In the letter, Tindal urges Canadians "to choose hope and action over despair and paralysis" in addressing what she calls "one of the most urgent moral challenges in human history."

"I believe this is a unique time in humanity's fretful reign on Earth, a rare moment that will have historic significance," writes Tindal.

"We need each other. We are emphatically, biologically not alone. As the carbon dioxide concentrations in the atmosphere rise, the planet will fail to provide for us. Life as we know it will die. Millions of human lives are on the line, rich and poor, old emitters and new, vulnerable and strong. There is no inoculation against this except all of us changing our behaviour all at once."