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How to use the carbon emissions estimate you see when booking flights

by Staff

Flight booking platforms are giving customers a new number to think about when they buy a plane ticket: the expected greenhouse gas emissions of their trip.

If you’ve searched for plane tickets on Google Flights in the past two years, you’ve probably seen a little green number that compares each route’s climate impact. Google began predicting flight emissions in 2022, using data about flight schedules, airplane models and how full a flight is expected to be to come up with an estimate for each passenger’s carbon footprint.

Lately, other platforms, including Expedia and Booking.com, have started using the same algorithm, called the Travel Impact Model (TIM), to tell customers whether their itinerary emits more or less carbon than usual.

But what, exactly, are these little green numbers — and how should an eco-conscious traveler take them into account when picking out flights? We asked the group that manages the model and a pair of economists what you should do with your emissions estimate.

Decoding the Travel Impact Model

To come up with an emissions estimate, the Travel Impact Model estimates how much fuel your plane will burn along its route, and then it estimates how much of those emissions you’re responsible for compared with other passengers.

Say you’re flying from Zurich to San Francisco on a Boeing 787-9.

Google’s model is one of the first to publicly estimate emissions for different flight options, according to Dan Rutherford, who heads an outside advisory board that checks Google’s work and suggests tweaks to make the model more accurate. “This is an effort to get data into the hands of consumers so they choose a less emitting flight,” he said.

On average, customers could cut their flight emissions about 20 percent if they picked the least polluting itinerary, according to a 2021 report from the nonprofit International Council on Clean Transportation (ICCT) based on an earlier emissions model that also compared available flights. They wouldn’t even have to pay much more in most cases: Greener flights also tend to be cheaper because they burn less fuel, according to the report.

What does a kilo of carbon cost?

So how should you weigh the benefit of lower emissions against other goodies, like picking the most convenient departure time or racking up miles on your favorite airline? One trick is to think about the “social cost of carbon,” an economic estimate of how much damage every additional bit of greenhouse gas pollution will do to the world.

The latest estimate, adopted by the Biden administration, puts the social cost of carbon at about $190 per metric ton — or about 19 cents per kilo. The flight from our Zurich to San Francisco example would do about $114 worth of climate damage if you flew in economy, or $456 in business class.

So if you want to consider the climate impact of your flight options, you could add the cost of carbon emissions onto the price of your plane ticket. Then compare your flight choices — departure times, layovers, airline points, etc. — with this new carbon-included price in mind and pick the option that seems like the best deal.

But you should take this carbon price with a grain of salt. It’s hard to put a dollar figure on climate damage, and economists disagree on the final number. To come up with an estimate, scientists model how much each additional ton of CO2 will heat the earth and contribute to sea level rise, droughts, wildfires and other calamities. Then economists determine how much those disasters will cost in terms of property damage, crop failures, hospital visits and so on.

“There are many, many links on this chain, and there’s a lot of uncertainty at each link,” said Lawrence Goulder, a Stanford University economist who studies climate change.

If you don’t want to deal with all that math, you can make your choice much simpler, according to Kenneth Gillingham, an economist at the Yale School of the Environment. “If people look at two flights and say, ‘The timing is the same, the cost is the same, why not grab the lower emissions one?’ Then that’s helpful,” he said.

Rutherford suggests following the ICCT’s informal rule on air travel: If you’re buying a plane ticket, consider at least three options and compare them on cost, convenience and emissions before making your choice. You don’t always have to pick the greenest flight — but if you make an effort to think about emissions, you’ll tend to make planet-friendlier choices.

“It’s about making informed decisions, not perfect decisions,” Rutherford said.

Of course, you could choose the greenest option of all, which is not to fly. But that’s not always practical in the United States, which lacks high-speed rail lines or other good alternatives to air travel. So if you’re going to fly, you might as well pick the least-emitting flight you can.

“You’re helping the world, it’s not affecting you very much, and it’s an easy thing to do,” Gillingham said. “We need more of these easy things to do in our lives.”

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