Globe and Mail: Michael Bloomberg is a former three-term mayor of New York and the UN Secretary-Generals Special Envoy for Cities and Climate Change. Gregor Robertson is mayor of Vancouver. ----------------------------- As the largest country in the Western Hemisphere, Canada has a special role to play in tackling the threat of global climate change. Canadian cities, which are on the front lines of this battle, must help lead the way. In recent years, destructive floods have caused billions of dollars in damage...