Picture Heathrow airport, west of London, on an ordinary day in 2025. About 1,300 planes take off or land. Across the year that was just under 480,000 flights and 84.5 million passengers, more than in any year before. Every one of those planes burns fuel, and the smoke goes up into the air.
A passenger pays for the ticket. The airline pays for the fuel. The airport is paid to land the plane. The smoke is different. It goes into the air that everyone breathes and the sky that everyone lives under. Nobody sends anybody a bill for that.
The ticketpaid
vsvs
The smokeno bill
The plane lands. Who gets sent the bill for the smoke it made on the way?
Nobody. The smoke goes into the air and no bill is written for it.
Right. The ticket, the fuel and the landing all have a price. The smoke has none, which is the whole reason this course exists.
The passenger, inside the ticket price.
Not yet. The ticket pays the airline for the seat and the fuel. No part of it is a bill for the smoke, because nobody is collecting one.
The airport, which has to clean the air.
Not yet. An airport cannot clean the sky, and it is not charged for what the planes put into it. The cost lands on everyone, with no bill attached.
A hundred yearscould it carry on?
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Other peoplewho pays?
People call the question behind this sustainability. The word just means whether a thing could carry on for a hundred years without using up what it needs, and without leaving a cost on other people. A thing can be good business and still fail that test. The smoke is where an airport is tested hardest.
Move the control through the years. Each frame is Heathrow's own count: passengers through the airport, and the carbon from its flights.
80.9 millionpassengers
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20.0 million tonnescarbon from flights
Year: 2019
2019: 80.9 million passengers, the busiest year before Covid, and 20.0 million tonnes of carbon from flights.
61.6 millionpassengers
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14.3 million tonnescarbon from flights
Year: 2022
2022: 61.6 million passengers as travel came back after Covid, and 14.3 million tonnes.
79.2 millionpassengers
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17.9 million tonnescarbon from flights
Year: 2023
2023: 79.2 million passengers and 17.9 million tonnes.
83.9 millionpassengers
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18.5 million tonnescarbon from flights
Year: 2024
2024: 83.9 million passengers, a new record, and 18.5 million tonnes.
84.5 millionpassengers
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18.6 million tonnescarbon from flights
Year: 2025
2025: 84.5 million passengers, another record, and 18.6 million tonnes. Still 7 in 100 below 2019, because newer planes burn less.
Passengers went from 61.6 million in 2022 to 84.5 million in 2025. What did the carbon do?
It fell, because planes are getting cleaner.
Not yet. Newer planes do burn less per seat, and that is why 2025 sits below 2019. But between 2022 and 2025 the extra flights outweighed that, and the carbon rose.
It stayed the same. An airport's carbon does not depend on passengers.
Not yet. It moved from 14.3 to 18.6 million tonnes over those three years. Passengers ride on flights, and flights burn fuel, so the two numbers move together.
It rose with them, from 14.3 to 18.6 million tonnes. More people means more flights, and more flights means more fuel.
Right. Carbon follows fuel, and fuel follows flights. The years with the most passengers are the years with the most carbon, which is why a growing airport is tested hardest.
Move the control.
Carboneveryone
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Noisenext door
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Dirty airnearby
An airport makes three kinds of cost that nobody is billed for. Carbon, which warms the whole planet and lands on everyone. Noise, which lands on the people under the flight paths. Dirty air, which lands on the people living and working nearby. The course takes them in that order: the far cost first, then the costs next door.
Each is a cost of running the airport. Who does it land on?
One card at a time. Tap the pile it belongs to.
Card 1 of 6
The fuel in a plane's tanks
The carbon the fuel turns into
The roar of a plane over a school at 6am
Wages for the people who load the bags
Exhaust from taxis queuing at the terminal
Electricity for the terminal lights
The fuel in a plane's tanks → Someone sends a bill
Not yet. Billed. The airline buys the fuel and pays for it, so this cost has a price and a payer.
The carbon the fuel turns into → Everyone, no bill
Not yet. Everyone, no bill. Carbon spreads through the whole sky and warms the whole planet, and no one is charged for it.
The roar of a plane over a school at 6am → The neighbours, no bill
Not yet. The neighbours, no bill. The noise lands on the people under the flight path, and they are not paid for it.
Wages for the people who load the bags → Someone sends a bill
Not yet. Billed. The handling company pays its staff, so this cost has a price and shows up in the accounts.
Exhaust from taxis queuing at the terminal → The neighbours, no bill
Not yet. The neighbours, no bill. The fumes stay near the airport, in the air people nearby breathe, and no one pays them for it.
Electricity for the terminal lights → Someone sends a bill
Not yet. Billed. The airport pays an electricity bill every month, so the cost is counted and paid.
3Billed
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1Everyone
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2Neighbours
Right. Three had a price and a payer. Three had neither. Sustainability is mostly about the second three: the costs that are real but never billed, so nothing pushes anyone to cut them.
A cost with no bill→Count it→Pick a baseline→Set a target→Report every year
Here is the whole course in one line. A cost nobody is billed for does not go away. It has to be counted some other way, and then somebody has to decide what counts, from which year, and against what target. Heathrow does all of that in public, every year. The next lessons read its count.
A new train line
↓→
Houses beside itshaken
A city opens a new fast train line. Which of these is a cost that nobody is billed for?
The vibration shaking the houses beside the track.
Right. The houses are shaken and nobody pays them. Like a plane's noise, it is real, it lands on neighbours, and no bill is written.
The electricity that runs the trains.
Not yet. The railway pays an electricity bill, so this cost has a price and a payer. It is counted the ordinary way.
The drivers' wages.
Not yet. Wages are paid every month and appear in the accounts. A billed cost is the opposite of what the lesson is about.
Lesson complete
A flight's smoke goes into everyone's air, and nobody is sent a bill for it.