Climate Central: April capped a 12-month period that tied the warmest such stretch on record, according to data released Tuesday. That period, going back to May 2014, tied the previous record holder, the 12 months from April 2014 to March 2015. Of the 10 warmest 12-month periods on record, nine occurred in the past two years, most of them in back-to-back stretches. The clustering of such warm periods is a marker of how much global temperatures have risen thanks to the human-driven buildup of heat-trapping gases...