Meteorologists have blamed El Niño and the polar vortex for record-breaking warm temperatures across the US this week, saying the pair of weather systems will likely keep 2015 warm enough to be the hottest year on record. This year’s El Niño, a recurring weather pattern caused by unusually warm water in the Pacific Ocean, is particularly strong and reaching its peak. The 2015-16 El Niño is expected to rank as one of the three strongest in half a century. --Alan Yuhas and Oliver Milman, The Guardian, 15 December 2015