Canada is having its worst fire season in modern history. The fires have burnt more than 20 million acres, casting hazardous smoke over parts of the U.S. and stretching Canadian firefighting resources thin. Public officials and many news headlines have declared the fires as "unprecedented," and in the modern-sense they are. But NPR climate correspondent Nate Rott has been talking to researchers who focus on the history of wildfire in Canada's boreal forests and they say the situation is not without precedent.Want more stories on the environment? Drop us a line at shortwave@npr.org.