|Credit: Gary Larson, "The Far Side"|
Fasten your seat belts. If your peanuts scatter and your coffee spills and heavy baggage threatens to burst from the overhead compartment while you jet across "the pond" between the US and Britain, you now have permission to blame anthropogenic climate change. According to simulations of a doubled carbon dioxide concentration carried out by Paul D. Williams and Manoj M. Joshi, of the Universities of Reading and East Anglia, respectively, and published in the April 8, 2013 issue of Nature Climate Change, average turbulence along transatlantic is projected to increase 10 - 40 percent, and the incidence of moderate to severe turbulence 40 - 170 percent. They write, "Our results suggest that climate change will lead to bumpier transatlantic flights by the middle of this century. Journey times may lengthen and fuel consumption and emissions may increase."
Unlike the turbulence associated with storms, landforms, and aircraft wakes, clear air turbulence (CAT) is notoriously difficult to detect in advance, offering little in the way of a radar signature, and little warning for pilots. Injuries are rare, and affect almost exclusively unbuckled passengers and crew.
Over the North Atlantic, the conditions that favor clear air turbulence are the eddies that form along the edges of a the jet stream. Williams and Joshi predict that the jet stream will become stronger, and move northward to affect more of the transatlantic air traffic. Their predictions are entirely model-based. Some historical measurements of the conditions that favor turbulence over the North Atlantic since 1980, during which time carbon dioxide has increased about 15%, have increased over the North Atlantic, but decreased over the Pacific. Historical conditions that favor turbulence do not correlate as well with carbon dioxide as with the North Atlantic Oscillation, according to a 2007 study by Jaeger and Sprenger in JGR. They concluded, "The interannual variability of CAT is significant as indicated by the CAT indicators and can be correlated with the two phases of the North Atlantic Oscillation as well as with the Pacific/North American flow pattern. The interannual variations of the TI and PV patterns are consistent with the variation of the jet position associated with the NAO, whereas the Ri and, especially, the N2 patterns are not markedly influenced by the jet stream position. During positive phases of the NAO, generally larger turbulence frequencies occur, which might be due to stronger jets, and associated with that, more frequent instabilities."
Williams and Joshi make no mention of the North Atlantic Oscillation in their report. It is a part of our climate, but not a part of their model simulation. Perhaps the rise in carbon dioxide will cause more turbulence for flight simulators than for actual flights. It is all part of the rush to supply the IPCC AR5 with alarmist fodder prior to the March 15, 2013 publication acceptance deadline. Expect many more reports of the hazards of carbon dioxide in the coming months.