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“I love to travel. But I hate the fact that something I love to do, creates so much pollution.” In this episode of TILclimate (Today I Learned: Climate), MIT professor Steven Barrett and host Laur Hesse Fisher dig into how — and why — air travel impacts our earth’s climate, and what solutions are on the horizon. They explore the surprising heating effect of condensation trails (“contrails”), how computer simulations of the earth’s climate system are built, and what scientists and engineers are doing to make flying, well, less bad for the planet.
Prof. Barrett is a professor of Aeronautics and Astronautics and the Director of the Laboratory for Aviation and the Environment. Through this lab, he coordinates the MIT Electric Aircraft Initiative, which aims to assess and develop sustainable electric aircraft technology. To read more about Barrett’s research on contrails, biofuels, fuel efficiency, electric aviation technology, and more, check out the links below.
For other short, climate-explainer podcasts, see: www.tilclimate.mit.edu on MIT’s Climate portal.
Laur Hesse Fisher, Host and Producer
Dave Lishansky, Editor and Producer
Cecelia Bolon, Student Production Assistant
Ruby Wincele, Student Researcher
Music by Blue Dot Sessions
Artwork by Aaron Krol
Special thanks to Tom Kiley and Laura Howells.
Produced by the MIT Environmental Solutions Initiative at the Massachusetts Institute of Technology.
Laur Hesse Fisher: [00:00:00] Welcome to Today I Learned Climate, the show where you learn about climate change from real climate scientists. Today's question is, what are those white lines that trail behind the airplanes and what do they have to do with climate change?
To get more insight around today's question, I reached out to Professor Steven Barrett, who leads MIT's Laboratory for Aviation, and the Environment.
Steven Barrett: [00:00:24] My name is Steven Barrett and I've been at MIT at eight years now, trying to improve scientific understanding of how aviation impacts the environment with a particular focus on climate change and air pollution.
Laur Hesse Fisher: [00:00:36] You may have read some headlines about why flying has become enemy number one, for many climate change activists. An article from the Washington Post from November last year is literally titled, For the Love of Earth, Stop Traveling. I don't know about you but I love to travel but I hate the fact that something that I love to do creates so much pollution.
Steven Barrett: [00:00:57] I mean a lot of people view environmental constraints as existential threat to aviation and I believe at least, aviation is positive and the more people can explore the world and experience different cultures and take up educational and work opportunities and see family and friends, the better.
Laur Hesse Fisher: [00:01:15] So Professor Barrett and his research team are not only working to better understand the problems of aviation and climate change but are also developing solutions.
Steven Barrett: [00:01:23] Like electric aircraft and also bio fuels and other policy changes.
Laur Hesse Fisher: [00:01:29] So we'll talk about those later.
But first, let's break down the problem.
Planes burn jet fuel, and when they do they release two gases. The most important are carbon dioxide and water vapor, water in its gas form. You're probably familiar with the climate impacts of CO2. This gas gathers in the atmosphere and forms a kind of blanket around the earth, trapping in heat and bumping up the average temperature of the planet. For hundreds of thousands of years this has created a very comfy place for humans and life to live, but as we've been adding more and more CO2 to the atmosphere, the blanket is becoming thicker and thicker, warming the planet more than we have in millennia. Just as a side note, I highly recommend checking out the climate primer that we've posted on our new MIT Climate Portal, Climate.mit.edu. You'll find the link to this in our show notes.
Okay, so the CO2 is creating this thick blanket making us warmer. The main issue with CO2 is that it sticks around in the atmosphere for a long time.
Steven Barrett: [00:02:32] CO2 has a lifetime atmosphere of hundreds of years. Now most of the CO2 that aviation's ever emitted is still in the atmosphere because it lasts so long.
Laur Hesse Fisher: [00:02:41] Think about fighter planes circling Europe in World War One, or Charles Lindbergh flying across the atlantic Ocean in 1927, the CO2 from those flights are still in the atmosphere.
Steven Barrett: [00:02:53] And so we're now experiencing the warming from all that accumulated CO2.
Laur Hesse Fisher: [00:02:58] Okay, so that's CO2, but planes also emit water vapor.
Steven Barrett: [00:03:03] When aircraft fly through a sufficiently cold or wet part of the atmosphere, it leaves behind it an artificial cloud called a contrail.
Laur Hesse Fisher: [00:03:09] Which is short for condensation trail, because the water vapor condenses into ice crystal in the cold air.
Steven Barrett: [00:03:16] Which are line shaped artificial clouds you sometimes see behind aircraft, and they form within a few seconds and they last a few hours if they form and persist.
Laur Hesse Fisher: [00:03:26] Understanding how contrails interact with heat and sunlight is gonna be really important in this episode, so let's break this down for a moment. So normally, heat and sunlight enters our atmosphere and warms the earth as we all know. Some of that heat bounces back off the surface of the earth and leaves the atmosphere. So contrails do two things inside of this process, they reflect incoming heat from the sun, so that heat ever reaches the earth's surface, and they also absorb the earth's heat, keeping in the heat that would normally never stay in our atmosphere. You could say that contrails act like both a jacket and a shade. They absorb heat radiating off of the earth, like how a jacket keeps in your body heat, and at the same time, they also act like a shade, preventing sunlight that would have normally warmed the earth from ever hitting the surface.
