In a blind test, economists reject the notion of a global warming pause

Oh how resilient myths can be, even in the face of facts. This past week saw the publication of the third strong refutation of the myth that global warming had somehow stopped a decade or two ago. You would think that with 2014 the hottest year on record and 2015 almost certain to exceed that, and 2016 to potentially set yet another heat record, people would use common sense to conclude that global warming continues. You’d also think with ocean heating breaking records (as discussed here) and loss of ice around the world, any lingering doubts would be put to rest. But alas, for some reason, even more proof is needed.

The first paper, which I covered here looked at the actual temperature trends and found no statistically significant reduction in the rate of warming. The latest paper, just published in the Bulletin of the American Meteorological Society by Stephan LewandowskyJames Risbey, and Naomi Oreskes, looks at the evolution of the terms “pause” and “hiatus.” The authors find that over the past decade or so, there has been a lot of interest in both the scientific community (as judged by papers covering the topic) and by the general public (as determined by web-search statistics). 

In particular, the authors found distinct increases in web searches related to the so-called pause just prior to two major climate-change meetings. The article then asks two questions. First, has there been a pause? Second, why has there been such an intense interest in this so-called event?

The authors show that there is no unique pause in the data. They also discuss biases in the measurements themselves which suggested a slowing in warming that actually did not occur once the data were de-biased. Finally, they reported on recent work that displayed a common error when people compare climate models to measurements (climate models report surface air temperatures while observations use a mixture of air and sea surface temperatures). With this as a backdrop, the authors take a step back and ask some seemingly basic questions. 

First, what is meant by a “pause”? According to its commonly defined meaning, a “pause” is the interruption or suspension of a process. With this context, the global warming “pause” means exactly what the contrarians intend it to mean, a halt to global warming, at least for some time. By this definition, the so-called pause is seen to be meaningless because warming has continued apace, particular by the near linear increase in ocean heat content. The data clearly shows no “pause.” However, the authors restricted themselves to surface temperatures and asked if the pause appeared there. 

The authors looked at temperatures since 1970 which they defined as long-term warming. They separately focused on15-year trends which they termed “fluctuations” because they represent short term fluctuations of temperatures about a long-term trend. They find clear fluctuations in the temperature trend, for instance the trend centered around 1999 was larger than the trend centered around 2005. By definition there must be certain time periods that are faster than the long-term average and certain time periods that are smaller than the long-term average. That is the meaning of an average. The real question is, can claims of a “pause” be distinguished from these fluctuations? 

The authors looked at all possible 15-year trends in global mean surface temperatures. They first find that every single 15-year period showed warming (all four major datasets). The authors also show that the most recent 15-year trend isn’t very different from other 15-year trends throughout the entire period. To quote from the paper,

For a ‘pause’ to be distinctive, it must deviate below the longer-term trend more than previous periods deviated above the longer-term trend – otherwise, it can be considered just a fluctuation like others observed in the past. 

While this sounds compelling so far, the authors went even further. They subjected the data to a blind expert test. They evaluated people’s forecasting judgement because it reveals human perception of a particular set of data. The authors asked a group of economists (each with a Masters or PhD degree in economics or an allied discipline) to evaluate the trend in global temperatures without awareness of the source of the data. 

The experts were told that the data referred to agricultural output and were asked questions about whether the agricultural output had “stopped”. In fact, the authors took exact statements from a climate contrarian, except they replaced words associated with global warming with statements associated with agricultural productivity.

In this blind test, the experts strongly rejected the agricultural “pause” conclusion. In fact, they found mention of a pause “to be misleading and ill-informed”. The experts were divided about whether the “pause” statement was also fraudulent. What is particularly convincing is that a blind test like this, which removes the effects of personal biases or preconceived opinions, is the gold standard for many research areas.

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Posted by John Abraham on Friday, 18 September, 2015

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