Is Global Warming Causing Aircraft Turbulence to Increase?

 After the turbulence encounter by a Singapore Airlines aircraft,  there has been a slew of articles claiming that severe turbulence incidents are rapidly increasing due to global warming.  The articles below in the Seattle Times and on the BBC are just two of hundreds of such stories

Several stories have been quite specific stating that turbulence has increased 55% in 50 years, based on a study by investigators at the University of Reading.

Is any of this true?   As I will describe below, there are substantial problems with these apocalyptic claims of radically more bumpy skies.

But before we discuss these turbulence claims, keep in mind that aircraft turbulence can be caused by several different mechanisms, and not all of them are related to changes in climate.

Much of the turbulence is due to convection associated with big cumulus clouds, like cumulonimbus, which produce thunderstorms. The turbulence on the Singapore Airlines flight was associated with tropical convection.

Other turbulence is associated with high-amplitude atmospheric waves associated with mountains (see figure).  Such turbulence is often observed downstream of major mountain barriers such as the Rockies and the Sierra Nevada.

Source: Whiteman (2000)

There is also turbulence associated with vertical wind shear:  if wind speed changes rapidly with height, the atmosphere can break down into turbulence eddies.  This kind of turbulence often occurs without clouds and thus is often called clear air turbulence and is generally the least severe of the major turbulence mechanisms.  Finally, it is possible to get turbulence near the surface, called mechanical turbulence, as air passes around objects.

How frequent are the different sources of turbulence?  The best records and observations are found in the United States and the contributions of various types of turbulence by month are shown below (from an FAA document) for 2009-2018.  

Convective turbulence is number one and dominates in spring and summer. Clear air or shear turbulence is number two and is most dominant during the cool season.

Is turbulence increasing rapidly over time as suggested by the media headlines?  

FAA records don't seem to suggest this.  Below is a graph showing the percentage of aircraft incidents/accidents caused by turbulence (dark line).   No increasing trend since the late 1980s!

So where are all these dramatic claims for increasing turbulence with global warming coming from?  

Answer:  from a handful of papers from one group at the University of Reading.  The key paper is here.

In this paper, they DON'T USE ANY TURBULENCE OBSERVATIONS.  Rather, they use analysis of weather data on a grid (called a reanalysis) and use a theoretical model that they suggest should indicate shear-forced turbulence.   But they really don't have the data to back this up or to demonstrate their approach is reliable.  

Even more problematic, the 55% increase in turbulence they cite is NOT for the entire world but just for one region in the North Atlantic.    If you read the actual paper (I have), there are all kinds of qualifiers that never made it into media stories.

To gain some perspective, their 55% increase in this one location represents an increase from 17.7 to 27.4h PER YEAR. Ten hours more a year is 0.0011 of a year.   For the least severe source of aircraft turbulence (vertical wind shear).

 The media did not tell you that the increase was so minor. Still worried?  And that paper said nothing about the other sources of strong turbulence, which in total are more important than the shear-induced, clear-air turbulence they considered.

And there are other technical problems with the above paper that could be a real problem.  For example,  the weather observational system has been greatly expanded and enhanced over the past 40 years, allowing observations to far better define atmospheric structures that can produce turbulence.  Massive additions of satellite data and aircraft observations have occurred during the period in which the Reading group claimed the threat was increasing the most.  Was the threat really increasing or was our ability to define strong wind shear layers getting better.  They didn't say.

In summary, I believe there are no reliable studies that indicate a substantial increase in aircraft turbulence due to changing climate conditions.  The media is needlessly causing folks to worry about flying.