A leading climate scientist says claims that there has been a slowdown or hiatus in the rate at which global warming is happening are not supported by statistical evidence.
The debate over the possible existence of a warming slowdown has sparked fierce scientific controversy over the past two decades. One group, including critics who question the gravity and even the reality of climate change driven by global warming, have insisted that the Earth has been warming more slowly since the end of the last century.
The IPCC appeared some years ago to have accepted the reality of the slowdown, at least as a short-term process, but the respected US agency NOAA, the National Oceanic and Atmospheric Administration, found no evidence for it. The then secretary-general of the World Meteorological Organisation, Michel Jarraud, dismissed the notion of any hiatus or standstill out of hand.
Now, researchers from Germany and the US, who examined global mean surface temperature (GMST) trends in the light of a recent series of three record-breaking years in a row in most data sets, have published the results of their study, which identified two important pitfalls in analysing GMST trends, in Environmental Research Letters.
The study’s lead author, Professor Stefan Rahmstorf, from the Potsdam Institute for Climate Impact Research, Germany, says: “Short-term fluctuations are unavoidable in global temperature. It is inevitable episodes will occur that visually seem to represent a change in the underlying trend.
“Because fluctuation is ever-present, it is important to tell the difference between genuine trend change and appearances that are merely the manifestation of ‘noise’.
“Many scientific publications have discussed an alleged hiatus or slowdown and its possible causes. But few have provided any statistical assessment of whether a significant trend change actually occurred.
“Indeed, discussion of these issues has unfortunately suffered from confusion generated even by some of our climate colleagues, who have fallen victim to common statistical errors.”
The team, which included two statisticians, Niamh Cahill and Grant Foster, examined five separate global temperature data sets – NASA’s GISTEMP, NOAA, HadCRUT4, the revision of HadCRUT by Cowtan and Way, and the Berkeley Earth Surface Temperature. Each dataset uses slightly different methods of calculation.
Although the global temperature data show short periods of greater and smaller warming trends, and even short periods of cooling, the team’s key question was whether or not these are statistically significant in showing a change in the form of a slowdown or acceleration of global warming, or whether they are merely expected fluctuations – or noise – in the data.
“We found that it’s all in the noise,” Foster says. “Neither an earlier slowdown nor a recent acceleration can be identified with any significance in the global temperature record, which is entirely consistent with a steady linear warming trend plus random noise.”
Cahill says: “Therefore, the public discussion of time intervals within the range 1998 to 2014 as somehow unusual or unexpected – indicated by terms like hiatus, pause and slowdown – has no support in rigorous study of the temperature data. Nor does recent talk of a sudden acceleration based on three record-hot years in a row and the exceptional value in 2016.”
Neither an earlier slowdown nor a recent acceleration can be identified with any significance in the global temperature record, which is entirely consistent with a steady linear warming trend plus random noise.
Grant Foster, statistician
Proponents of slowdown
Possible explanations advanced for the slowdown include the effect of small volcanic eruptions, the absorption of extra heat by the oceanic depths and the juxtaposition of two natural ocean cycles.
Refutations cover possible pressure from climate deniers, a statistical illusion and evidence that average temperatures have anyway continued to climb during this century as fast as they were doing 20 or 30 years ago.
Professor Rahmstorf leaves little room for doubt. He says the slowdown’s proponents are simply muddled: “What we’ve found points to much of the public (and scientific) discussion on this topic being misguided.
“It is unfortunate that a major public and media discussion has revolved around an alleged significant and unexpected slowdown in the rate of global warming, for which there never was a statistical basis in the measured global surface temperature data.”
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