Global warming hasn't slowed down — we were just measuring it wrong, according to a new study published today in the journal Science.
In 2013, a landmark UN report on climate change described a strange contradiction: greenhouse gases were still on the rise, but Earth's surface temperatures hadn't increased as fast as expected. Researchers have spent years trying to explain this phenomenon, dubbed a "global warming hiatus," while skeptics have seized upon it as evidence that warnings about human-driven climate change were overblown. But a new report suggests that there was no significant decrease in global warming over the last 15 years. In short, the hiatus never happened.
This result directly challenges what the UN's Intergovernmental Panel on Climate Change (IPCC) concluded in 2013, when it reported that global temperatures rose at a far lower rate between 1998 and 2012 than from 1951 to 2012 (0.05°C per decade compared to 0.12°C per decade). According to the authors of today's study, however, the IPCC's data was flawed. Using corrected measurements and more recent data, they find that temperatures rose at a rate of 0.106°C per decade between 1998 and 2014, more than twice the rate they reported with older data and without adjusting for biases. That's also on par with the 0.113°C rate they report for 1950 to 1999.
"There was never a hiatus to begin with."
"Our analysis tells us that there was never a hiatus to begin with," says Russell Vose, head of climate science at the National Climatic Data Center at the National Oceanic and Atmospheric Administration (NOAA) and a co-author of today's study.
In its 2013 report, the IPCC acknowledged that 15-year hiatuses are "common" in historical records, suggesting that the apparent decline may be due to natural weather variations. But the evidence was perplexing given the ongoing rise in heat-trapping greenhouse gases, and because it contradicted most computer projections. Several studies have suggested that the increased heat absorbed by deep oceans may be to blame, while others have pointed to volcanic activity, lower solar energy levels, or air pollution.
The NOAA study would appear to undermine these hiatus studies, but Vose says that's not the case. "If those various factors weren't operating during that period of time, we might actually have seen even more warming," he says.
(NOAA's National Centers for Environmental Information)
The results come as surface temperature datasets are expanding to cover more weather observation stations, resulting in more accurate readings. The authors analyzed more than double the amount of data previously compiled by the NOAA, from buoys and commercial ships as well as the land surface. In doing so, they took into account the variations on how ships and buoys collect ocean temperatures as well as the recent increase in land observation stations — factors that the IPCC did not consider.
"The fact that such small changes to the analysis make the difference between a hiatus or not merely underlines how fragile a concept it was in the first place," says Gavin Schmidt, a climate scientist at the Columbia University Earth Institute who was not involved in today's study. For some climate scientists, the notion of a recent hiatus was always tenuous, if not a red herring. As they have pointed out, overall temperatures still rose during the supposed hiatus period, and the 15-year time frame was too short to separate signal from noise.
"This paper will have very significant policy implications."
"I think for scientists, the hiatus was not regarded as something that was fundamentally changing the picture of global warming," says Ronald Prinn, a professor of atmospheric science at MIT, who was not involved in the NOAA study. But he says it could have stronger repercussions at the political level, since climate change skeptics may no longer be able to point to the hiatus to make their case. "I think that this paper will have very significant policy implications," he adds.
But there are still some major gaps that remain to be filled. Experts say the relative lack of temperature data from the Arctic is most glaring, as the authors acknowledge, since that region has warmed much faster than the rest of the globe. Others argue that using 1950 as a baseline year is misleading, since global warming didn't accelerate until the 1970s. But for those who never bought into the hiatus to begin with, today's findings are a welcomed affirmation.
"I hope that this study helps to put this false idea of a hiatus to rest," says Stefan Rahmstorf, a professor of ocean physics at Germany's Potsdam Institute for Climate Impact Research, who was not involved in the study. "It didn't have any merit in the first place."