Global Response of Clear-Air Turbulence to Climate Change. Professor Paul D. Williams University of Reading, UK

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1 Global Response of Clear-Air Turbulence to Climate Change Professor Paul D. Williams University of Reading, UK

2 Climate change impacts on aviation Rising sea levels and storm surges threaten coastal airports Shifting wind patterns modify optimal flight routes and fuel consumption Warmer air imposes take-off weight restrictions Stronger jet-stream wind shears increase clear-air turbulence More extreme weather causes disruptions and delays Puempel & Williams (2016) ICAO Environmental Report

3 The acceleration of the jet stream Pacific Atlantic Jet-stream changes driven by CO 2 in IPCC climate simulations Stronger eastward winds & windshears at flight cruising altitudes u z T y C20 (10 m/s contours) C21 C20 (0.25 m/s contours) Delcambre et al. (2013)

4 Changing LHR JFK flight times Likelihood of taking under 5 h 20 min more than doubles from 3.5% to 8.1% Likelihood of taking over 7 h 00 min nearly doubles from 8.6% to 15.3% Williams (2016)

5 Changing LHR JFK flight times Have these changes already begun? The North Atlantic jet stream wind speeds reached 250 mph on 8-12 January 2015 An eastbound JFK LHR crossing took only 5 h 16 min, which is the current non-concorde record Westbound LHR JFK crossings took so long that two flights had to make unscheduled refuelling stops in Maine Extrapolation to all transatlantic traffic (600 crossings per day) suggests that aircraft will collectively be: airborne for an extra 2,000 hours each year burning an extra 7.2 million gallons of jet fuel at a cost of $22 million emitting an extra 70 million kg of CO 2 into the atmosphere, equating to 7,100 British homes Williams (2016)

6 Turbulence Aircraft encounter moderate turbulence (>0.5g) 65,000 times and severe turbulence (>1.0g) 5,500 times annually in the USA. These encounters: cause about 40 fatalities and 100s of serious injuries cause structural damage to planes cause flight diversions and delays cost airlines $150m $500m Statistics from: Ralph et al. (1997)

7 FAA (2006) Turbulence injury trends Number of serious injuries (including fatalities) caused by turbulence, per million flight departures (US carriers)

8 Tvaryanas (2003) Turbulence injury trends Number of flight attendant injuries caused by turbulence, per million hours flown (US carriers)

9 Clear-air turbulence (CAT) CAT occurs in clear skies at cruise altitudes, above weather systems like clouds and storms CAT is difficult to avoid, because it cannot be seen by pilots or detected by satellites or on-board radar Aircraft spend about 3% of their cruise time in light CAT (Watkins & Browning 1973) and about 1% of their cruise time in moderate CAT (Sharman et al. 2006) CAT is forecast operationally by computing various diagnostic measures from weather prediction models, e.g. those due to Colson & Panofsky (1965), Brown (1973), and Ellrod & Knapp (1992) World Area Forecast Centres (in London and Washington) use such diagnostics to issue global CAT forecasts every six hours (Gill 2012) These forecasts show significant skill when evaluated against pilot reports of turbulence

10 What causes CAT? height Stratification inhibits CAT Wind shear encourages CAT CAT occurs if the wind shear is stronger than the stratification. Climate change is strengthening the wind shear

11 Is CAT increasing? PRE-INDUSTRIAL DOUBLED CO2 TI1 u z u x v y 2 v x u y 2

12 Is CAT increasing? 100% Number of turbulence diagnostics indicating an increased median 0% Williams & Joshi (2013)

13 Williams (2017) 149% (36-188%) 59% (43-68%) 75% (39-96%) 94% (37-118%) 127% (30-170%) Slight strain against seat belts; unsecured objects may be displaced slightly; food service may be conducted with little difficulty walking Definite strain against seat belts; unsecured objects are dislodged; food service and walking are difficult Occupants are forced violently against seat belts; unsecured objects are tossed about; food service and walking are impossible

14 Is CAT increasing? Storer, Williams & Joshi (2017)

15 Is CAT increasing? Storer, Williams & Joshi (2017)

16 Summary A basket of CAT measures diagnosed from climate simulations is significantly modified if the CO 2 is increased At cruising altitudes on transatlantic flights in winter, the diagnostics show a 59% / 94% / 149% increase in the prevalence of light/moderate/severe CAT, with similar results on other flight routes and in other seasons We conclude that, all other things being equal, climate change will lead to bumpier flights later this century Flight paths may become more convoluted to avoid stronger and more frequent patches of turbulence, in which case journey times will lengthen and jet fuel consumption will increase

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