Blame global warming for about 75 percent of the world’s unusually hot days and 18 percent of its extreme snow or rain, according to a new paper in the journal Nature Climate Change. Heat waves and heavy storms are occurring at least four times more often than they did before carbon pollution started driving up thermometers. Global average temperatures are now about 0.85 degrees Celsius (1.4 Fahrenheit) higher than before industrialization. Additional heat and precipitation are expensive. Severe weather costs the U.S. economy as much as $33 billion a year, according to a U.S. Energy Department report released April 21. And those figures will increase as the planet continues to warm, as climate change may not be smooth or gradual, according to the new paper. At 2 degrees Celsius — United Nations climate negotiators’ avowed upper limit — extremely hot days may be twice as likely as at 1.5 […]