Scientists have long labored to explain what appeared to be a slowdown in global warming that began at the start of this century as, at the same time, heat-trapping emissions of carbon dioxide were soaring. The slowdown, sometimes inaccurately described as a halt or hiatus, became a major talking point for people critical of climate science.
Now, new research suggests the whole thing may have been based on incorrect data.
When adjustments are made to compensate for recently discovered problems in the way global temperatures were measured, the slowdown largely disappears, the National Oceanic and Atmospheric Administration declared in a scientific paper published Thursday. And when the particularly warm temperatures of 2013 and 2014 are averaged in, the slowdown goes away entirely, the agency said.
“The notion that there was a slowdown in global warming, or a hiatus, was based on the best information we had available at the time,” said Thomas R. Karl, director of the National Centers for Environmental Information, a NOAA unit in Asheville, N.C. “Science is always working to improve.”
The change prompted accusations on Thursday from some climate-change denialists that the agency was trying to wave a magic wand and make inconvenient data go away. Mainstream climate scientists not involved in the NOAA research rejected that charge, saying it was essential that agencies like NOAA try to deal with known problems in their data records.
At the same time, senior climate scientists at other agencies were in no hurry to embrace NOAA’s specific adjustments. Several of them said it would take months of discussion in the scientific community to understand the data corrections and come to a consensus about whether to adopt them broadly.
“What you have is a reasonable effort to deal with known biases, and obviously there is some uncertainty in how you do that,” said Gavin A. Schmidt, who heads a NASA climate research unit in New York that deals with similar issues.
Some experts also pointed out that, depending on exactly how the calculation is done, a recent slowdown in global warming still appears in the NOAA temperature record, though it may be smaller than before. “These trends are very sensitive to the time periods you use to compute them,” said Gerald A. Meehl, a senior scientist at the National Center for Atmospheric Research in Boulder, Colo.
Scientists like Dr. Meehl never accepted the notion, put forward by some climate contrarians, that the slowdown disproved the idea that global warming poses long-term risks. But they said they believe it is real and demands an explanation.
A leading hypothesis to explain the slowdown is that natural fluctuations in the Pacific Ocean may have temporarily pulled some heat out of the atmosphere, producing a brief flattening in the long-term increase of surface temperatures.
NOAA is one of four agencies around the world that attempts to produce a complete record of global temperatures dating to 1880. They all get similar results, showing a long-term warming of the planet that scientists have linked primarily to the burning of fossil fuels and the destruction of forests. A huge body of physical evidence — notably, that practically every large piece of land ice on the planet has started to melt — suggests the temperature finding is correct.
Yet the temperature record is plagued by many problems: thermometers and recording practices changed through time, weather stations were moved, cities grew up around once-rural stations, and so on. Entire scientific careers are devoted to studying these issues and making corrections.