New research shows that condensation trails from aircraft exhaust are playing a significant role in global warming. Experts are concerned that efforts to change aviation engine design to reduce CO2 emissions could actually create more contrails and raise daily temperatures even more.

White wispy trails across the blue sky on a sunny day are one of the few attractive features of air travel. But they have a darker side, especially at night. For the condensation trails produced by the exhaust from aircraft engines are creating an often-invisible thermal blanket of cloud across the planet. Though lasting for only a short time, these “contrails” have a daily impact on atmospheric temperatures that is greater than that from the accumulated carbon emissions from all aircraft since the Wright Brothers first took to the skies more than a century ago. More alarming still, researchers warned late last month that efforts by engineers to cut aircraft CO2 emissions by making their engines more fuel-efficient will create more, whiter, and longer-lasting contrails — notably in the tropics, where the biggest increases in flights are expected. In a paper being widely praised by other experts in the field, Lisa Bock and Ulrike Burkhardt of the Institute of Atmospheric Physics in Oberpfaffenhofen, Germany, forecast a near-tripling in the “radiative forcing” from contrails by 2050. Aircraft emissions are rising up the climate agenda. With renewable energy taking over from fossil fuels in power generation, and the rise of electric cars, the continued surge in flights is increasingly seen as potentially the worst future threat to the climate, not least because there are as yet no carbon-free replacement technologies or international regulations to bring down emissions. Civilian aircraft currently emit about 2 percent of anthropogenic CO2 and, once the effects of contrails are included, cause 5 percent of warming. But there is a key difference. While CO2 accumulates in the atmosphere and has a long-lasting effect, contrails last a matter of hours at most, and their warming impact is temporary.

Like regular cirrus clouds, contrail clouds trap heat radiating from the earth’s surface, causing warming in the air below.

Contrails are human-made clouds. They form in air above about 25,000 feet, when that air is moist and colder than -40 degrees Celsius. Like regular clouds, they arise when water vapor, in this case from the engine exhausts, forms into droplets by condensing onto particles in the air, in this case soot from the engines. Within a second, the water droplets freeze to make tiny ice crystals that show up visually as contrails. If the air is not cool or moist enough, contrails may not form or may disappear quickly. But at other times, they stick around – either as tight, white lines in the sky, like chalk marks, or gradually spreading to create thin layers of ice clouds. They are similar to natural cirrus clouds and are often called contrail cirrus clouds. Bernd Kärcher, also of the Institute of Atmospheric Physics in Oberpfaffenhofen, reckons contrail cirrus clouds cover around 0.6 percent of the global skies at any one time — nine times the amount covered by contrails themselves. In areas with high amounts of air traffic, they can merge to cover as much as 38,000 square miles, roughly the size of Indiana, and last for many hours or even days. The concern is not new. Patrick Minnis of NASA’s Langley Research Center reported more than two decades ago at a NASA meeting that vapor trails turned up on 40 percent of days over New York state, and later argued that “increased cirrus coverage, attributable to air traffic, could account for nearly all of the warming observed over the United States for nearly 20 years starting in 1975.” But understanding of the role of contrail cirrus clouds to climate is growing. Though sometimes too thin to spot easily from the ground, these will-o-the-wisps are the biggest component of the warming from contrails, says Kärcher.

An infrared satellite image showing dozens of contrails over the southeastern United States during a single morning in January 2004. NASA

Like regular cirrus clouds, contrail cirrus clouds have two competing effects on climate. They shade us by reflecting incoming sunlight back into space. But they also trap heat radiating from the earth’s surface, so causing warming in the air below. During the day, cooling compensates part of the warming. But at night, with no sunlight, only the warming effect operates. Red-eye flights are a red light for climate. That’s the theory, and observational evidence backs it up. Research in the American South and Midwest has concluded that when contrails are around, they raise night-time temperatures sufficiently to reduce the day-night differences by 3 degrees C. And after 9/11, when all commercial flights in the U.S. were grounded for three days, the diurnal temperature difference increased by up to 1.8 degrees C. The increase was strongest where air traffic was normally densest, said the study’s author, David Travis of the University of Wisconsin. So how does this play out globally? What does it mean for climate change? The calculations are complex, and researchers warn it is not clear whether all the warming at cruising altitudes translates to warming at the earth’s surface. But in the new study, Bock and Burkhardt put the average effect on the earth’s radiation balance of contrails and contrail cirrus at 50 milliwatts per square meter of the earth’s surface. The figure is for 2006, the base year for the U.S. Federal Aviation Administration dataset used by the authors. It was double the 24 milliwatts from the CO2 that had accumulated in the atmosphere from a century of aviation (and is a significant part of a total anthropogenic effect at the time of around 1,600 milliwatts).

If the world wants a big short-term contribution from aircraft for keeping below a temperature target, contrails can provide it.

