Did planet-warming carbon dioxide from fossil fuels contribute to this event? No one knows for sure. One controversial notion suggests that the rapid retreat of Arctic sea ice is making the polar jet stream wavier, allowing weather systems to persist longer. A more accepted theory pegs this heat to normal atmospheric fluctuations in the jet stream triggered by warmth in the tropics. But even researchers subscribing to that theory don’t necessarily rule out a secondary role for climate change. “Is long-term warming somehow the puppeteer controlling things in the background?” asks Nate Mantua, at NOAA’s Southwest Fisheries Science Center in Santa Cruz, California. “I haven’t seen proof, but it’s clearly a prime suspect.”

Source: This story appears in the September 2016 issue of National Geographic magazine.