Global warming ‘hiatus’ never happened, Stanford scientists say

New study reveals that evidence for recent pause in global warming lacks sound statistical basis

New York | Heidelberg, 17 September 2015

An apparent lull in the recent rate of global warming that has been widely accepted as fact is actually an artifact arising from faulty statistical methods, Stanford scientists say. Their findings, published in Springer’s journal Climatic Change, provide a comprehensive assessment of the purported slowdown, or hiatus, of global warming. “We translated the various scientific claims and assertions that have been made about the hiatus and tested to see whether they stand up to rigorous statistical scrutiny,” says lead author Bala Rajaratnam, an assistant professor of statistics and Earth system science.

The findings call into question the idea that global warming “stalled” or “paused” during the period between 1998 and 2013. Accounting for the hiatus was a major focus of the 2013 climate change assessment by the Intergovernmental Panel on Climate Change (IPCC).

Using a novel statistical framework that was developed specifically for studying geophysical processes such as global temperature fluctuations, Rajaratnam and his team of collaborators methodically examined not only the temperature data but also the statistical tools scientists were using to analyze the data. A look at the latter revealed that many of the statistical techniques climate scientists were employing were ones developed for other fields such as biology or medicine, and not ideal for studying geophysical processes. “The underlying assumptions of these analyses often weren’t justified,” Rajaratnam said.

“Our results clearly show that, in terms of the statistics of the long-term global temperature data, there never was a hiatus, a pause or a slowdown in global warming,” said Noah Diffenbaugh, a climate scientist in the School of Earth, Energy & Environmental Sciences and co-author of the study.

The scientists say their findings should go a long way toward restoring confidence in the basic science and climate computer models that form the foundation for climate change predictions.

“Global warming is like other noisy systems that fluctuate wildly but still follow a trend,” Diffenbaugh said. “Think of the U.S. stock market: There have been bull markets and bear markets, but overall it has grown a lot over the past century. What is clear from analyzing the long-term data in a rigorous statistical framework is that, even though climate varies from year-to-year and decade-to-decade, global temperature has increased in the long term, and the recent period does not stand out as being abnormal.” 

Reference: Rajaratnam, B. et al (2015). Debunking the climate hiatus, Climatic Change, DOI 10.1007/s10584-015-1495-y