Did global warming 'pause'? Depends how you define 'pause'

After seemingly being put to rest last year, the controversy over a supposed “hiatus” has resurfaced, but this time with a key change in terminology that could temper the pitch of the discussion.

The idea that global warming took a hiatus in the 2000s has stoked political debate in the United States over the validity of climate models and scientists' near-universal consensus that global warming has been man-made. If a pause in global warming occurred even as global industries continued to rely on fossil fuels, then perhaps the changes seen in climate scientists could be attributed to natural variation after all, critics of climate change science have argued.

The debate would seem to have been put to rest last year, after scientists from the National Oceanic and Atmospheric Administration (NOAA) recalculated climate data to correct for known biases and found that global warming had not paused as proponents of the hiatus theory had suggested. Last November, the online science journal Scientific Reports published an article that affirmed the NOAA’s claim.

According to their research, the authors of the article stated that, “Even putting aside possible artifacts in the temperature record, there is no substantive evidence of a 'pause' or 'hiatus' in warming.”

Now, a commentary article penned by several climate scientists and published this month in the journal Nature Climate Change, takes issue with NOAA's conclusion.

“The interpretation [the NOAA group] made was not valid,” said John Fyfe, a climate scientist at Environment and Climate Change Canada. “The slowdown is there, even in this new updated data set.”

However, there is a significant difference in Dr. Fyfe's choice of language and in the language used by global warming skeptics, Nature commentator Jeff Tolferson says.

"Fyfe uses the term 'slowdown' rather than 'hiatus' and stresses that it does not in any way undermine global-warming theory."

Fyfe and his colleagues point out a "mismatch" between climate models predictions and actual observations, but stress that the data does not indicate that global warming reversed or even halted during that time, as has been suggested by the so-called hiatus theory. 

Fyfe said that he and his fellow authors believe that the Pacific Decadal Oscillation (PDO) was to blame for the slowdown since 2000. The PDO goes through positive, neutral, and negative cycles. When the PDO is negative, the Earth cools down, slowing global warming. The PDO was negative in the early 2000s.

Fyfe points out that the models are not perfect. The PDO may not be the best explanation for the slowdown they have identified. But, he says, after record warming years in 2014 and 2015, the slowdown is definitely over.

After climate change skeptics attempted to use the last round of “hiatus” evidence to claim that global warming had stopped, there may be fears that the new study will be seized upon by the same people. The climate change hiatus theory was the subject of congressional hearings led by Rep. Lamar Smith (R) of Texas last year. On Friday, NOAA reported receiving new requests from Representative Smith for all documents referring to "climate" and "temperature."

Penn State University climatologist and study co-author Michael Mann said that such concerns should not deter scientists from pursuing the truth.

“As scientists, we must go where the evidence takes us,” said Mann, “we can’t allow our worries about climate contrarians and how they might seek to misrepresent our work to dictate what we do and do not publish.”