2015 was hottest year in recorded history

PHOTOGRAPHER: The Washington Post

Overall, NOAA said, 2015 was 1.62 degrees Fahrenheit (0.9 degrees Celsius) above the 20th century average, a substantial rise.

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, Colorado.

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.

Michael E. Mann, a climate scientist at Pennsylvania State University, calculated that if the global climate were not warming, the odds of setting two back-to-back record years would be remote.

Two U.S. government agencies, NASA and NOAA, the National Oceanic and Atmospheric Administration, compile separate analyses of the global temperature, based upon thousands of weather stations and ocean buoys scattered around the world.

They released their results Wednesday, showing 2015 as the warmest year in a global record that began in 1880. Preliminary data from the Japan Meteorological Agency also show record warmth for 2015, and a British monitoring program is expected to report a similar result in coming weeks.

NOAA previously reported that 2015 was the second-warmest year for the continental United States, after 2012.

The strong El Niño has continued into 2016, raising the possibility that this year, too, will set a global temperature record. The El Niño pattern is also disturbing the circulation of the atmosphere, contributing to worldwide weather extremes that include a drought in southern Africa, threatening the food supply of millions.