Steven Barrett: [00:04:21] At nighttime, they're always warming because there's no incoming solar radiation but there is outgoing infrared which gets trapped. And then in the daytime they can either be warming or cooling.
Laur Hesse Fisher: [00:04:31] That's because it also matters where the contrail is. The balance of absorbing versus reflecting heat changes depending on if the contrail's over a darker area like the ocean, which absorbs more heat than it reflects, or over brighter areas like ice, which reflects more than it absorbs. If you wanna know more about this, check out our show notes on climate.mit.edu.
Overall, just like your jacket, scientists think that contrails have a warming effect, trapping in more heat than they reflect. And the models show that this warming effect is dramatic.
Steven Barrett: [00:05:08] So you have as much warming from the last six hours of contrails as you do from the whole history of aviation CO2 emissions.
Laur Hesse Fisher: [00:05:15] whoa, so contrails contribute a lot to warming but only temporarily, whereas CO2 lingers for hundreds of years. In fact, after 9/11, all planes were grounded for three days, and scientists studied and were able to see and measure how the lack of contrails really did impact the planet's temperature, which brings up another question. How do scientists actually study this stuff?
Steven Barrett: [00:05:40] Yeah, I mean in some ways a lot of climate science is difficult because we don't have a spare planet to do a control experiment on and that makes life much harder, so if we could create one, that would be ideal. But failing our ability to do that, we've got to approach problems in a more piecewise way. So that means building up models from rigorously verified pieces of evidence, so say for example, creating models of atmospheric chemistry, verifying those models of atmospheric chemistry, including verifying that experimentally in say smog chambers.
Laur Hesse Fisher: [00:06:16] So Professor Barrett and his team build and use climate models that try to simulate what's happening ten miles about us.
Steven Barrett: [00:06:22] A model is a computer representation of equations that govern physics, so they're equations that are transformed into computer code, and these things usually have millions of lines because you're trying to model or trying to capture in a computer code, what's going on from chemicals reacting, to emissions into the atmosphere, to clouds forming, winds, rain; a huge number of different processes that all get put into climate and atmospheric models.
Laur Hesse Fisher: [00:06:50] Most computer models can take weeks, months or even more than a year to run on super computers, because they require so much computational power.
Steven Barrett: [00:07:00] So you can run hypothetical cases and use the answers to understand what the effect is of aviation even now or in the future or if you were to change it in some way. You have generations of researchers who contribute a piece to the work, and in this case, often they'll work on modifying, improve or create computer codes that represent or improve the representation of some kind of physics or chemistry process. And the models that get built that represent the atmosphere and how it responds, are the product os hundreds of PHDs across scores of universities over decades, so this atmospheric and climate models represent the sum totals of generations of people's work towards building them.
Laur Hesse Fisher: [00:07:42] Okay so CO2 is still lingering and will still be lingering for hundreds of years. And contrails also trap heat depending on how many planes are flying at any given time. So how much does this actually matter? Well if you include both the CO2 and contrails, aviation contributes about six per cent of the warming we're experiencing today. Six per cent might sound small but it's actually a really big number. The country of India contributes six per cent of the world's greenhouse gases, and it's the world's third largest emitter. And aviation is on the rise.
Steven Barrett: [00:08:22] The current forecasts are that aviation would double or triple by mid century, and at the same time most scientists say that you want to reduce CO2 emissions by about 80%. So even though today aviation's only about six per cent, if we want to reach something like an 80% or more reduction of CO2 emissions, while enabling growth in aviation because of the positive effect it has on society, that creates a huge challenge that is very difficult to answer.
Laur Hesse Fisher: [00:08:50] These are hard questions but many people around the world are working on solving them. Airline industries are always looking at more and more fuel efficient planes, largely because it's in their economic interest to do so. Researchers like Professor Barrett are developing super efficient plane technologies. Companies are manufacturing lower carbon fuels like bio fuels made out of plant matter. There are a lot of solutions being pursued and there are great challenges with each of these solutions. But one thing is for sure, because of how long CO2 lasts in the atmosphere, the decisions that we make now, have an impact far into the future.
To see some of the work that MIT and others we know, are doing to reduce aviation's impact on climate change and other cool climate science explanations, check out tilclimate.mit.edu. That's tilclimate.mit.edu.
Thanks to Professor Barrett for coming in and speaking with us and thank you for listening.
