December 18, 2018 – The reality of ongoing climate warming might seem plainly obvious today, after the four warmest years on record and a summer of weather extremes in the whole northern hemisphere. A few years back however, some media and some experts were entangled in debates about an alleged pause in global warming – even though there never has been statistical evidence of any “hiatus”, as new research now confirms. In two recent studies, a group of international scientists joined forces to thoroughly disentangle any possible “hiatus” confusion, affirming that there was no evidence for a significant pause or even slowdown of global warming in the first place.
“Claims of a presumed slowdown or pause in global warming during the first decade of the 21st century and an alleged divergence between projections from climate models and observations have attracted considerable research attention, even though the Earth’s climate has long been known to fluctuate on a range of temporal scales,” says James S. Risbey from CSIRO in Australia, lead author of one of the new studies. “However, our findings show there is little or no statistical evidence for a pause in global warming. Neither current nor historical data support it.”
“The alleged pause in global warming was at no time statistically conspicuous or significant, but fully in line with the usual fluctuations”, explains Stefan Rahmstorf from the Potsdam Institute for Climate Impact Research, a co-author to both studies. “The results of our rigorous investigation in both studies are as simple as unambiguous: There was no pause in global warming. And global warming did not fall short of what climate models predicted. Warming continued as predicted, together with the normal short-term variability. There has been no unusual slowing of warming, as our comprehensive data analysis shows.”
There was no pause in global warming
Published in Environmental Research Letters, the first new study analyzes variations in global surface temperature in historical context, while the second compares model projections to observations. They scrutinized all available global temperature data sets in all available earlier and current versions and for all alleged time periods of a “hiatus”, looking for statistical significance. In no data set and for no time period could a significant pause or slowing of global warming be detected, nor any discrepancy to climate models.
Statements claiming the contrary were based on premature conclusions, partly without considering statistics at all, partly because statistical analysis were faulty.
A common problem for instance was the so-called selection bias. Simple significance tests generally only apply to randomly drawn samples. But when a particular time interval is chosen out of many possibilities specifically because of its small trend, then this is not a random sample. “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,” explains Stephan Lewandowsky from University of Bristol in the UK, lead author of the second study.
Reduced momentum for action to prevent climate change
One reason for the attention that the alleged “global warming pause” received in the public may have been that interest groups used this idea to argue against the urgency of ambitious climate policies to reduce CO2-emissions from the burning of fossil fuels. This in turn may have contributed to delays in action to halt global warming, the scientists argue.
“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”, adds Naomi Oreskes from Harvard University in the USA and co-author of the second study. “This may have caused scientists to take positions they would not have done without such opposition.”
James S. Risbey, Stephan Lewandowsky, Kevin Cowtan, Naomi Oreskes, Stefan Rahmstorf, Ari Jokimäki, Grant Foster: A fluctuation in surface temperature in historical context: reassessment and retrospective on the evidence. Environmental Research Letters. [DOI: 10.1088/1748-9326/aaf342]
Stephan Lewandowsky, Kevin Cowtan, James S. Risbey, Michael E. Mann, Byron A. Steinman, Naomi Oreskes, Stefan Rahmstorf: The ‘pause’ in global warming in historical context: Comparing models to observations. Environmental Research Letters. [DOI: 10.1088/1748-9326/aaf372]