Scientists slam door on the alleged 'pause' in global warming

Today, climate change deniers will resurrect the tired old argument that Earth's global warming stalled sometime at the beginning of this century. 

The evidence for such a slowdown, however, doesn't exist. 

A diverse group of global researchers published two papers in the scientific journal Environmental Research Letters on Tuesday, affirming why such claims of a global warming hiatus are and always were misleading, at best.

"We find there never was any statistical evidence for it," Stefan Rahmstorf, head of Earth System Analysis at the Potsdam Institute for Climate Impact Research and coauthor of the research, said over email.

Image: nasa giss

The planet has gradually warmed over the last century, and beginning around 1980, started warming at an accelerated pace. 

When the supposed hiatus is brought up, it's usually said to have started after 1998 and lasted over a decade. 

Here's what really happened: In 1998, Earth's warming trend got a potent boost from the normal climatic event, El Niño, in which warmer sea surface temperatures over a large swath of the Pacific Ocean can ultimately amplify overall global temperatures (it happened again in 2016). But in the decade after 1998, many years didn't exceed that notably robust year of warming, and the rate of warming didn't rise as much as climate models predicted. 

This led to mostly unfounded notions of a "hiatus." 

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The problem is, if one looks exclusively at select years or just the decade after 1998, one could convince themselves that the greater warming trend had indeed stalled, study coauthor Stephan Lewandowsky, a cognitive scientist at Bristol University who studies human decision-making, said in an interview.

But that's a statistical blunder, which ignores the greater warming trend.

"Every scientist knows you can’t make any claim about the climate based upon a few years of data," Naomi Oreskes, professor of the history of science at Harvard University and a study coauthor, said in an interview. 

Still, climate-contrarians promoted the hiatus. And to address the question, some scientists pursued the question, giving it some — if limited — life. 

"It seemed obvious that this was a contrarian bone that had been thrown to get scientists to chase after it," said Oreskes, who noted that over 200 scientific studies either cited "the pause" or researched it. 

With these newly-published papers, however, international scientists have shown that there's nothing unusual the global warming trend in the decade or so after 1998, and filled in some missing data about warming in the Arctic. 

Image: environmental Research letters/J S Risbey et al

Indeed, the rate of warming between around 1998 to 2010 fluctuated more slowly than some scientific models predicted, but the critical point is that such up and down variations are an expected part of the greater overall trend. The long-term trend, as scientists emphasize, is what matters.

After all, the year-to-year climate is quite variable.

"The study asks whether the recent period was statistically unusual," John Fasullo, who researches climate variability at the National Center for Atmospheric Research, said in an interview.

"The answer to that question would be 'No'," said Fasullo, who had no role in the research. "There are other periods that have behaved this way."

As the years go by, the reality of a continued, long-term warming trend becomes increasingly clear. After all, 17 of the 18 warmest years on record have now occurred since 2001.

"Over the last 15 years or so we have learned a hell of a lot," said Lewandowsky. "If you look at the issue now from today’s perspective using the best available knowledge, there is no evidence for a pause in warming."  

Perhaps the best evidence against the purported hiatus, however, is in the oceans — a place immune to the whims and variability of atmospheric weather.

The vast oceans absorb most of Earth's accumulating heat. This resulting heat expansion in the seas, combined with the melting of the planet's great ice sheets, results in consistent sea level rise — with little fluctuation.

"We knew global warming didn’t stop all along," said Fasullo. "It was obvious. You just look at sea level measurements."

But although the contrarian argument of an early-century warming hiatus is dead in the water, that doesn't mean climate-deniers will stop arguing it.

"Does this mean the contrarians will go away?" asked Harvard's Oreskes. 

"Of course not," she said. "They don’t let facts get in the way of argument."

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