The wildfires sweeping Canada have become the largest in its modern history. Across the country, 30 million acres of forest have burned — three times as much land as in the worst American fire in the past 50 years. The scale has forced an international response and a re-evaluation of how the world handles wildfires. Firefighters on the front lines discuss the challenges they face, and David Wallace-Wells, a climate columnist for The Times, explores how climate change has shifted thinking about wildfires. Guest: David Wallace-Wells, a climate columnist for The New York Times.