How do national governments calculate their share of greenhouse gas emissions from international air travel?

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.