A recent rise in deadly, debilitating, and expensive heatwaves was caused by climate change, argues a new statistical analysis published in the Proceedings of the National Academy of Sciences (PNAS). Climatologists found that extreme heatwaves have increased by at least 50 times during the last 30 years. The researchers, including James Hansen of NASA, conclude that climate change is the only explanation for such a statistical jump. “This is not a climate model or a prediction but actual observations of weather events and temperatures that have happened,” Hansen, a prominent scientist and outspoken climate change activist, wrote in an op-ed in the Washington Post.