4 may 2017

Another study disputes uncertainty over global warming 'pause'

Scott Waldman, E&E News reporter, Thursday, May 4, 2017

A growing body of evidence has diminished a favorite talking point of Washington politicians who reject mainstream climate science.

For conservatives who oppose environmental regulations, the appearance of a "pause" in global warming between 1998 and 2012 is evidence that the understanding of climate science is limited and that the U.S. should not restrict its fossil fuel industry. However, a new analysis of dozens of peer-reviewed scientific studies shows that confusion around the public's understanding of the hiatus in global temperatures was really just because researchers were using different data sets and different definitions of a pause.

What's more, the current rate of the planet's warming, which has broken high temperatures for the last three years, erases any notion that there is confusion around our understanding that increased levels of atmospheric carbon dioxide are rapidly warming the planet, said Iselin Medhaug, an author of the analysis and researcher at the Institute for Atmospheric and Climate Science in Zurich, Switzerland.

She said some of the previous research on the pause seemed to contradict itself, with some scientists claiming there was a pause and others disproving it. She found the discrepancy could largely be attributed to three factors: Scientists used different data sets, used different definitions of what constituted a pause and tracked a different time period. She found that, when the full range of factors was considered, including heat uptake by oceans, natural variability and inconsistencies in the climate models, the evidence all pointed in the same direction, which is that the warming trends observed over decades are continuing apace.

"It's not controversial at all that people came to different conclusions," she said.

"Having a period of warming that is less than the global warming that we saw, that's also kind of arbitrary because there will always be years that are warmer than the mean and there will be years that will be colder than the mean. The temperature will not always increase every year, it varies from year to year."

Any notion of a meaningful pause in global warming is further eradicated by the observed temperatures of the last three years, which each broke records for the warmest in recorded history, she said.

'No evidence of a pause at all'

The revelation builds on previous research that has shown a pause may not have even happened.

In 2015, scientists from the National Oceanic and Atmospheric Administration found that the slowdown in global warming never happened and published their results in the journal Science to much controversy. Earlier this year, a study published in the journal Science Advances confirmed that the NOAA scientists were correct and there never was a global warming pause. Instead, researchers found ocean temperatures were rising 0.22 degree Fahrenheit per decade since 2000, which was consistent with the rate of warming dating back to 1970.

For years, conservative lawmakers who reject basic climate science, which holds that humans are warming the planet at the unprecedented rate through the burning of fossil fuel, have highlighted the pause as evidence that our understanding of climate change was limited. On the 2016 campaign trail, Sen. Ted Cruz (R-Texas) repeatedly claimed that there was "no significant warming whatsoever for the last 18 years," by focusing on just one data set while ignoring others that contradicted his claim. Since 2015, when the NOAA study debunking the pause was first published, House Science Chairman Lamar Smith (R-Texas) has accused federal scientists of fraud and has been investigating their findings for more than a year.

In any event, looking at such a relatively limited time period as a decade to understand long-term climate trends was always problematic, said Zeke Hausfather, author of the Science Advances study and a researcher at the University of California, Berkeley's Energy and Resources Group.

The pause started in 1998, during one of the biggest El Niño events in recorded history, which drives up temperatures. Starting at that unusually hot year could falsely create the impression that the successive years, where temperature rose more slowly, were not part of the overall warming that has been observed for decades, he said. Because 2016, the hottest year on record, was so warm, it's likely the next couple of years will be more moderate, but that, too, does not indicate a pause in warming, he said.

"Certainly, when you add the last four years to it, there's no evidence of a pause at all," Hausfather said. "And 2017 is shaping up to be probably the second warmest on record, though there is a non-negligible chance it may even beat 2016 as the warmest. There is no evidence that after the big El Niño event of last year, global warming is somehow slowing down again or going back to a lower state."

He said one issue is that researchers were focused too much on observations as the full picture of warming when the reality of measuring global warming is much broader and needs modeling to test hypotheses. The models show too much warming while the observations show too little warming, but combining both techniques accounts for the deficiencies of using just one. Since the first studies of the pause were conducted, more research points to a sustained pattern of warming, she said.

"When people have been bashing at your research for years and years, and they cannot falsify what you've done or they cannot falsify your claims, then that's when you say this is strong science," Hausfather said. "When the paper comes out, that's when you get all of the attention, but if it's still robust 20 years later, then that is robust science."

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