A new study has just been prepared for an upcoming climate meeting of the US Climate Variability and Predictability Program. This group has an annual summit and this year will have a special science session with papers and presentations devoted to the so-called “hiatus”. The “hiatus” has taken many meanings. In the popular press, it is often used to falsely claim that global warming stopped. As I’ve written many times, global warming has not stopped; the Earth has been continuing to gain energy because of human emissions of greenhouse gases.

In other cases, the “hiatus” refers to a reported slowdown in temperature increases. This too is not seen in the ocean data or in sea level rise. It is only seen in surface temperatures (temperatures of the surface of land and ocean regions).



What my colleague and I wanted to know was, is this slowdown real or not? Specifically, we wanted to know whether it passed mathematical tests for statistical significance. My colleague, who is an expert is statistics, and operates a climate website, downloaded the surface temperature data from NASA and detrended it (removed the long term increase in temperatures). The difference between the red trendline and the black dots is called the residual. We wanted to increase the odds that we would find a “hiatus” by stopping our analysis in 2013 (omitted the hottest year on record, 2014).

NASA temperature anomalies from Foster and Abraham, 2015.

If you plot the residual over this time period, it looks like the image below.

NASA temperature residuals from Foster and Abraham, 2015.

We wanted to know whether there is any trend in these residuals. If the global warming trend had slowed recently, you would expect residuals to become more negative. We found that there was no statistically significant change to the residuals since 1970.

Next, we grouped years together into groups that ranged from 3 years long to 20 years long and searched for changes to the residuals in those groups. Again, we found no significant change. Our findings are best summarized by our concluding paragraphs.

A barrage of statistical tests was applied to global surface temperature time series to search for evidence of any significant departure from linear increase at constant rate since 1970. In every case the evidence not only failed to establish a trend change with statistical significance, it failed by a wide margin. Our results show that the widespread acceptance of the idea of a recent slowdown in the increase of global average surface temperature is not supported by analytical evidence. We suggest two possible contributors to this. First, the natural curiosity of honest scientists strongly motivates them to investigate issues which appear to be meaningful even before such evidence arrives (which is a very good thing). Second, those who deny that man-made global warming is a danger have actively engaged in a public campaign to proclaim not just a slowdown in temperature increase, but a complete halt to global warming. Their efforts have been pervasive, so that in spite of lack of evidence to back up such claims, they have effectively sown the seeds of doubt in the public, the community of journalists, and even elected politicians. An unfortunate habit in public discourse has been to graph only the data since the supposed “pause” began and state only the trend estimate since that moment, in order to avoid having to show that such a practice implicitly models temperature with a “broken” trend like that of Figure 3. Claims based on failing to reveal what happened before a purported trend change, are inevitably misleading. It is certainly possible that some change in the trend has occurred since 1970, and it is very beneficial to look for causes, whether it is present or not. But we suggest that scientists should stop speaking of a slowdown in temperature increase as though it were a known fact, when it simply isn’t.



Our paper is freely available here.