It will take a few more years to know for certain, but the back-to-back records of 2014 and 2015 may have put the world back onto a trajectory of rapid global warming, after period of relatively slow warming dating to the last powerful El Niño, in 1998.
Politicians attempting to claim that greenhouse gases are not a problem seized on that slow period to argue that “global warming stopped in 1998” and similar statements, with these claims reappearing recently on the Republican presidential campaign trail.
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.