A study published in Science argues that the next glacial period had already begun in the Canadian Arctic, with the Laurentide Ice Sheet’s remaining Barnes Ice Cap starting to grow again about 2,000 years ago. That expansion continued for roughly two millennia, reaching its greatest size around 1960, when warmer summers began driving the ice back.
The Laurentide Ice Sheet covered much of Canada and the northern United States between about 95,000 and 20,000 years ago. Its last substantial remnant is the Barnes Ice Cap, on Baffin Island in Nunavut. Researchers led by Nicolás Young of Columbia Climate School investigated whether the ice sheet had previously shrunk to a smaller size than it is today before growing again.
To reconstruct the ice cap’s history, the team dated plants exposed as the ice retreated, using carbon-14. Because the Barnes Ice Cap is frozen to the rock beneath it, advancing ice can bury vegetation rather than sweep it away. That preserved material gave researchers evidence of earlier ice margins. They combined these findings with data from other, recently melted ice caps in the region.
The results indicate that the ice cap reached its smallest extent more than 3,600 years ago and began expanding again around 2,000 years ago. Young and his colleagues define that initial regrowth as the start of the next glacial period. This differs from models that place the next glaciation tens of thousands of years in the future, with estimates of about 50,000 years even without global warming. The disagreement partly reflects different definitions of when a glaciation begins and what evidence should establish its onset.
The researchers say their findings also offer evidence relevant to claims made by some scientists in the 1970s that a new ice age was approaching. Young stresses, however, that the study does not mean global warming “saved humanity from an ice age.” The researchers’ argument is that ice growth had started, although a significant cooling trend would have taken millennia to become apparent and the Laurentide would have needed about 30,000 years to spread across the continent again.
Since around 1960, the Barnes Ice Cap has lost more ice in summer than it gains in winter. Model projections suggest it could disappear within roughly 300 years. The study warns that this loss may affect future glaciations: the Laurentide Ice Sheet historically provided a starting point for ice to build and expand as temperatures fell.
The timing of any future glaciation is now difficult to determine in an atmosphere with high greenhouse gas concentrations. In a commentary accompanying the study, glaciologist Shaun Marcott of the University of Wisconsin–Madison says future emissions could postpone one by hundreds of thousands of years. If continental ice begins expanding again, the Barnes Ice Cap may no longer remain as a remnant of the Laurentide Ice Sheet.
Sources: Focus.it.