Two days before the presidential inauguration, scientists are expected to announce that 2016 was the hottest year on Earth since record-keeping began in 1880 ​- news that will test national, state and economic leadership on climate change. Even as Washington, D.C., and California bundle up against winter chill, records ​will ​show that extreme heat blanketed large areas of every continent this year, pushing up average temperatures for the third year in a row.