Visiting Alaska on Tuesday to highlight the dangers of climate change, President Barack Obama summoned up powerful images of the hundreds of wildfires that have burned across the state this summer: "More than five million acres in Alaska have already been scorched by fire this year—that's an area about the size of Massachusetts."

The Alaska megafires have helped make this an unprecedented wildfire season. Fires raging across the American West have set records for size and property damage, badly stretched firefighting resources, and cost the lives of three firefighters. So far this year, according to the National Interagency Fire Center, eight million acres have burned nationwide, well ahead of the past decade's average of five million at this point.