Flights from Honolulu to the mainland are the basis of a new study showing that the weather phenomenon El Nino and a warming atmosphere have a profound influence on time spent in the air.
And the longer the flights are, the more they pump carbon dioxide into the atmosphere — a feedback loop that scientists are exploring for the first time.
Separately, there’s a huge amount of money at stake.
If each of the nearly 50,000 daily commercial round-trip flights worldwide were to be one minute longer, that translates to 304,000 hours over the course of a year. That would require about 1 billion more gallons of jet fuel — $3 billion worth, the researchers calculate — and in turn emit 22 billion pounds of carbon dioxide into the atmosphere.
"Upper-level wind circulation patterns are the major factor in influencing flight times," says Kris Karnauskas, an associate scientist at the Woods Hole Oceanography Institution in Massachusetts. "Longer flight times mean increased fuel consumption by airliners. The consequent additional input of CO2 into the atmosphere can feed back and amplify emerging changes in atmospheric circulation."
Karnauskas is the lead author of a study on air travel and climate published this week in Nature Climate Change.
The study began when Massachusetts Institute of Technology-WHOI student and co-author Hannah Barkley experienced a speedy flight from Honolulu to Newark, N.J.
She mentioned it to Karnauskas, who checked a National Oceanic and Atmospheric Administration website and saw that the jet stream was especially strong that day.
"There was just a big swath of extra-fast westerly winds stretching from Honolulu, Hawaii, to Newark," Karnauskas said in a release Monday. "It was just serendipitous, as if she was part of some kind of golden mileage club where the atmosphere just opens up for you."
Through a database maintained by the Department of Transportation, the researchers downloaded departure and arrival data for every flight on four airlines from Honolulu to Los Angeles, San Francisco and Seattle from 1995 to 2013. The airlines were United, American, Delta and Hawaiian.
Because the upper-level winds blow from west to east, the eastbound leg of a round-trip flight is generally faster. Karnauskas plotted the differences in duration between eastbound and westbound flights and noticed that they were largely the same for each carrier over those 19 years.
"That the observed fluctuations in flight times are synchronous throughout the sector regardless of route or carrier suggests a common driver, which we hypothesize to be climate variability," Karnauskas and his colleagues conclude.
The result also pointed toward the influence of El Nino, characterized by warmer sea surface temperatures in the eastern tropical Pacific.
As the temperature of the equatorial Pacific rises and falls, atmospheric waves are propagated toward the higher latitudes, where they change wind circulation patterns.
Karnauskas found that just by looking at the temperatures in the tropical Pacific, he could predict the Honolulu-West Coast flight times.
"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 said.
The difference in eastbound and westbound leg durations was greater than 30 minutes during the 1997-98 and 2009-10 El Ninos, the researchers found.
The analysis also determined that the difference in flight times between eastbound and westbound flights on any given route didn’t cancel each other out.
Rather, when an eastbound flight became 10 minutes shorter, the corresponding westbound flight became 11 minutes longer. So in a strong jet stream, a westbound plane burns more additional fuel than it saves going east.
The net additional flying time for a pair of eastbound and westbound flights between, for example, Honolulu and Los Angeles is only a couple of minutes for every 10 mph speed-up of the prevailing wind.
"Multiply those couple of minutes by each flight per day, by each carrier, by each route, and that residual adds up quickly," Karnauskas said. "We’re talking millions of dollars in changes in fuel costs."
Karnauskas contends this information could be useful for the airline industry to plan for future fuel costs, reallocate fuel resources, refine the predicted flight durations for customers and better manage the inconveniences and manpower related to flight delays.
"The airline industry keeps a close eye on the day-to-day weather patterns, but they don’t seem to be concerned with cycles occurring over a year or longer," he says. "They never say, ‘Dear customer, there’s an El Nino brewing, so we’ve lengthened your estimated flight duration by 30 minutes.’ I’ve never seen that."
NOAA said there is a greater than 90 percent chance that the current El Nino will persist through the winter.
Global air travel contributes around 3.5 percent of the greenhouse gas emissions behind human-influenced climate change, according to the International Panel on Climate Change. But current global climate change models do not factor in air travel, Karnauskas said.
