text:  The court's in session again today and tomorrow. A big source of last year's air pollution in the U.S., Canada's wildfires. CBS's Cammie McCormick has that. The Canadian wildfires lasted for months. The smoke traveled across the globe, leading to air quality alerts in major cities here in the U.S. Researchers now estimate the fires put more than 3 billion tons of heat-trapping carbon dioxide into the air. That's three times the carbon emissions from airplanes each year.