Global Warming 'Hiatus' Never Happened, Scientists Say

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.

The study, titled "Debunking the climate hiatus" and published online in the journal Climatic Change, is 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," said study lead author Bala Rajaratnam, an assistant professor of statistics and of Earth system science.

The finding calls into question the idea that global warming "stalled" or "paused" during the period between 1998 and 2013.  Reconciling 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 Stanford collaborators have shown that the hiatus never happened.

"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 a co-author of the study.

Faulty ocean buoys
The Stanford group's findings are the latest in a growing series of papers to cast doubt on the existence of a hiatus.  Another study, led by Thomas Karl, the director of the National Centers for Environmental Information of the National Oceanic and Atmospheric Administration (NOAA) and published recently in the journal Science, found that many of the ocean buoys used to measure sea surface temperatures during the past couple of decades gave cooler readings than measurements gathered from ships.  The NOAA group suggested that by correcting the buoy measurements, the hiatus signal disappears.

While the Stanford group also concluded that there has not been a hiatus, one important distinction of their work is that they did so using both the older, uncorrected temperature measurements as well as the newer, corrected measurements from the NOAA group.

"By using both datasets, nobody can claim that we made up a new statistical technique in order to get a certain result," said Rajaratnam, who is also a fellow at the Stanford Woods Institute for the Environment.  "We saw that there was a debate in the scientific community about the global warming hiatus, and we realized that the assumptions of the classical statistical tools being used were not appropriate and thus could not give reliable answers."

More importantly, the Stanford group's technique does not rely on strong assumptions to work. "If one makes strong assumptions and they are not correct, the validity of the conclusion is called into question," Rajaratnam said.

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