Scientists reported Wednesday that 2015 was the hottest year in the historical record by far, breaking a mark 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 contiguous 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 releasing 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.
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 a 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,” with these claims and similar statements reappearing recently on the Republican presidential campaign trail.
Statistical analysis suggested all along that the claims were false, and that the slowdown was, at most, a minor blip in an inexorable trend, perhaps caused by a temporary increase in the absorption of heat by the Pacific Ocean.
“Is there any evidence for a pause in the long-term global warming rate?” said Gavin A. Schmidt, head of NASA’s climate-science unit, the Goddard Institute for Space Studies, in Manhattan. “The answer is no. That was true before last year, but it’s much more obvious now.”
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, about one chance in every 1,500 pairs of years. Given the reality that the planet is warming, the odds become far higher, about one chance in 10, according to Dr. Mann’s calculations.
Two American government agencies - NASA, the National Aeronautics and Space Administration, and NOAA, the National Oceanic and Atmospheric Administration - compile separate analyses of the global temperature, based upon thousands of measurements from weather stations, ships and ocean buoys scattered around the world. Meteorological agencies in Britain and Japan do so, as well. The agencies follow slightly different methods to cope with problems in the data, but obtain similar results.
The American agencies released figures on Wednesday showing that 2015 was the warmest year in a global record that began, in their data, in 1880. British scientists released figures showing 2015 as the warmest in a record dating to 1850. The Japan Meteorological Agency had already released preliminary results showing 2015 as the warmest year in a record beginning in 1891.
On Jan. 7, NOAA reported that 2015 was the second-warmest year on record, after 2012, for the lower 48 United States. That land mass covers less than 2 percent of the surface of the Earth, so it is not unusual to have a slight divergence between United States temperatures and those of the planet as a whole.
The end of the year was especially remarkable in the United States, with virtually every state east of the Mississippi River having a record warm December, often accompanied by heavy rains.
A warmer atmosphere can hold more water vapor, and an intensification of rainstorms was one of the fundamental predictions made by climate scientists decades ago as a consequence of human emissions. That prediction has come to pass, with the rains growing more intense across every region of the United States, but especially so in the East.
The term global warming is generally taken to refer to the temperature trend at the surface of the planet, and those are the figures reported by the agencies on Wednesday.
Some additional measurements, of shorter duration, are available for the ocean depths and the atmosphere above the surface, both generally showing an inexorable long-term warming trend.
Most satellite measurements of the lower and middle layers of the atmosphere show 2015 to have been the third- or fourth-warmest year in a 37-year record, and scientists said it was slightly surprising that the huge El Niño had not produced a greater warming there. They added that this could yet happen in 2016.
When temperatures are averaged at a global scale, the differences between years are usually measured in fractions of a degree. In the NOAA data set, 2015 was 0.29 degrees Fahrenheit warmer than 2014, the largest jump ever over a previous record. NASA calculated a slightly smaller figure, but still described it as an unusual one-year increase.
The intense warmth of 2015 contributed to a heat wave in India last spring that turns out to have been the second-worst in that country’s history, killing an estimated 2,500 people. The long-term global warming trend has exacted a severe toll from extreme heat, with eight of the world’s 10 deadliest heat waves occurring since 1997.
Only rough estimates of heat deaths are available, but according to figures from the Center for Research on the Epidemiology of Disasters, in Brussels, the toll over the past two decades is approaching 140,000 people, with most of those deaths occurring during a European heat wave in 2003 and a Russian heat wave in 2010.
The strong El Niño has continued into 2016, raising the possibility that this year will, yet again, 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.