US study finds link between climate and flight times results in higher fuel consumption and emissions
(photo: Honolulu International Airport)
Fri 24 Jul 2015 – The contribution of emissions from commercial aviation to climate change has been widely researched, although uncertainties still remain over the warming impact of aircraft at altitude. But what effect may a warming planet have on air travel itself? Scientists at the Woods Hole Oceanographic Institution (WHOI) in Massachusetts and the University of Wisconsin Madison believe they have found a connection between climate and longer airline flight times that adds significantly to fuel consumption and emissions. In turn, says WHOI’s Kris Karnauskas, who led the study that has just been published in the journal Nature Climate Change, the consequent additional input of CO2 into the atmosphere can feed back and amplify emerging changes in atmospheric circulation. The study looked at every flight between Honolulu and three US West Coast airports over the past 20 years and compared data with flight times and daily wind speeds at cruising altitudes.
The study began after a flight from Honolulu to the US East Coast taken by Hannah Barkley, a doctoral student in the MIT-WHOI Joint Program in Oceanography and subsequent co-author of the study, arrived at its destination well ahead of schedule. Together with Karnauskas, an associate scientist in WHOI’s Geology and Geophysics Department, they discovered the jet stream that day was extra fast.
Through a database maintained by the US Department of Transportation, they downloaded departure and arrival data for four different airlines on every flight between Honolulu and Los Angeles, San Francisco and Seattle over the 20-year period. As upper level winds blow from west to east, the eastbound leg of a roundtrip flight is generally faster than the westbound leg. What they found was that regardless of the carrier, the difference in flight times looked the same.
“Whatever was causing these flights to change their duration was the exact same thing, and it wasn’t part of the airline’s decision-making process,” says Karnauskas, who concludes that climate variability and not just day-to-day weather determines flight times.
When he overlaid the plots of the airlines’ differences in flight times with graphs of wind variability at climatic time scales, Karnauskas found them to be virtually identical, even after smoothing out seasonal differences due to the jet stream being stronger in winter and weaker in summer. The results also pointed towards the influence of the El Niño effect, in which the temperature of the equatorial Pacific Ocean rises and falls and sets off atmospheric waves towards higher latitudes of both hemispheres to change circulation patterns.
Karnauskas, who has studied El Niño extensively, found that just by looking at the state of the tropical Pacific Ocean, he could predict what the difference in the airlines’ flight times had been. “We’re talking about anomalies happening down at the equator that are affecting the atmosphere in such a spatially broad way that it’s probably influencing flights all around the world,” he says.
An important finding of the study is that the difference in flight times between eastbound and westbound flights on any given route did not cancel each other out, although minor.
“The net additional flying time for a pair of eastbound and westbound flights between, for example, Honolulu and LA is only a couple of minutes for every 10mph speedup of the prevailing wind,” says Karnauskas. “But the wind really fluctuates by about 40mph, so multiply those couple of minutes by each flight per day, by each carrier and by each route, and that residual quickly adds up.”
Based on what they had learned about residual flight times on the routes, the researchers explored how climate models predict the atmospheric circulation to change and to make some estimates of how much more CO2 would be emitted by the airline industry in the face of those changes. Global climate models do incorporate projected emissions from global aviation but, they say, if the atmospheric circulation changes, those initial assumptions would miss the potential feedback.
From a simple extrapolation on a global basis, applying an extra minute to the 102,470 flights per day on 49,871 routes served by commercial airlines in 2014, would mean aircraft spending an extra 300,000 hours per year in the air. This translates, says the study, to around 1 billion gallons of jet fuel costing $3 billion (at $3/gallon) and CO2 emissions of 10 billion kilograms of CO2 (at 9.6kgCO2/gallon) on an annual basis.
Karnauskas believes the study could be useful for the airline industry to more efficiently plan for future fuel costs, reallocate fuel resources, refine the predicted flight durations for their customers, and better manage all the inconveniences and manpower related to flight delays. However, he is surprised the industry is not more aware of flight-time patterns beyond the day-to-day.
“The industry keeps a close eye on the day-to-day weather patterns but they don’t seem to be addressing cycles occurring over a year or longer,” he says.
He now plans to extend the scope of the research to total global airline traffic, which he says will be “a massive undertaking”. To work with such large datasets, he has been granted access to Azure, a powerful cluster of networked supercomputers operated by Microsoft, under a special research grant jointly offered between Microsoft Research and the White House Climate Data Initiative.