Read more about:
Aviation solutions developed at MIT & beyond:
MIT Electric Aircraft Initiative research (MIT Electric Aircraft Initiative)
Electric planes that have no moving parts (MIT News)
Alternative jet fuels that reduce CO2 emissions (Energy Policy Journal)
Prof. Steven Barrett and his work:
The earth’s reflectivity based on color (the albedo effect):
MIT research on how urbanization is changing Earth’s albedo (MIT Concrete Sustainability Hub)
Articles mentioned in the podcast:
For the Love of Earth, Stop Traveling (Washington Post)
For advice on personal air travel:
“I feel guilty about flying… help!” (Yale Climate Conversations - Climate Advice)
An overview of climate change:
Climate Science and Climate Risk: A Primer (MIT’s Kerry Emanuel)
Created by Olivia Burek, Alyssa Farkas, and Aaron Krol, with thanks to Sarah Hansen and MIT Open Learning
HIGH SCHOOL AND HIGHER EDUCATION
The following questions can be used to encourage your students to reflect on, extend, and apply what they’ve learned from the podcast episode. Re-use and remix them as writing prompts, discussion guides, or ideas for project-based learning in your classroom.
- What people, groups, and industries are contributing most to aviation-based climate warming? Are these the same people, groups, and industries who are most affected? Who needs to learn and understand more about the impacts of aviation? Why should people know more about this? How could you make people in your community more aware?
- Read about some potential solutions to reduce air travel emissions, including those in the episode notes. How would these solutions impact the aviation industry? What about the customers of the industry? Can you think of alternative ways to address this issue? What can be implemented immediately, and what might take more time? How could you allow air travel to increase while still reducing greenhouse gas emissions?
- Think about the analogy used in the podcast: “You could say that contrails act like both a jacket and a shade. They absorb heat radiating off of the earth, like how a jacket keeps in your body heat, and at the same time, they also act like a shade, preventing sunlight that would have normally warmed the earth from ever hitting the surface.” Did you find this analogy to be useful in explaining how contrails work? In general, do you think analogies are useful rhetoric devices to communicate about climate change? How have you seen them used in science communication? How might you use your own skills (artistic, musical, written, etc.) to share this analogy with others in your own community in a way that would help them understand the gravity of the situation?
- Research how condensation trails form. How do they affect the atmosphere and contribute to the greenhouse effect? Why do their effects vary in different conditions, and what does this variation mean for their overall effect on the atmosphere? In what other ways does aviation have an impact on the climate?
- Research how airline companies are trying to be more climate-friendly by reducing greenhouse gas emissions. Are these popular, well-known airlines? Have they shown previous commitment to sustainability and fighting climate change? Do you think the scale of these initiatives is fair, relative to these companies’ contributions to climate change?
- Which countries use air travel the most, whether by commerce or personal travel? Are these the same countries that are most affected by climate change?
- Are greenhouse gas emissions something you should take into consideration when planning air travel? Why or why not? Is this already something you think about? How have you benefited from air travel in the past, or how has air travel been a challenge for you? What do you think you can do, as an individual, to lessen the negative impacts of air travel?
- How do you foresee travel changing? Will different modes of travel grow to be more popular, or less popular? How do you think new policies could affect the popularity and accessibility of different modes of transportation? Consider other popular modes of transportation, such as train travel in Europe and Asia, or highways in the U.S. and Canada.
- Suppose you are the CEO of a major airline. You want to make improvements to your company, but you have a limited amount of money to do so. In order to stay within your budget, you have to choose your priorities: making planes more eco-friendly, more comfortable, cheaper or more expensive to fly, or more in number, to name a few examples. Organize your top priorities and decide how you would allocate your resources. Pay attention to how one choice may influence another.
The following infographic was used to develop many of the critical thinking questions in this guide. You may also find it helpful:
Watanabe-Crockett, L. (2016, December 12). The critical thinking skills cheatsheet [Infographic] [Web log post]. Retrieved May 1, 2019, from https://www.wabisabilearning.com/blog/critical-thinking-skills-cheatsheet-infographic
Open Teaching Materials
Need additional open educational resources related to the topics of planes and climate change? You may find these free teaching materials from MIT OpenCourseWare (ocw.mit.edu) helpful:
Climate Physics and Chemistry
This course, taught by Professors Carl Wunsch, Edward Boyle, and Kerry Emanuel, introduces students to climate studies, including internal feedback mechanisms involving ice, aerosols, water vapor, clouds, and ocean circulation. Educators have access to the reading list, assignments, a list of potential project topics, and PDF versions of the lecture notes, with six lectures focusing on the role of the atmosphere in climate.
Experimental Atmospheric Chemistry
This course, taught by Professors Ronald G. Prinn and Shuhei Ono along with Dr. Karin Ardon Dryer, provides an introduction to the atmospheric chemistry involved in climate change, air pollution, and biogeochemical cycles. Educators have access to the reading list, PDF versions of selected lecture notes, and data and supporting files required for three lab assignments, including one on aerosols and clouds.
Aircraft Systems Engineering
This course, taught by Professors Earll Murman, John Hansman, and Robert Liebeck along with Allen Haggerty, explores the aircraft as a system whose subsystems must meet demanding customer and value requirements. Educators have access to project assignments, a list of study materials, and PDF versions of the lecture notes, with two lectures on environmental factors including the role of contrails in climate change.