"We are at a critical moment," says the letter. "In the words of the U.S. National Academy of Sciences, 'each additional ton of greenhouse gases emitted commits us to further change and greater risks.' The longer we delay the transition to low-carbon economy, the more drastic, disruptive and costly that transition will be." The letter is dated May 7 and signed by a dozen scientists at universities across Canada.

Canada's natural resources minister insists the federal government believes climate change is an urgent matter and says his recent comments on climate science will not erode Canada's credibility or chances at U.S. approval for the Keystone XL pipeline.

In Ottawa, John Stone, a professor of environmental studies and geography at Carleton University, said time is rapidly running out. "I don't think he fully appreciates what the science behind climate change is saying," said Stone. "We are offering to help better inform a much needed debate on the biggest problem we are facing."