Climate Change: The View From 35,000 Feet

 

On November 29, Dr. Steven Barrett blew through the doors to KSJ — his flight home from London to Boston had been delayed, and he was in a rush. Scheduling snafus aside, Barrett is no stranger to aviation; he’s an aeronautics and astronautics professor here at MIT. An expert on the relationship between aviation and the environment, Barrett spoke to the KSJ fellows about the ways in which high-altitude transportation accelerates climate change.

Barrett explained that as countries become wealthier, modes of high-speed transport, like trains and planes, become more popular. This trend is especially pronounced in Asia, where aviation is growing at an extremely fast pace. During his talk, Barrett forecast that carbon dioxide emissions from global aviation could double or even triple by midcentury. And even though air travel accounts for just 4 percent of the CO₂ produced today, that kind of increase could undermine hopes of reducing emissions 80 percent by 2050.

The most obvious remedy, he said, is to build more efficient planes. But they would have to be much more efficient — burning 90 percent less fuel to meet the 2050 emission goals — and the technology to do that is still a long way off. So experts like Barrett are looking at alternatives.

For instance, “biofuels” are made from natural plants, which already contain CO₂ taken from the air during photosynthesis. This plant-based carbon can be converted into fuel, burned, and then returned to the atmosphere whence it came.

“In theory it’s a closed loop,” said Barrett. “However, in practice it’s not quite that simple; there a few problems.”

Planting biofuel crops in previously open swaths of land decreases the Earth’s available surface area to reflect sunlight. As a result, the planet retains more heat than it expels — a phenomenon known as the albedo effect. Clearing forests to make room for biofuels could “unlock” the CO₂ stored within the decimated trees, adding more carbon to the atmosphere.

To make matters even more complicated, Barrett emphasized that CO₂ is just one of many factors that contribute to aviation-induced climate change. “On the ground, one ton of fuel gives you about three tons of CO₂,” he said. “Up at high altitudes, you still get the same three tons of CO₂, but you also get a bunch of other chemical effects too, like contrails.”

These white, puffy streaks form in the wake of aircraft if the atmosphere is cold and wet enough. As the airplane burns fuel, it emits water vapor that condenses onto the carbon soot in the exhaust. This creates billows of ice crystals suspended in the sky — the contrails we see from down below.

Just like greenhouse gases, contrails are worrisome because they trap the heat given off by the surface of the Earth. But not all contrails are created equal. Those generated during the day aren’t as concerning; they trap radiation from the planet but simultaneously block incoming heat from the sun. While these two temperature effects nearly balance each other out during the day, at night there is no solar radiation to reflect. As a result, nighttime contrails serve to hold in the Earth’s warmth like a blanket of insulation. To make matters worse, contrails are most likely to form at 35,000 feet — the optimal altitude for air travel.

Contrails last for only about three hours before dissipating, which means their warming effects fade shortly thereafter as well. By comparison, CO₂ persists in the atmosphere for thousands of years. While contrails don’t sound so bad in this context, Barrett expressed that “the last three hours of flights actually produce as much warming through contrails as CO₂ emissions have over the last 60 years.”

By altering the combustion process, Barrett thinks daytime contrails could be brightened so they reflect more sunlight and nighttime contrails could be thinned out to permit heat from the Earth to escape. “Over all, that strategy could reduce contrail warming by 50 percent,” he said.

KSJ fellow Iván Carrillo called Barrett’s talk “quite depressing, but realistic at the same time.” “I hope we’ll be tough enough to face the challenge, and change our air transportation system in the future,” he said. “It’s a big issue for humanity.”

Asked if there was reason for hope in the face of such daunting challenges, Barrett told the fellows, “Lots of us do still have hope,” and added, “At the same time, this isn’t the sort of trivial challenge that can be solved instantly when a politician signs an agreement.” It’s much bigger than that.