This story has been updated.
It’s probably the biggest debate in the climate science arena of the last half decade.
Starting in at least early 2013, a number of scientific and public commentators have suggested that the rate of recent global warming has slowed or even stopped. The phenomena has been variably termed a “pause,” a “slowdown,” and a “hiatus.” Pointing to it is a favorite technique of climate change doubters — recently employed by none other than presidential candidate Ted Cruz — but it certainly doesn’t stop with them.
In its 2013 mega report, no less than the United Nations’ Intergovernmental Panel on Climate Change appeared to validate the idea of a global warming slowdown when it noted that “the rate of warming over the past 15 years [from 1998 to 2012], which begins with a strong El Niño, is smaller than the rate calculated since 1951.” Meanwhile, a bevy of scientific studies have emerged in recent years seeking to explain the slowdown with reference to various modes of natural variability within the climate system, including volcanic eruptions and — probably the most popular account — the temporary burial of heat deep within the vast Pacific ocean.
But as a team of federal scientists report today in the prestigious journal Science, there may not have been any “pause” at all. The researchers from the National Oceanic and Atmospheric Administration’s National Centers for Environmental Information (NCEI) adjusted their data on land and ocean temperatures to address “residual data biases” that affect a variety of measurements, such as those taken by ships over the oceans. And they found that “newly corrected and updated global surface temperature data from NOAA’s NCEI do not support the notion of a global warming ‘hiatus.’”
In other words, maybe it wasn’t about natural wobbles in the climate system at all — but rather, simply about flaws in the data.
“When we took a look at trying to update and improve our dataset, the results of that show that the notion of a slowdown in global warming over the last two decades is no longer valid,” says NCEI director Thomas Karl, who led the research.
Here’s a figure to drive the point home:
The details of the data adjustments quickly get complicated — and will surely be where global warming doubters focus their criticism. But the most important change involves accurately measuring the surface temperatures of the world’s oceans. Karl said the agency has made a “major update” to NOAA’s temperature dataset, which for the oceans relies on measurements from buoys and ship thermometers that measure the temperature of water taken in to cool engines. (For an overview of these data sources, see here). Noting that “buoy data have been proven to be more accurate than ship data,” the new study applies a new “bias correction” to address the difference between them.
This, in addition to the integration of new temperature data from more land-based thermometers around the world, brought the temperature trend from the first 15 years of this century into line with the trend from the second half of the 1900s. “Our new analysis now shows the trend over the period 1950-1999, a time widely agreed as having significant anthropogenic global warming, is 0.113°C [per decade], which is virtually indistinguishable with the trend over the period 2000-2014 (0.116°C [per decade]),” the study reports.
The global warming pause or hiatus narrative has often been helped by the way proponents of this idea conduct their analysis. By starting with 1998, a record hot El Niño year, the warming trend naturally appears to be smaller.
But the new NOAA analysis finds that even if you start with 1998 and track temperatures forward to the present, a trend persists that is not much different from the trend in the second half of the 20th century.
Furthermore, as Karl noted in an interview, even the new analysis may not adequately take into account the rapidly warming Arctic, where many scientists say there is still not enough data coverage to capture accurate temperature trends in the vast but sparsely populated region.
“It’s pretty obvious that there’s missing warming in the Arctic that we’re not including,” says Karl. Because of this, he says, “we think our dataset is still under-representing the total temperature rate of warming.”
Asked about the new research, Gavin Schmidt, who directs the Goddard Institute for Space Studies at NASA — which also keeps a leading temperature dataset — said in an e-mail, “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.” Schmidt said that NASA is currently discussing which aspects of NOAA’s analysis it will “incorporate” in its own dataset.
The new study appeared to almost instantly spark debate. “NOAA fiddles with climate data to erase the 15-year global warming ‘hiatus,'” wrote the Daily Caller, quoting numerous skeptical scientists.
Our own Capital Weather Gang quoted John Christy, a satellite researcher at the University of Alabama, who observed that temperatures in the lower part of the atmosphere as measured by satellite don’t show a matching trend. “If Karl’s work holds up…this will only add to the puzzle of diverging surface and atmospheric temperatures which stands in contrast to model expectations,” he said.
One question raised by the research is whether other global temperature datasets will see similar adjustments. One, kept by the Hadley Center of the UK Met Office, appears to support the global warming “hiatus” narrative — but then, so did NOAA’s dataset up until now.
“Before this update, we were the slowest rate of warming,” said Karl. “And with the update now, we’re the leaders of the pack. So as other people make updates, they may end up adjusting upwards as well.”
Another major issue is what all the published studies seeking to explain the “hiatus,” with respect to natural changes in the climate system that suppressed warming, were actually doing, if there wasn’t actually a hiatus.
Karl’s answer is that these researchers were studying real natural phenomena that did suppress warming — meaning that without such phenomena, the last 15 years might have been a blockbuster warming period.
“Those things won’t persist, and when they’re gone, that means the rate of temperature is free to increase even more than it would have,” he says. “And you can make the case that had those factors not been operating, we might be talking now about why the temperatures have been warming more rapidly.”
By these lights, the work remains valuable. But some are a bit more critical of scientists for seeming to validate the notion of a slowdown, including by publishing such a thick shelf of studies of a phenomenon that, now, NOAA is saying may not exist.
Harvard science historian Naomi Oreskes recently co-authored a paper depicting research on the “hiatus” as a case study in how scientists had allowed a “seepage” of climate skeptic argumentation to affect the formal scientific literature. Of the new NOAA study, she said in an e-mail: “I hope the scientific community will do a bit of soul searching about how they got pulled into this framework, which was clearly a contrarian construction from the start.”
Soul searching may be one response. But with NOAA’s new temperature analysis appearing to render moot a highly contentious multi-year discussion — both scientific and also public — the range of reactions, and emotions, will certainly be much broader.
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