Climate Change: Ice-Age Canada, Why It Melted

We are zooming in close to the last ice age, looking at the massive Laurentide ice sheet that covered Canada then, with new research from Oregon State University. What happened may provide ideas as to how Greenland and Antarctica's ice sheets may respond to a changing climate, scientists say.

Here's what happened first: the Laurentide ice sheet began calving icebergs, then abruptly shifted to a focus on melting on the continent, say Oregon State researchers, who were funded by the National Science Foundation, with support from the National Aeronautics and Space Administration (NASA). That shift, caused by "radioactive forcing," began prior to 9,000 years ago and greatly sped up deglaciation.

Two mechanisms cause ice sheets to diminish, says David Ullman, a postdoctoral researcher at Oregon State and lead author on the study, according to the university press release. Either they diminish dynamically, by jettisoning icebergs at their edges, or by using negative "surface mass balance," regarding snow accumulation and melting. That is, when more snow melts than accumulates, as happened after the last glacial maximum, surface mass balance is negative.

Interestingly, during most of the deglaciation of the Canadian continent, the surface mass balance of the Laurentide Ice Sheet was generally positive, Ullman said, according to the release. "But there was a shift before 9,000 years ago and the deck became stacked, as sunlight levels were high because of the Earth's orbit and CO2 increased."

Here's what happened then: When melting took off, the ice sheet began pulling back from the coast and iceberg calving diminished. Surface melt permeated the ice sheet, driving the final deglaciation, Ullman said, according to the release.

The CO2 level then was near the top of pre-industrial measurements, though it was much less than it is today, Ullman said in the release. The solar intensity then was higher than today, he noted.

Here's another interesting part: Big shifts in the surface mass balance occur from only very small changes in radioactive forcing, Ullman said in the release. "It shows just how sensitive the system is to forcing, when it might be radiation or greenhouses gases," he noted. 

In the past, scientists have examined ice cores dating back some 800,000 years, documenting any times when increases in summer isolation took place, though not all of them resulted in the heavy deglaciation that is occurring now. "It just might be that the ice sheet needed an added kick from something like elevated CO2 levels to get things going," Ullman said in the release.