Let’s start with the bottom line: you can’t judge climate over a 15-year interval. Climate is almost universally defined as weather averaged over a long period, usually 30 years or more. Therefore, a deviation from trend of 15 years or less, including the so-called global warming hiatus, may be the beginning of a possible change, but it is not proof of a change in trend. This is true whether it’s a few colder years or a few hotter years.
What is the global warming hiatus?
The hiatus is usually defined as 1998-2012 or 1998-2013, an interval during which the rate of global surface warming decreased or temperature actually declined. Various analyses disagree on the amount of decrease.
The temperature curve below is based on data from NASA. The hiatus is circled in red. Other temperature reconstructions differ but would not be greatly different in trend.
There is a great deal of up and down in the curve. It is obvious that there are numerous other short intervals that could be chosen to show declines or accelerations. They are easily seen in the graph below of measured temperatures and several reconstructions from the Intergovernmental Panel on Climate Change’s Fifth Report. Despite these short-term fluctuations, the overall trend has been upward since the early 1900s.
Is the hiatus real?
Some have claimed the hiatus never existed. Arguments to support this include that arctic warming was underrepresented in the temperature data, and that improper corrections to the data made the increase appear less than it really was.
The discussion in the technical literature is arcane. Some papers delve into semantic argument about what constitutes a pause. Others redefine the hiatus interval to drop the anomalously warm starting point or add subsequent warmer years. Some argue that it is not statistically significant.
Despite these mental gymnastics, the preponderance of data shows there was a decrease in the warming rate from 1998 to 2012. A 2016 analysis by Lewandowsky, et al. concludes this period is not statistically significant but shows it to have the slowest warming rate of any 15-year period since 1981.
The fact that it is not statistically significant does not mean the hiatus doesn’t exist. However, it does support the main premise of this article: that it is not meaningful in assessing the long-term trend.
Technical explanations for the hiatus
According to one count, over 200 peer-reviewed papers have been written to explain the hiatus. Explanations include the El Nino/La Nina phenomenon, the Atlantic Multidecadal Oscillation, greater transfer of heat to the ocean, lower sunspot activity, volcanic eruptions and warming occurring in the upper troposphere rather than at the surface. Some of these are short-term effects and some are partitioning of heat between the lower atmosphere and other parts of the Earth system.
To the extent these explanations are correct, they explain why surface warming was lower. However, they do not negate the longer upward trend.
Are the models wrong?
The hiatus has been used to argue climate models are wrong. In a nutshell, this conclusion is wrong. However, it is a more complex question than whether the hiatus is real.
One cannot simply say “the models” are wrong. There have been published projections from numerous different models using numerous different assumptions. In order to validate a model, one must compare a projection based on assumptions that match the actual conditions that have occurred since the projection was made. In practice, this means a projection based on assumed CO2 concentration that is closest to observed.
Zeke Hausfather has done such comparisons for a number of published models, including those of the IPCC reports. His illustration below for the models in the IPCC’s Fourth Assessment Report is typical.
The matches on the order of a few years are not very good and the model overestimates temperature during the hiatus, but the gross fit is pretty good. Other comparisons differ, but are generally similar.
This is a rather simplistic look at a complex subject; however, it is fair to conclude that most models do a reasonable job of forecasting intermediate-term temperatures, on the order of 20 to 40 years.
We should know better
It doesn’t matter whether the global warming hiatus did or didn’t exist because data from 2014 to 2018 show warming has resumed. It should have been known at the time, and should be known in the future, that a few years deviation from a longer-term trend does not prove the trend has ended. It is an indication of the emotion involved in the subject of climate change that the “hiatus” generated so much controversy.
We all want to know the answers to questions, and we want the answer right now. But we have to make decisions on the best information available to us at the time. That includes keeping new information in perspective.
Earl J. Ritchie is a retired energy executive and teaches a course on the oil and gas industry at the University of Houston. He has 35 years’ experience in the industry. He started as a geophysicist with Mobil Oil and subsequently worked in a variety of management and technical positions with several independent exploration and production companies. He retired as Vice President and General Manager of the offshore division of EOG Resources in 2007. Prior to his experience in the oil industry, he served at the US Air Force Special Weapons Center, providing geologic and geophysical support to nuclear research activities. Ritchie holds a Bachelor of Science in Geology–Geophysics from the University of New Orleans and a Master of Science in Petroleum Engineering and Construction Management from the University of Houston.