Global temperature hiatus claims ‘based on statistical errors’

25 Apr 2017

Claims of a ‘slowdown’ or ‘hiatus’ in rising global temperatures are not supported by an in-depth analysis of statistical evidence, a new study has shown.

The study, conducted by researchers from Germany and the USA, 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.

Their results, which identified two important pitfalls in analysing GMST trends, are published today in the journal Environmental Research Letters.

Lead author Professor Stefan Rahmstorf, from the Potsdam Institute for Climate Impact Research, Germany, said: “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 – the NASA GISTEMP, NOAA, HadCRUT4, the revision of HadCRUT by Cowtan and Way, and the Berkeley Earth Surface Temperature.

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. 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,” Mr Foster said

Ms Cahill said: “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.”

“What we’ve found points to much of the public (and scientific) discussion on this topic being misguided,” said Professor Rahmstorf.

“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.”