Recent papers have begun to chip away at the idea of the slowdown. Now, a new analysis in the journal Nature brings together many of those arguments to show that the hiatus may not have been quite what it seemed.
“A combination of changes in forcing, uptake of heat by the oceans, natural variability and incomplete observational coverage reconciles models and data,” the researchers from the Institute for Atmospheric and Climate Science in Switzerland wrote. “Combined with stronger recent warming trends in newer datasets, we are now more confident than ever that human influence is dominant in long-term warming.”
Below, we sort through a few of the issues with the hiatus — and the lessons scientists learned from it.
What was the hiatus, anyway?
That’s a good question. Part of the problem is that there doesn’t seem to be an agreed-upon definition. The hiatus has been defined in three main ways in the scientific literature:
A period in which there is no significant positive trend in global mean surface temperatures (when it’s essentially flat)
A shorter-term slowdown in rising temperatures compared to the preceding long-term warming trend
The rate of increase in temperatures is lower than scientific models predicted
So whether you call it the ‘hiatus’ or the ‘pause,’ both are arguably misnomers, depending on which meaning is used. After all, in the latter two definitions, temperatures are generally still rising — just not as much as expected. And depending on which definition you use, different models will be more accurate than others.
“All three of those are very different arguments …. so it often makes it fairly confusing to talk about this hiatus discussion,” said Zeke Hausfather, a climate scientist at Berkeley Earth and PhD student at UC Berkeley who was not involved in the paper.
“The only sort of ‘true’ definition of a hiatus would be if global warming actually stopped,” he added, “and there’s no evidence in any operational datasets today that that happened.”
Regardless of the definition, to whatever the extent the global warming slowdown existed, it’s definitely over now. Independent analyses by NASA and NOAA show that 2016 was the hottest year on record, marking the third year of record-breaking heat in a row. The causes remain linked to human-produced greenhouse gas emissions.
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So the hiatus is over? Then why do scientists care?
Scientists had to care because everyone else seemed to. The hiatus, which is marked from 1998 to 2012, was generally treated by climate scientists as a reflection of short-term variations that didn’t affect long-term trends.
“In leaked drafts of the Intergovernmental Panel on Climate Change (IPCC) Working Group I (WG1) Fifth Assessment Report (AR5), the global warming hiatus was considered to be consistent with natural variability, and hence not in need of a detailed explanation,” the study authors wrote. “At the time of the first draft, there was almost no literature on the hiatus to be assessed anyway. Scientists knew from observations and models that global temperatures fluctuate on timescales of years to decades.”
But the deviation from expectations was seized upon by the media as well as global warming skeptics, the study authors noted, turning it into a political football around the time that major policy decisions about how to deal with climate change were under discussion.
“The interest of the media and public grew, and groups with particular interests used the case to question the trust in both climate science and the use of climate models,” the authors wrote.
(The Times has reported on fossil fuel companies’ support of climate skepticism.)
Those short-term trends may be considered relatively minor fluctuations — but they matter on human timescales, and they’re still poorly understood.
“Now, in 2017 … after a wave of scientific publications and public debate, and with GMSTs [global mean surface air temperatures] setting new records again, it is time to take stock of what can be learned from the hiatus,” the authors wrote.