Global warming ''hiatus'' was never real

MELBOURNE

There has never been a statistically significant 'pause' in global warming, scientists say.

Many studies over the past decade have claimed to find a pause or slowdown in global warming and have typically posited this as evidence that is inconsistent with our understanding of global warming.

The study, published in the journal Environmental Research Letters, looked at how the 'pause' had been defined, the time intervals used to characterise it, and the methods used to assess it.

The study then tested historical and current versions of the Earth's global mean surface temperature (GMST) datasets for pauses, both in terms of no warming trend and a substantially slower trend in GMST.

"Our findings show there is little or no statistical evidence for a 'pause' in GMST rise," said James Risbey from the Commonwealth Scientific and Industrial Research Organisation (CSIRO) in Australia.

"Neither the current data nor the historical data support it. Moreover, updates to the GMST data through the period of 'pause' research have made this conclusion stronger. But, there was never enough evidence to reasonably draw any other conclusion," Risbey said.

He said that "global warming did not pause, but we need to understand how and why scientists came to believe it had, to avoid future episodes like this."          

The climate-research community's acceptance of a 'pause' in global warming caused confusion for the public and policy system about the pace and urgency of climate change, researchers said.

That confusion in turn might have contributed to reduced impetus for action to prevent greenhouse climate change, they said.

"The full costs of that are unknowable, but the risks are substantial. There are lessons here for the science, and for the future," Risbey said.

The group's companion study looks at the alleged mismatch between the rate of global warming in observations and climate models.

The team carried out a systematic comparison between temperatures and projections, using historical GMST products and historical versions of model projections from the times when claims of a divergence between observations and modelling were made.

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

"We found the impression of 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," said Professor Stephan Lewandowsky from the University of Bristol in the UK.

Despite this, the researchers 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.

"This broad range may indicate a lack of formal or scientific procedures to establish the onset of the 'pause'," Lewandowsky said.

"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," he said. — PTI