Thirty-six million people across the western United States are under heat advisories today, with forecasters predicting record temperatures of up to 25 degrees above normal. Phoenix, Arizona, reached 113 degrees Fahrenheit on Tuesday — the hottest October day in its history, coming after Arizona logged its hottest summer on record this year. The federal government’s most recent National Climate Assessment found the number of heat waves that major U.S. cities are experiencing each year has doubled since the 1980s due to climate change driven by human activity.