Air travel responsible for 4-8 percent of global climate change, says study

That flight back to Stanford may have had a bigger impact on climate change than previously thought.

According to a study by civil and environmental engineering Prof. Mark Jacobson, commercial aircraft flights have contributed four to eight percent of global surface warming since air temperature records began in 1850.

The first study of its kind analyzing commercial airlines, the results also showed that aircraft flights are responsible for an estimated 15 to 20 percent of warming in the Arctic.

Jacobson ran simulations using the aircraft emissions data for every commercial flight in the year 2004 and repeated the process for data from 2006. These numbers were entered into a computer model that also analyzed background pollution, clouds and radiation fields to predict how the emitted particles interacted chemically with atmospheric gases and aerosols.

“It’s a really complex soup of interactions occurring, but in the end it’s pretty relatively organized,” Jacobson said.

“We can then look at differences between running simulations with and without aircraft to see the changes in temperature and atmospheric composition of the clouds that result,” he added.

Aircraft engines emit particles that serve as sites on which clouds condense, forming the trail of vapor, known as “contrails,” that are sometimes seen behind planes. These contrails reflect sunlight but also absorb infrared radiation – due to the black carbon found in emitted soot – which changes atmospheric radiation balance.

Once contrails dissipate they also affect larger-scale cloud formation because the remnants of the contrails serve as nuclei on which other clouds form.

The impact of these emissions is magnified in the Arctic, where contrails absorb infrared radiation from both direct sunlight and light reflected back upward by the highly reflective snow and ice. Once the edges of the ice melt to reveal the water below, the ocean absorbs even more radiation, accelerating climate change.

Aircraft emission pollutants are short-lived, but they can still have a significant effect on the environment, Jacobson explained.

“You can ramp up those temperature changes with short-lived pollutants,” he said. “I mean, you can have that temperature effect due to brief periods of emissions – you don’t need hundred-year emissions to get that temperature change,” he added.

World airline passenger traffic fell 3.1 percent amid last year’s global financial crisis, the biggest drop in aviation industry history, according to figures from the International Civil Aviation Organization. Even so, black carbon emissions would have to be reduced 20-fold to halt Arctic warming, Jacobson said.

“There’s a general statement that can be made about all pollution sources – we should take action to reduce all pollution,” Jacobson explained. “All combustion causes some kind of air pollution or climate problem, so we should always be aggressively trying to reduce combustion toward alternatives that don’t combust or combust much cleaner materials.”

To meet this challenge, airplane manufacturer Boeing recently designed a lighter-weight, more energy efficient plane, according to Jacobson. The company is also developing lower sulfur emission fuels.

Still, even cleaner-burning fuel is possible, Jacobson said.

“If you really were to take it to the next level, you would want to go to maybe liquid hydrogen airplanes,” he said.

Studies from the European Commission have shown that a hydrogen-fueled airplane is possible, but the technology must still undergo rigorous safety tests, Jacobson said. Though hydrogen fuel has been used for the space shuttle, Jacobson predicts that hydrogen-fueled planes are still 10 to 20 years away.

“We have plenty of examples of hydrogen being used, but it’s a question of whether you can do it at low cost or not,” he said. “There’s also transforming the whole fleet – it’s a long process.”

Jacobson presented his study at the Geophysical Union’s annual meeting in December. The paper is currently being peer reviewed and is awaiting publication.