Scientists reported Wednesday that 2015 was the hottest year in recorded history by far, breaking a record set only the year before — a burst of heat that has continued into the new year and is roiling weather patterns all over the world.
In the continental United States, the year was the second-warmest on record, punctuated by a December that was both the hottest and the wettest since record-keeping began. One result has been a wave of unusual winter floods coursing down the Mississippi River watershed.
Scientists started predicting a global temperature record months ago, in part because an El Niño weather pattern, one of the largest in a century, is dumping an immense amount of heat from the Pacific Ocean into the atmosphere. But the bulk of the record-setting heat, they say, is a consequence of the long-term planetary warming caused by human emissions of greenhouse gases.
“The whole system is warming up, relentlessly,” said Gerald A. Meehl, a scientist at the National Center for Atmospheric Research in Boulder, Colo.