Friday, June 5, 2015

1872. Global Warming ‘Hiatus’ Challenged by NOAA Research

By Justin Gillis, The New York Times, June 4, 2015

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.

In their paper published online Thursday by the journal Science, and in interviews, scientists at NOAA said that in coming months they would roll out new versions of their temperature record that incorporate numerous improvements.

The previous record showed that temperatures from 2000 to 2014 had warmed at about two-thirds the rate of temperatures from 1950 to 1999. In the new analysis, the rate of warming in those two time periods is basically identical.

NOAA said the improvements in its data set included the addition of a huge number of land measurements from around the world, as a result of improving international cooperation in sharing weather records. But the disappearance of the slowdown comes largely from adjustments in ocean temperatures.

The ocean covers 70 percent of earth and thus the temperature at its surface has a huge influence on the overall record. Yet ocean measurements in particular are rife with difficulties.

For many decades, into the mid-20th century, the main measurements came from sailors hauling up buckets of seawater and plopping thermometers into them.
The buckets varied, the thermometers varied, and some of the sailors were more diligent than others about following instructions. On average, scientists believe, the water tended to cool off a bit before the temperature was recorded.

NOAA had long believed the data glitches from the buckets had largely disappeared after World War II, but new information suggests that bucket measurements continued on some commercial vessels long after the war. The new NOAA data set attempts to correct for this and other problems in the ocean records.

The Cato Institute, a libertarian think tank in Washington that is critical of climate science, issued a statement condemning the changes and questioning the agency’s methodology.

“The main claim by the authors that they have uncovered a significant recent warming trend is dubious,” said the statement, attributed to three contrarian climate scientists: Richard S. Lindzen, Patrick J. Michaels and Paul C. Knappenberger.

However, Russell S. Vose, chief of the climate science division at NOAA’s Asheville center, pointed out in an interview that while the corrections do eliminate the recent warming slowdown, the overall effect of the agency’s adjustments has long been to raise the reported global temperatures in the late 19th and early 20th centuries by a substantial margin. That makes the temperature increase of the past century appear less severe than it does in the raw data.

“If you just wanted to release to the American public our uncorrected data set, it would say that the world has warmed up about 2.071 degrees Fahrenheit since 1880,” Dr. Vose said. “Our corrected data set says things have warmed up about 1.65 degrees Fahrenheit. Our corrections lower the rate of warming on a global scale.”

Even if the warming slowdown in the early 21st century was real, there seems to be little question that it is ending. By a small margin, the global temperature hit a record in 2014, and developing weather patterns suggest that record will likely be broken by a larger margin in 2015.

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