One round‑trip flight between New York and San Francisco generates two to three tons of carbon dioxide emissions per passenger, more than 10 percent of the annual carbon footprint of the typical American. The aircraft further heats up the climate through the cloudlike contrails (condensation trails) that form in its wake, and by cruising at an altitude where the warming effect of greenhouse gases is magnified. Air travel is now responsible for about five percent of global warming, but that number is expected to rise as demand—expected to triple by 2050—outpaces efficiency improvements in airliners.
Concerned about this trend, MIT’s Steven Barrett aims to better understand the impact of aviation on the environment, from local air pollution to global climate change, and to explore technological, operational and regulatory options to reduce that impact. An associate professor of Aeronautics and Astronautics, director of the Laboratory for Aviation and the Environment, and Joint Program faculty member, Barrett has published around 50 papers covering aviation and its impact on the environment and public health.
Barrett’s research in this field stems from an early interest in airplanes and environmental science while growing up in London and the Scottish Highlands. At Cambridge University, he earned his bachelor’s degree and PhD in aerospace engineering—both involving stints at MIT as an exchange and visiting student. After completing his doctoral thesis on public health impacts of aviation in 2009, he spent a year on the Cambridge University faculty before assuming his current position at MIT, where he is part of the Joint Program, the MIT Center for Environmental Health Sciences, and the SMART Center for Environmental Sensing and Modeling. The associate director of the Partnership for Air Transportation Noise and Emissions Reduction from 2012 to 2015, he has served as an independent expert on green aviation, and on advisory committees on aviation and the environment and emissions mitigation.
Reducing aircraft emissions enough to make a dent in the local and global environment is a daunting challenge. The fastest growing mode of transport, aviation is expanding at about five percent a year. At the current rate, aviation CO2 emissions—90 percent of which are produced at cruise altitude—are expected to double or triple by 2050. Unlike in the automotive industry, in which emissions can be cut by shifting to smaller cars or electric vehicles, obvious substitutes don’t exist for today’s aircraft, and even if they did, it can take decades to deploy them.
“Even if we took a recent MIT design for an aircraft with 70 percent reduction in fuel consumption, that might take 10 or 15 years to complete, well into the 2020s,” Barrett explains. “To penetrate the market, it might take 30 years, to the 2050s. Even with the most aggressive technological scenario, we might hold CO2 emissions constant. The problem is hugely challenging due to the expected growth rate of CO2, the lack of substitution options and the technological inertia of the system.”
Nonetheless, Barrett is among perhaps less than a dozen academicians whose primary focus is the environmental impact of aviation—research that’s largely supported by the Federal Aviation Administration, NASA and the Environmental Protection Agency. Much of his work centers on evaluating the economic and environmental viability of replacing conventional jet fuel with biofuel.
On first glance, using biofuels seems like a no‑brainer. Producing a biofuel entails growing biomass, which through photosynthesis, extracts CO2 from the atmosphere, and then converting the biomass to fuel, which when burned in flight, releases CO2 back into the atmosphere. But the process is not exactly a zero‑sum game. Converting biomass into liquid jet fuel requires a lot of energy, far more than it takes to process biomass as a coal substitute in power plants, where—since jet fuel burns much cleaner than coal—it could offset twice the CO2 emissions at a much lower cost.
“That tells us that biofuels for aviation might make sense in the long run, but in the short run, there is more to gain by offsetting coal‑fired power generation,” says Barrett.
Another potential emissions reduction strategy that Barrett is pursuing is to minimize the impact of contrails. According to a recent study in Nature Climate Change, the radiative forcing, or warming, from aircraft contrails exceeds that of all CO2 emissions produced by the entire 113‑year history of aviation. One way to reduce the impact of contrails is to alter flight paths so that aircraft avoid very cold or wet regions of the atmosphere where they typically form, or fly at altitudes where they are less likely to emerge.
“The typical commercial jetliner altitude today is close to the most efficient altitude to create contrails, so higher or lower‑flying aircraft could reduce their incidence,” says Barrett, who has also determined that a switch from conventional, petroleum‑based jet fuels to alternative fuels may result in thinner contrails. “If you could figure out a way to significantly mitigate contrails, you could perhaps reduce aviation’s climate footprint by one‑third or more.”
Barrett is also evaluating strategies to reduce aviation’s impact on air pollution and public health through new technologies, flight operations rules and/or cleaner jet fuels. Delivered primarily at cruise altitude, health‑hazardous emissions from aircraft include sulfur dioxide, oxides of nitrogen, and particulate matter.
In a study published in 2012, he determined that removing sulfur from jet fuel would avert about 2,000 deaths per year in the U.S. for just three cents a gallon, but it would also increase the warming effect of aviation by about 10 percent. That’s because sulfur in airplane exhaust is emitted in the form of reflective sulfate particles, which cool the planet by reducing the amount of sunlight that reaches the surface.
Barrett is currently analyzing the effects of aircraft‑borne lead emissions in a joint EPA‑funded study with MIT Associate Professor Noelle Selin.
In February 2016, the aviation industry agreed to the first global carbon emissions standards for commercial aircraft. Established by the U.N. and applicable to new aircraft starting in 2028, the standards could lower carbon emissions by more than 650 million metric tons between 2020 and 2040.
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