"It's another nail in the coffin of the idea that we're not blame," said Graham Cogley, a Trent University geographer, who has co-authored research that appeared Thursday in the journal Science.
For decades, scientists and anyone who spends time in the mountains have been able to see that high-altitude glaciers are shrinking. The start of that recent melt dates back to the middle of the 19th century, with the end of what's been called the Little Ice Age.
Glaciers respond slowly to shifting climates and much of their shrinkage over the decades has been due to natural warming after several unusually cold centuries.
"(Glaciers) take decades or centuries to catch up with the climate," Cogley said. "They're always chasing the climate, trying to get to the right size."
But some of the melt is due to human-caused climate change. To find out how much, Cogley and his colleagues performed a Herculean feat of calculation.
They took 12 different computer models of the Earth's climate and made two versions of each. One version included all known natural and artificial factors that influence climate — from land-use practices to solar activity to volcanoes to greenhouse gas emissions. The other included only natural factors.
They then turned those models loose on the Randolph Glacier Inventory, a database of nearly 200,000 glaciers from around the world. Each glacier was run through both versions of all 12 models.
"It's a fair amount of number-crunching," Cogley said.
They found that both versions of their climate models produced very similar results until about 1960, results that also matched observations from the field. But after that, the lines on the graphs began to diverge — and the models that lined up most closely with observed values were the ones that accounted for greenhouse gas emissions.
"We see a more and more intense anthropogenic forcing of the glaciers as time passes."
Although the effect of human activity is indistinguishable from natural warming in the mid-19th century, by 1991 human-caused warming was responsible for at least half of the annual glacier melt, the study suggests.
The paper also breaks down glacier melt by region. Although Cogley is less confident that he can show the human influence in some regions than others, that's not the case for Western Canada.
The massive ice sheets in Banff, Jasper and Yoho national parks clearly show anthropogenic melting, he said. The paper gives that conclusion at least an 80 per cent confidence level.
It's appropriate that his work so clearly describes what's been happening to those icy castles, because the Angel Glacier along the side of Mount Edith Cavell in Jasper was the first one Cogley ever saw.
"It's a spectacular sight," he said.
"It's going to be too late for a lot of (the glaciers). But if we could figure out a way for our grandchildren's grandchildren so they can still get a look at Angel Glacier, that would be better than if they couldn't."