It is the most rare and extreme events for which the largest fraction is anthropogenic, and that contribution increases nonlinearly with further warming.Fischer explains:
Climate change doesn’t ‘cause’ any single weather event in a deterministic sense. But a warmer and moister atmosphere does clearly favor more frequent hot and wet extremes.Update (July 17): Two studies show that climate change is already making extreme weather events worse.
Update (May 30, 2019): A preliminary count shows 500 tornado observations in the past 30 days.
Bill Bunting, the [U.S. Storm Prediction Center's] chief of forecast operations, told Bloomberg on Wednesday that only four other 30-day periods in the official record — in 2003, 2004, 2008 and 2011 — saw an excess of 500 tornado reports.