“Pause” in global warming was never real, new research proves

The comparisons were made with a variety of statistical techniques to correct for problems in previous work.

Professor Stephan Lewandowsky, from the University of Bristol’s School of Psychological Science, is this paper’s lead author. He said: “We found the impression of a divergence – i.e. a divergence between the rate of actual global warming and the model projections – was caused by various biases in the model interpretation and in the observations. It was unsupported by robust statistics.”

Despite this, the authors point out that by the end of 2017, the ‘pause’ was the subject of more than 200 peer-reviewed scientific articles. Many of these articles do not give any reason for their choice of start year for the ‘pause’, and the range spans 1995 to 2004.

Professor Lewandowsky said: “This broad range may indicate a lack of formal or scientific procedures to establish the onset of the ‘pause’. Moreover, each instance of the presumed onset was not randomly chosen but chosen specifically because of the low subsequent warming. We describe this as selection bias.

“This bias causes a problem. If a period is chosen because of its unusually low trend, this has implications for the interpretation of conventional significance levels (“p-values”) of the trend. Selection of observations based on the same data that is then statistically tested inflates the actual p-value, giving rise to a larger proportion of statistical false positives than the researcher might expect. Very few articles on the ‘pause’ account for or even mention this effect, yet it has profound implications for the interpretation of the statistical results.

“This is important, because some of the biases that affect the datasets and projections were known, or knowable, at the time.”

When the researchers reanalyzed the data, accounting for the selection bias problem, they found no evidence for a divergence between models and observations existed at any time in the last decade.

They also offer some possible explanations why some scientists believed climate warming lagged behind modelled warming.

Co-author Professor Kevin Cowtan, from the University of York, UK, said: “One cause may be a that surface temperature data providers struggle to communicate the limitations of the data to climate scientists. This is difficult because users need to focus their expertise in their own problem areas rather than on the temperature data.

“Additionally, there can be delays of several years in updating surface temperature datasets. It takes time to find a bias, find a solution, and then for a paper to be published before most providers update their datasets.  This process is good for transparency, but it may leave users in the position where they download data with knowable biases and unwittingly draw incorrect conclusions from those data.

Co-author Professor Naomi Oreskes, from Harvard University, USA, added “A final point to consider is why scientists put such emphasis on the ‘pause’ when the evidence for it was so scant. An explanation lies in the constant public and political pressure from climate contrarians. This may have caused scientists to feel the need to explain what was occurring, which led them inadvertently to accept and reinforce the contrarian framework.”

University of Bristol climate scientist Dr. Dann Mitchell, from the School of Geographical Sciences, who was not involved with either study, said: “As climate scientists we often look back at previous bodies of evidence and wonder why certain topics were so prominent in discussion; the so-called climate hiatus being an excellent example of this. Given the fast pace of increasing climate change understanding, the conclusions of this paper will be very relevant for the inevitable future ‘apparent’ climate contradictions that emerge over time.”

— Read more in Stephan Lewandowsky et al., “The ‘pause’ in global warming in historical context: (II). Comparing models to observations,” Environmental Research Letters (19 December 2018) (DOI: 10.1088/1748-9326/aaf372); and James S Risbey et al., “A fluctuation in surface temperature in historical context: reassessment and retrospective on the evidence,” Environmental Research Letters (19 December 2018) (DOI: 10.1088/1748-9326/aaf342)