The global warming pause that never was

CSIRO’s Ecos Newsletter shared this important article written by Jenni Metcalfe, that highlights the need for us to be informed about what is really happening in global warming…

In recent years, there has been significant public discussion about a so-called ‘hiatus’ or ‘pause’ in global warming that has supposed to have spanned part of the last one or two decades.  These claims have led scientists to study and report extensively on the phenomenon over the past five years. Even the latest Intergovernmental Panel on Climate Change (IPCC) Assessment Report references a hiatus in global warming.

But researchers examining the scientific literature and data on trends in global mean surface temperatures (GMST) have found there is no meaningful statistical evidence to support the notion of a pause in global warming.

“There was no pause,”

says CSIRO climate researcher Dr James Risbey, co-author of a paper published in Scientific Reports this week.

“The warming trends in GMST over the period of claimed pausing are not uniquely different to any other global warming trends over the period of modern global warming.”

Dr Risbey and his colleagues, Stephan Lewandowsky at the University of Bristol and Naomi Oreskes at Harvard University, firstly looked at the way ‘hiatus’ or ‘pause’ was defined in 44 scientific papers on the issue published between 2009 and 2015.  They found that there was a diversity of views on when the global warming pause was meant to have started (from 1993 to 2003) and how long it lasted for (from 10 to 20 years).  This lack of scientific agreement and criteria for a pause is a concern when statistical analysis aiming to detect short-term trends is sensitive to start and end dates, says lead author, Professor Lewandowsky.

“When we analysed the statistics for short term trends against each paper’s claimed start and end dates for a pause, we found no exceptional differences from the overall trends for global warming,” Professor Lewandowsky adds.

Typically, about 17 years of data are needed to have enough surface temperature data to detect any statistically significant trends in GMST data.

“When we considered 17-year long sets of data, we found significant warming trends throughout the entire period of modern global warming, including those alleged pause periods,” says Dr Risbey.

A graph showing coloured squares indicating warming over the years 1984 to 2014
Observable warming trends in GMST between 1984 and 2014, over periods of 3 to 25 years. The graphic shows warming trends throughout the entire period, including alleged ‘pause’ periods. A black dot in a square is indicative of significant warming and the dotted line marks a period of 17 years, the number of years that needs to be included for a trend to be statistically significant. The circles represent years that have been used in the literature to identify a ‘pause’. Image: Lewandowsky et al. (2015)

 

The researchers tested their conclusions using different statistical assumptions and with different sets of data and gained similar results.  Yet the question remains: why has so much research been directed at this perceived global warming pause? And what has been the effect on public perceptions of the apparent scientific acceptance of a pause?

The power of ‘pause’ rhetoric

Stephan Lewandowsky says that the notion of a pause most likely entered scientific discourse and consideration in part through scientists’ honest attempts to detect and explain it in the terms in which it had been framed for them.

“The perceived scientific acceptance of a pause in global warming is likely to have caused damage to communication and action on climate change.  When scientists talk about a pause, they don’t mean that global warming has stopped. However, some climate change sceptics use the notion of a pause to claim that global warming is due to natural climatic variation rather than human actions.”

The differences between how scientists and the public talk about research has exacerbated the issue.

“Some scientists talk about a pause when referring to the slower than average warming periods,” says Dr Risbey. “But the public may believe the terms pause or hiatus imply global warming is actually stopping, which is not the case.”

The response of GMST to increasing greenhouse gases is not smooth on short time scales and includes some periods of both faster and slower than average warming, noted James Risbey in an earlier paper in Nature.

In this latest research, the team looked at the trends in observable surface temperature data.

“Our focus was exploring the temperature trends in the actual data, as this is what would best tell us if there was a genuine pause in warming,” says Stephan Lewandowsky.

He says the notion of a global warming pause has possibly delayed actions to mitigate climate change: “This may impact negatively on some people, such as those most likely to suffer as a result of rising sea levels and temperatures.”

The authors of the report believe continued talk of a pause could become an ethical matter, or even a point of law if the public are not adequately informed of the full and identifiable risks of global warming.

Naomi Oreskes points out that such concerns are not remote:

“The legal aftermath of the earthquake in L’Aquila, Italy included charges against scientists for their alleged failure to communicate with all due diligence.”

Stephan Lewandowsky concludes: “We must avoid the unsubstantiated use of terms like pause and hiatus when talking about global warming.”


More information

Scientific Reports paper: ‘On the definition and identifiability of the alleged “hiatus” in global warming

Nature paper: ‘Earth science: Free and forced climate variations’