Such comparisons are potentially misleading, because they don’t reflect the very different long-term effects of the warming influences from different aircraft emissions. Contrails and the cirrus clouds they create last for only a few hours at most. “If aviation was halted today, the contrails would be gone in a matter of hours,” says Keith Shine of the University of Reading. They do not accumulate and leave no lingering effect, whereas much of the CO2 emitted by aviation sticks around in the atmosphere for centuries. As more and more is emitted, it keeps building up — even if aviation were halted, the CO2 would remain for a long time. So in the longer run, if we want to shut down global warming, there is no alternative to curbing CO2 emissions. But if the world wants a big short-term contribution from aircraft to keeping us below some specific temperature target, such as 1.5 degrees C, then action on contrails can provide it. Within days it would take some of the heat out of the atmosphere. It could, says Kärcher, “buy time to more substantially reduce aircraft CO2 emissions.” Can it be done? Researchers spoken to for this article offer three approaches, though they disagree about which has the most potential. One is to divert aircraft away from air where contrails are likely to form. This can be done vertically by changing altitude, or horizontally by detouring around the problem air. But aircraft currently fly the shortest routes and at altitudes that minimize fuel burn, so, Kärcher says, “controlling contrail formation in this way… will almost certainly lead to increases in aircraft CO2 emissions.” There is a potential trade-off. In retrospective studies of real flights, Banavar Sridhar, a senior scientist at the NASA Ames Research Center in California, found that changing altitude – usually by flying lower where the air is warmer — could produce a reduction of 35 percent in contrails and contrail cirrus for an extra fuel burn of only 0.23 percent. Even so, if done right, there could be a net overall reduction in warming. Volker Grewe of the Institute of Atmospheric Physics looked at 800 real transatlantic flights across the seasons and how their flight paths could have been altered to cut their combined warming effect from both contrails and CO2. He found that there could have been a 10 percent reduction in warming for only a 1 percent increase in operating costs.

Contrails over Lisbon, Portugal, in February 2000. NASA/JPL/UCSD/JSC

Of course, figuring out optimum flight paths after the event is different from doing it in a busy air-traffic control room. “You would have to forecast with high precision where atmospheric conditions are conducive to contrail formation, so you can plan re-routes with confidence,” says Shine, a strong advocate of re-routing. “If you get it wrong you likely emit more CO2 for no gain.” But he reckons that with the right research, it could be done real-time within a decade. Less attention has been given to the idea of altering the timing of flights by drastically reducing flying at night, when contrails have their greatest warming potential. This would not be entirely effective since daytime contrails produce cirrus clouds that can last long into the night. But it could help. A second approach to minimizing contrails is to change fuels — from kerosene-based fuels to biofuels, hydrogen, liquid natural gas, or even electricity. Electric propulsion from batteries could one day end all contrails, but is far off, especially for long-haul flights. But if burned “neat,” the other alternative fuels could cut the soot emissions around which water vapor condense in contrails by 90 percent or more, according to Fabio Caiazzo of the Massachusetts Institute of Technology. Even a 50:50 mix of biofuel and kerosene-based fuel could halve soot. Reducing soot may not necessarily reduce contrails. But a modeling study by Burkhardt and Bock, with colleague Andreas Bier, concluded last year that less soot would “strongly reduce” both the extent and lifetime of cirrus contrails. There is potentially a third approach: changing engine design to generate more propulsive energy from a given fuel burn. This is already an overriding priority of aviation engineers seeking to cut costs, and it would also reduce CO2 emissions. But there is problem as regards contrails, say Bock and Burkhardt. Their new paper warns that increasing the efficiency of fuel burning will waste less heat in the engine, and so result in cooler exhaust gases. Contrail production depends critically on cold temperatures in the surrounding air. So colder exhaust “leads to an increase in the contrail formation probability and contrail radiative forcing,” they say.

Even if fuel changes reduce soot emissions, researchers estimate contrail production will increase by a factor of 2.8 by 2050.

Shine agrees. “Improvements in engine efficiency can lead to more contrails, if the heat of combustion is used more efficiently to propel the aircraft rather than heat the exhaust.” That creates a growing risk that efforts to reduce fuel costs and CO2 emissions will result in more contrails. The stakes are high. Left to their own devices, airlines are expected to increase global traffic four-fold by 2050. Improvements in engine performance will likely limit the increase in warming from CO2 to a factor of 2.4, Bock and Burkhardt predict. But even if fuel changes reduce soot emissions, they estimate that contrail production will increase by a factor of 2.8. This is enough, says Kärcher, to add as much as 0.1 degrees C to the global warming caused by contrails. And yet, even as it makes the contrail problem worse, the international aviation industry has yet to come up with proposals for addressing the issue. Its current climate mitigation plan, known as the Carbon Offsetting and Reduction Scheme for International Aviation (Corsia), is to offset any increase in the industry’s CO2 emissions after 2020, for instance by planting trees. But the offsets only cover CO2 — creating a perverse incentive to add to contrails. The International Civil Aviation Organization, a United Nations agency that has been coordinating the industry’s response to the climate emergency, offers no prospect of a change in its approach. Its head of communications, William Raillant-Clark, said it could only change tack “on the basis of a technical/scientific consensus, which currently does not exist. There are no commonly accepted numbers” for the climate impact of contrails. He refused to comment on the new paper. But Kärcher dismisses this approach. “Uncertainty is being used as an excuse to remain inactive,” he said. There are certainly uncertainties in the measurement of the climate impact of contrails. And it is true that things are complicated because of the very different timescales over which the warming effects of contrails and CO2 play out. Arguably, contrails and contrail cirrus clouds are less important in the big picture of halting climate change, because they don’t accumulate in the way CO2 does. But they are adding significantly to global warming on every day that planes fly. And they could be adding “as much as a tenth of a degree [to global temperatures] by mid-century,” says Kärcher.