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Global warming ‘pause’ never happened, scientists say

Calved icebergs from the nearby Twin Glaciers. (Joe Raedle/Getty Images)

The climate science sphere has been wrapped up in a major debate for the past several years: over the global warming “pause.” But now, a growing body of research has some scientists saying that the case may, in fact, be closed. They’re arguing that the pause never existed.

The notion of a global warming pause, or “climate hiatus,” suggests that the rising of global surface temperatures has significantly slowed or even stopped during the past 15 years. The idea, which experts believe cropped up sometime in 2013, has been seized upon by many climate skeptics, but also has managed to cause unexpected controversy within the scientific community. Since then, a flurry of scientific studies have emerged attempting to explain why such a pause might be occurring, pointing to natural climatic factors such as volcanic eruptions or changes in oceanic patterns.

But in the past few months, a handful of scientists have taken a different approach by asking not why the hiatus is occurring, but whether it’s occurring at all. And two new studies, released within days of each other, are adding to the evidence that the pause may not exist.

A new statistical approach. One of the studies, published Thursday in the journal Climatic Change, introduces a new statistical method designed specifically to help tackle the hiatus problem.

To figure out whether the pause exists, scientists have to compare two data sets: global temperatures from the past 15 years vs. all the decades leading up to that point. Naturally, the data set from the supposed hiatus period is much smaller — so small, in fact, that it would be hard to get accurate results using most classical statistical methods, says Bala Rajaratnam, the study’s lead author and an assistant professor of statistics and Earth system science at Stanford University.

“We had to develop a new method which takes into account the idea that the sample size during the hiatus is really not very large,” Rajaratnam says. Using the new method, the scientists tackled four hypotheses related to the idea of a hiatus: first, that warming stopped entirely in the past 15 years; second, that the rate of warming just slowed down; third, that there has been a change in average global temperature in the past 15 years; and fourth, that there was a difference in the year-to-year temperature changes between the two data sets.

Including all four hypotheses was a way to make sure all the bases were covered, says Noah Diffenbaugh, an associate professor of Earth system science at Stanford University and the study’s senior author. The supposed climate hiatus has been defined in a variety of ways within both the general public and the scientific community, and each of the hypotheses represents a slightly different definition. In each case, Diffenbaugh says, he and his colleagues were able to conclude “with high statistical confidence” that a pause was not occurring.

The study partly builds on another high-profile paper, published in June in Science by researchers from the National Oceanic and Atmospheric Administration (NOAA), that also investigated the hiatus question. That paper suggested that biases or errors in the measurements used to compile global temperature data sets could have produced something resembling a warming pause. By correcting these biases, the researchers found that the temperature data did not support the idea of a pause:

Rajaratnam and Diffenbaugh’s analysis, using the new statistical method, resulted in the same conclusion using the uncorrected temperature data. “I think it further enhances the robustness of the notion that we really do not have strong evidence for a slowdown or a stoppage of global warming,” says Thomas Karl, director of NOAA’s National Centers for Environmental Information and the lead author of the June NOAA paper.

Rajaratnam adds that the success of the new statistical methods support the idea that “it’s really critical the appropriate scientific method [is used] to do the investigation,” and that collaboration between Earth scientists and statisticians is a necessary partnership.

Although it’s one of the most comprehensive statistical analyses performed, it isn’t the only recent study questioning the pause. Another paper supporting the same conclusion was published Tuesday in the Bulletin of the American Meteorological Society.

A “blind” expert test. This paper also includes a statistical analysis of global temperature data and concludes that “there have been 6 occasions since 1970 when a 15-year trend would have failed to reach significance.” The authors argue that each of these cases, including the most recent 15-year stretch, constitutes a mere fluctuation in a much larger overall warming trend, and that none of these fluctuations statistically alters the trend.

The paper also describes a separate experiment that lead author Stephan Lewandowsky, a cognitive psychologist at the University of Bristol, says is “the most exciting part.” The researchers subjected the idea of a warming pause to something called a blind expert test.

They presented the climate data to a group of 25 professional economists, but told them that the data represented world agricultural output, not temperature. They did this to prevent any personal biases the economists might have, essentially “dressing up the data as something different that doesn’t have any political or emotional connotations,” Lewandowsky says.

They then asked the economists whether a pause in output had occurred in the period after 1998. The experts rejected this idea, and some even agreed that such a statement was “fraudulent.”  

“When you do a blind study and it comes out that strongly … that’s a pretty compelling view,” says John Abraham, a professor of thermal sciences at the University of St. Thomas. Abraham also published a study earlier in the summer investigating the climate hiatus. He and co-author Grant Foster concluded, through statistical tests, that it “not only failed to establish a trend change with statistical significance, it failed by a wide margin.”

And their paper was published around the same time as another study, conducted by researchers from the University College Dublin and the Potsdam Institute for Climate Impact Research, which came to similar conclusions.

Multiple studies, one conclusion. It’s telling that so many studies have emerged in such a short time, each using different statistical methods but all drawing similar conclusions, Abraham says, adding that “any argument that global warming stopped 18 or 20 years ago is just hogwash.”

The question that remains is how the idea of a warming pause became so deeply entrenched. Part of the original confusion may stem from the fact that, although there has been no statistically significant climate pause, a climate fluctuation probably has been occurring — a brief blip on the radar, so to speak, that doesn’t statistically alter the overall long-term warming trend.

“When one looks at the long-term record of global temperature, there are wiggles up and down and there are wiggles year to year and wiggles decade to decade,” says Diffenbaugh, one of the Stanford researchers. “What we’ve seen is not unusual for a noisy up-and-down time series.”

Lewandowsky, the cognitive psychologist, argues that the idea of an actual warming pause probably first “started out in the media and on the Internet as a contrarian sort of meme.” He says this language — the use of the words “pause” or “hiatus” — was later accepted by climate scientists, probably partly out of a sort of societal pressure.

“Climate scientists are constantly under scrutiny at the least, or unfortunately under attack,” Lewandowsky says. Over time, exposure to so much hostility can change the way people think and talk about their own science, he thinks. And in this case, he says, “climate scientists have basically adopted the language that was created by contrarians when they talk about a pause, and by adopting that language climate scientists have actually ultimately changed their interpretation of the data.”

At this point, however, it seems that the warming pause debate may be drawing to a close for several reasons. On the one hand, there’s the mounting evidence that the pause never existed. But even those scientists who accepted the idea of a climate hiatus are starting to express the opinion that the pause is now ending. This view has been spurred by an unusual period of warmth: 2014 was the warmest year on record, and 2015 may be shaping up to top it.

At the very least, it has been one of the climate science’s more instructive controversies. The issue has spurred genuine debate among researchers, inspired philosophical discussions about the propagation of myths and the behavior of climate scientists and has led to the development of novel research methods. And it has shed new light on the value of better understanding climate fluctuations.

“We want to understand why the climate sometimes warms more slowly and sometimes faster than at other times,” Lewandowsky says. “But that research has to be done with awareness, and the awareness has to imply that we’re very careful about the language we use.”

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