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A New Ice Age Was Beginning. Then Humans Started Burning Fossil Fuels
Burning fossil fuels may have stopped the Earth from entering another ice age, a new study suggests. The research, published Thursday in the journal Science, uses samples of ancient vegetation and rock records to analyze the growth—and sudden retreat—of an ice cap that is the last vestige of an ice sheet that once covered much of North America.
The findings shows the Barnes Ice Cap, which covers parts of Canada’s Baffin Island, started growing a few thousand years ago before rapidly reversing course beginning in the 1960s as carbon pollution ramped up.
“A question that’s in the popular consciousness is, have we caused enough global warming to stop the next ice age from happening?” says Andrew Wickert, an associate professor of environmental science at the University of Minnesota Twin Cities, who was not involved in the research. The new research shows that “even as we were just starting to ramp up emissions, we’ve already done enough damage to cause the major North American ice sheet to already start retreating. That’s significant.”
The planet regularly goes through phases where ice extends down from the high latitudes—what we colloquially call "ice ages"—due to natural shifts in the Earth's rotation and orbit, known as Milankovitch Cycles. Those are followed by interglacial periods where ice retreats toward the poles.
The last ice age ended 12,000 years ago. At its peak, what's known as the Laurentide Ice Sheet covered all of Canada and extended as far south as New York and the Midwest. It retreated to form the Barnes Ice Cap, which covers an area the size of Delaware on Baffin Island.
Unlike glaciers, which can move, the Barnes Ice Cap is frozen to the bedrock, preserving the vegetal record of what grew around it under the ice. That quirk allowed the scientists leading the new study to glean information about how much it has grown—and shrank.
To determine when it first reached its modern size, they used an exposure dating technique to calculate roughly how long rocks several meters from the glacier’s edge had been exposed to the sky. Using measurements of Beryllium-10, an element that forms on the surface of exposed rock, the authors calculated that the ice sheet first reached the size it is today around 3,600 years ago.
Next, they wanted to see how much the ice cap had regrown from that point. To do this, they turned to the humble denizens of the tundra in front of the glacier: mosses and lichen.
“After the ice cap retreated, mosses eventually colonized this newly exposed land,” says Nicolás Young, a paleoclimatologist at the Lamont-Doherty Earth Observatory at Columbia University and lead author of the new paper. “These mosses are living there, and things are normal—then, eventually, the ice cap starts to re-expand. The ice covers the moss, and kills it.” The traces of the dead moss are preserved in newly reexposed land, allowing Young and his team to piece together a timeline of how the ice has moved across the landscape.
Thanks to these records, Young and his coauthors estimate that the glacier started growing again around 2,000 years ago. This may have signaled the start of another glaciation period.