Global warming halted budding ice age, study finds

The last surviving piece of the ice sheet that once covered much of North America started growing again about 2,000 years ago, according to a study published October 22 in the journal Science. The authors say this regrowth marked the start of Earth's next glacial period, and that rising temperatures cut it short around 1960.lamont.columbia.edu

The study was led by geochemist Nicolás Young and climate geochemist Joerg Schaefer of Columbia University's Lamont-Doherty Earth Observatory. It focuses on the Barnes Ice Cap, a Delaware-sized remnant of the Laurentide Ice Sheet on Baffin Island in Nunavut. "It was just getting started, everything was normal, the next glaciation was about to happen—and yet it stopped," Young said. "It's a very powerful example of how we have jolted the system".lamont.columbia.edu

Dating the ice with dead moss

The Barnes Ice Cap is frozen to its bedrock, so it does not slide. Plants it buried as it advanced stayed in place under the ice, and they are now melting out at its edge. The team measured beryllium-10 in nearby rocks and found the ice shrank to roughly its current size about 3,600 years ago. Radiocarbon dates from the buried moss and lichen point to renewed growth starting around 2,000 years ago. The University of Colorado Boulder, home of coauthor Gifford Miller, says the plants at the Barnes margin itself show growth beginning at least 1,000 years ago.wired.com+2

By the 1960s, the ice cap was losing more ice each summer than it gained each winter. Since the 1980s, its edges have pulled back by an average of 4 meters a year. Model studies had suggested the next glaciation was thousands or even tens of thousands of years away. The authors argue instead that a glacial period begins when ice first starts to regrow.phys.org+1

Revisiting 1970s warnings

The findings shed new light on 1970s scientific warnings of a coming ice age, which climate skeptics later held up as evidence that climate science was wrong. "I think this study actually helps show that scientists were right about a pending glaciation," Young told Wired. Miller recalled summers cooling from the 1950s to the mid-1970s, but said that "by the late '70s, the debate was over. Greenhouse gas forcing was clearly dominant".colorado.edu+1

Andrew Wickert of the University of Minnesota, who was not involved in the study, called the result "significant." Dartmouth's Meredith Kelly praised the data but said she would like to see comparisons with other sites around the world.wired.com

An ice cap's final centuries

Models show the Barnes Ice Cap melting completely within a few centuries. That could change how future glaciations start, because the Laurentide has acted as a seed point where new ice builds up. "What has been an extremely natural and predictable phenomenon is now gone," Young said.phys.org