Those thin white clouds that jet engines draw across the sky are leaving their mark on the climate. A new study warns that the global heat-trapping effect of contrail clouds will triple by 2050 unless airlines and airplane builders dramatically reduce emissions or air traffic patterns change.

Air travel is growing so fast that current efforts to curb the climate-harming effects of airplane pollution won't be able to keep up with the expected increase in the formation of heat-trapping clouds, scientists wrote in a study published Thursday in the European Geosciences Union (EGU) journal Atmospheric Chemistry and Physics.

"Given the increasing rate of air traffic we see now, there's not much we can do to keep the climate impact constant," said Ulrike Burkhardt, a co-author of the study and a climate researcher at DLR, the German Aerospace Center.

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Air travel affects the climate in two distinct ways: through the greenhouse gases released as airplanes burn fuel, and through the heat-trapping effect of the condensation that forms as hot gases and soot from partially burned jet fuel activate water particles that then freeze and form contrails.

Those clouds can persist for more than half a day, and under certain atmospheric conditions, they can merge and spread across thousands of square miles, expanding the heat-trapping effect across wide areas.

"Usually people say clouds are cooling the surface. For lower clouds that's true. They reflect sunlight. But high clouds that are optically thin are most likely to warm the atmosphere," Burkhardt said.

The prevalence of tiny ice crystals intensifies the heat-trapping effect. In the short term, the climate impact of the clouds formed by jet exhaust is greater than the warming effect of aviation's greenhouse gas emissions.

The impact of contrail cirrus clouds on global warming will be stronger over North America and Europe, the busiest air traffic areas on the globe, but it will also significantly increase in Asia as air travel increases, the researchers said. In 2005, air traffic was responsible for about 5 percent of human-caused climate warming. Contrail cirrus clouds are already the largest contributor to aviation's climate impact, and the study found that their warming effect will be three times larger in 2050 than in 2006.

How Can Airlines Reduce the Impact?

The commercial aviation industry is targeting a 50 percent cut in greenhouse gas emissions by 2050 from 2006 levels, but the plans don't address the climate warming caused by contrail cirrus.

The new study found that reducing soot pollution would be the best way to reduce the non-CO2 part of aviation's climate warming effect.

Cleaner aircraft emissions would solve part of the problem, Burkhardt said. Reducing the number of soot particles emitted by aircraft engines decreases the number of ice crystals in contrail clouds, which in turn reduces their climate impact. But even a 90 percent reduction of soot would probably not be enough to limit the climate impact of contrail clouds to 2006 levels, she added.

The researchers also showed how improvements in fuel and propulsion efficiency, adjusting flight paths and the use of alternative fuels could shake up the equation. For example, rerouting flights or changing altitude to avoid regions most sensitive to the effects of contrail formation could limit their short-term heating impacts—but could also lead to increases in emissions of long-lived greenhouse gases.

"There is no feasible carbon-free technological replacement for commercial aviation on the horizon," said Peter Kalmus, a climate scientist with NASA's Jet Propulsion Lab who also launched a website to advocate for less air travel. "We need to recognize the full projected impacts of aviation on our climate, and that includes factoring in the non-CO2 forcings, which are very significant."

EPA Is Considering New Rules

Air travel is not going to stop. It's growing fast and will continue to grow for the foreseeable future, said Bill Hemmings, aviation and shipping program director for the NGO Transport and Environment. The non-CO2 impacts from aviation is the elephant in the room nobody wants to talk about, he said.

"The traditional view of the airlines is, the science is too uncertain, and because of that, we shouldn't do anything," he said, adding that the new study will bolster current efforts by European lawmakers to reduce aviation emissions.

In the U.S., the EPA is also in the process of proposing new rules on commercial airplane emissions, and science showing how those emissions harm the environment could be used to strengthen regulations, or as evidence against the agency in future legal challenges if new rules are weak.

"The new study is a large improvement to the data on the climate impacts of clouds caused by jet pollution," said David Fahey, director of the Chemical Sciences Division at the National Oceanic and Atmospheric Administration's Earth System Research Laboratory in Boulder.

Even though aviation emissions are small compared to those from energy or housing, the current debate around cutting them is meaningful because every ton of CO2, every tenth of a degree of warming matters in the attempts to stay within the bounds of the global carbon budget, he said.

Kalmus argues that what is really needed is demand reduction, preferably by putting a price on carbon.

"It would also help shift us to a new climate-aware way of thinking about travel, as something we don't take for granted and that we're willing to spend more time on—slower trips over land and ocean, staying at our destinations longer with more meaningful trips."