Stanford scientists debunk the notion of a lull in global warming

This view from Beetle Rock in Sequoia National Park shows that trees surrounding the marshy meadow is sucking the marshes dry. Glaciers are melting into mere ice fields. Wildflowers are blooming earlier. This view from Beetle Rock in Sequoia National Park shows that trees surrounding the marshy meadow is sucking the marshes dry. Glaciers are melting into mere ice fields. Wildflowers are blooming earlier. Photo: Tracie Cone, Associated Press Photo: Tracie Cone, Associated Press Image 1 of / 4 Caption Close Stanford scientists debunk the notion of a lull in global warming 1 / 4 Back to Gallery

A suspected 15-year lull in global warming, seized upon by climate-change skeptics as evidence that climate change had ground to a halt, never happened, say Stanford researchers, who blame the costly misunderstanding on faulty statistical methods.

Their findings debunk the idea that global warming stalled or slowed down between 1998 and 2013, despite the fact that numerous climate studies purported otherwise.

“We find compelling evidence that recent claims of a ‘hiatus’ in global warming lack sound scientific basis,” the authors said in the study released Thursday in the journal Climatic Change.

The researchers performed a full analysis of statistics gathered by scientists to measure temperature changes in the Earth, breaking down and testing four core claims behind what was interpreted as a global warming lull. The claims were similar but technically different, that:

•The average rate of global warming had slowed down.

•The average global temperature had not changed.

•There had been a change in the variation of year-to-year temperatures around the globe.

•The steady increase in global temperatures, first reported in 1880, had stopped.

The group started its research after the hiatus was a major focus of the 2013 climate change assessment by the U.N. Intergovernmental Panel on Climate Change.

“We started to hear a lot about the hiatus in everything, from the news media to peer-reviewed journals,” said Bala Rajaratnam, an assistant professor of statistics and of Earth system science at Stanford.

A new approach

To test whether it was actually occurring, he and his team used a new statistical framework designed specifically to look at processes like global temperature fluctuations. Their intent was to examine temperature data and the statistical tools scientists had used previously to reach their conclusions about a lull or hiatus.

One by one, the researchers found all four claims had no statistical backing. They found that in many cases, those scientists had used “faulty statistical methods.”

“It doesn’t mean those studies are wrong,” said Noah Diffenbaugh, a climate scientist in the university’s School of Earth, Energy & Environmental Sciences and a co-author of the study. “They have generated a lot of important understanding that look at the processes of how global temperatures vary over a long-term period.”

And, he said, the new study should not be used to question a body of scientific work that shows climate change is occurring.

“Our analysis shows that there hasn’t been any slowdown or pause,” Diffenbaugh said. “Rather, it showed that global temperature is a noisy system that goes up and down, but that the long-term trend of global warming persists.”

He and his colleagues hope their findings go a long way toward restoring confidence in the basic science and climate computer models that form the foundation for future climate change.

Their study is just the latest in a number of findings that have cast doubt on the existence of the global warming lull.

Skewed by ocean buoys

For example, a study led by Thomas Karl, a director at the National Oceanic and Atmospheric Administration, in June showed that ocean buoys that read cooler temperatures than their counterparts on ships accounted for false signs of the hiatus. The researchers for that team corrected their data, adjusting for the buoys’ cooler temperature readings, and subsequently removed all signs of the warming hiatus.

Anthony Arguez, a climate scientist at NOAA and a co-author of that June paper, said the new Stanford study looks at global warming in a different, but very robust way.

“Based on the current state of the science, I would consider this to be a significant step forward mostly because I share the authors’ bewilderment that the scientific community has made statements about a so-called hiatus without more rigorous statistical considerations,” Arguez said in an e-mail.

Rajaratnam wants the studies to move the debate beyond a belief in a climate change lull. Researchers, he said, have spent countless dollars and performed numerous studies to investigate potential causes of the hiatus.

Instead, he said, funding should be used directly for dealing with global warming.

Allocating resources

“A lot of money has been poured into global warming mitigation efforts,” he said. “The question is, because there was this purported slowdown, are we spending these funds the best way possible? Did we allocate our resources as best as possible because it didn’t seem as severe as it was at the time?”

The release of the Stanford study coincided with a monthly climate update from NOAA, which showed August as the warmest month on record for overall land and ocean temperatures.

Kevin Schultz is a San Francisco Chronicle staff writer. E-mail: kschultz@sfchronicle.com Twitter: @KevinEdSchultz