice sheet changed before the end of the last glacial era. These cores contained distinct layers of sand and silt deposited by icebergs split from the ice sheet on four different occasions in the last 42,000 years. The team then used radiocarbon dating to determine the chronology of events, finding that the Cordilleran's ice purges "surprisingly" preceded Laurentide's periods of sudden ice loss, known as the "Heinrich events," between 1,000 and 1,500 years each. time.
“We’ve known for a long time that these Heinrich events are a big problem,” says co-author Maureen Walczak, also a paleoceanographer at Oregon State University. "They have global climate consequences associated with increases in atmospheric CO2, warming in Antarctica … and weakening of the Asian monsoon in the Pacific. But we don't know why they happened."
Although scientists can now point the finger at the North Pacific, the exact mechanism remains unclear. Mix proposes several theories about how the Cordilleran ice loss eventually translated into massive ice delivery along the east coast of North America. It is possible, he says, that fresh water deposited in the North Pacific would travel north through the Bering Strait, across the Arctic, and as far as the North Atlantic. There, the floating fresh water served as a “plug” to the densest salt water in the ocean, preventing it from tipping over. This process could lead to hotter water, destabilizing the adjacent ice sheet.
Another theory postulates that the low elevation of the diminished Cordilleran ice sheet altered the way surface winds entered North America. Normally, the ice sheet would act as a fence, diverting the winds and their water vapor southward as they entered North America. Without this barrier, the transport of heat and fresh water between the Pacific and Atlantic Ocean basins is disrupted, changing the salinity of the Atlantic waters and eventually delivering more heat to the ice there.
Today, Alaska glaciers serve as the last remnants of the Cordillera ice sheet. Many are in rapid decline due to climate change. This melted ice also drains into the Pacific and Arctic oceans, raising sea levels and interfering with the normal mixing processes of the oceans. “Knowing the ice failure in the North Pacific seemed to portend a very rapid ice loss in the North Atlantic, that’s something to worry about,” Walczak says.
Researchers suggest that if melted ice in the North Pacific follows patterns similar to the past, it could produce major global climate events. But Mix warns that the amount of freshwater runoff needed to trigger changes elsewhere in the global ocean and climate is unknown. "We know enough to say those things have happened in the past, it's true, they're real and they could happen again."
It is unclear, however, what the timing of these global changes would be. If ice losses in the Atlantic occurred in the past due to a change in the dynamics of the deep ocean triggered by the Pacific melt, that signal would probably take hundreds of years to reach the other remaining layers of ice. However, if those losses were caused by a change in sea level or winds, other layers of ice could be affected a little faster, though not yet in this century.
Of course, the Laurentide ice sheet disappeared. But there are two more, in Greenland and Antarctica (SN: 30/09/20, 23/09/20). Both have numerous glaciers that end in the ocean and drain the interior of the ice sheets. This makes the ice sheets susceptible to both warmer ocean water and sea level.
Alaska’s melting glaciers have already fed about 30 percent of the global sea level rise. “One of the hypotheses we have is that rising sea levels are going to destabilize the ice shelves at the mouths of those glaciers, which will break like champagne corks,” Walczak explains. When this happens, the idea goes, the ice sheets will start to fall faster and faster.
Records of climate change in the Pacific, such as the one compiled by Walczak and colleagues, were difficult to find, says Richard Alley, a glaciologist at Pennsylvania State University who did not participate in the study. “This new data can generate more questions than it answers,” he says. "But by linking the circulation of the North Pacific Ocean … with the global model of climate oscillations, the new work gives us a real breakthrough in understanding all of this."