Picture Heathrow with a third runway. Its 2025 plan would take the airport from about 480,000 flights a year to 756,000, and from 84 million passengers to 150 million. Runway and airfield: £21 billion; the whole plan £49 billion. The government set 2035 for the first flights; in September 2026 it was reported the airport now expects 2039.
Number of flights→times carbon per flight→equals total carbon
The carbon from flights is a multiplication: the number of flights, times the carbon of each one. Chapter 2 so far has been about the second number. A third runway changes the first. If flights rise faster than each flight's carbon falls, the total goes up, however good the planes.
Flights today480 thousand a year
With a third runway756 thousand a year
756,000 flights against 480,000. By how much do flights grow? Give it in 100.
in 100
Divide 756 by 480.
Right. Fifty-seven or fifty-eight in 100 more flights. The 2030 target cuts total flight carbon by 15 in 100, and with flights near 2019's level that is about 15 per flight. The two numbers are not close in size.
Move the control through the number of flights. Each flight's carbon is set 15 in 100 below 2019, which is what the 2030 target amounts to per flight at today's flight numbers. The total is given against 2019 = 100.
2019 total100
Total at this many flights85
Flights a year: 480,000
480,000 flights, about today's number: 85. The target cut, delivered.
2019 total100
Total at this many flights97
Flights a year: 550,000
550,000 flights: 97. Nearly back to the 2019 level.
2019 total100
Total at this many flights110
Flights a year: 620,000
620,000 flights: 110. Above 2019, with every flight 15 in 100 cleaner.
2019 total100
Total at this many flights122
Flights a year: 690,000
690,000 flights: 122.
2019 total100
Total at this many flights134
Flights a year: 756,000
756,000 flights, the full third runway: 134. A third above the baseline year, on the target's own cleaner planes.
At 756,000 flights with the 2030 per-flight cut, where does the total land against 2019?
Fifteen in 100 lower. The target is the target.
Not yet. The target is 15 in 100 off the total, set with flights capped at 480,000. Fifteen in 100 off each of 756,000 flights is still far more carbon than 480,000 flights made in 2019.
About level. The two roughly cancel.
Not yet. They would cancel at about 565,000 flights. At 756,000 the growth is nearly four times the size of the cut.
About a third higher. The growth in flights is bigger than the cut per flight.
Right. Flights up 58 in 100, carbon per flight down 15. Multiply and the total is 134 against 100. The cut is real and the total still rises.
Move the control.
SAF to 22 in 100by 2040
++
Enough for a third more flights?nobody knows
Heathrow's answer is more of lessons 5 to 7: more SAF, newer planes, and the industry's goal of net zero flying by 2050, which Heathrow has signed up to. The UK rule reaches 22 in 100 SAF by 2040. Whether fuel and removals can cover a third more flights by 2050 is a question nobody can yet answer.
Each is a claim about the third runway. Is it a count we have today, or something nobody can yet count?
One card at a time. Tap the pile it belongs to.
Card 1 of 6
Flights would rise from 480,000 to 756,000
The runway and airfield are priced at £21 billion
SAF will be 22 in 100 of UK fuel by 2040
In 2025 flight carbon was 7 in 100 below 2019
Flights from Heathrow will be net zero by 2050
Enough carbon can be taken out of the air to cover the rest
Flights would rise from 480,000 to 756,000 → Counted today
Not yet. Counted today. It is the plan's own number, on paper now.
The runway and airfield are priced at £21 billion → Counted today
Not yet. Counted today, by Heathrow in its 2025 plan, up from £14 billion in 2018.
SAF will be 22 in 100 of UK fuel by 2040 → Nobody can count yet
Not yet. Nobody can count yet. It is a rule, not a supply; the factories to make most of it are not built.
In 2025 flight carbon was 7 in 100 below 2019 → Counted today
Not yet. Counted today, in Heathrow's own data book, checked by its accountants.
Flights from Heathrow will be net zero by 2050 → Nobody can count yet
Not yet. Nobody can count yet. It depends on fuel, planes and removals that do not exist at that scale.
Enough carbon can be taken out of the air to cover the rest → Nobody can count yet
Not yet. Nobody can count yet. Removing carbon at that scale is an idea with small trials behind it, not a count.
3Counted today
vsvs
3Not yet
Right. Three are counts on paper today. Three are promises about 2040 and 2050 that nobody can count yet. A plan is made of both, and it helps to know which sentence is which.
Slide to your guess. If flights grow 58 in 100 and each flight's carbon is cut 15 in 100, how much must each flight be cut instead for the total to stay level?
Slide the marker to where you think the answer is, then press Lock. Nothing is scored.
12
your guess · 0 in 100 per flight
the answer · 36.5 in 100 per flight
03060
in 100 per flight
115, the 2030 target15 in 100 per flight
2Half50 in 100 per flight
Your guess: —
About 36 or 37 in 100 off every flight, more than twice the 2030 target, just to stand still. Guesses tend to land near the target itself, because the target feels like the answer. It is the answer to a different question, with today's flights.
Cleaner per year1 in 100
Runway growth58 in 100
Growth has eaten savings before. Heathrow's flights have risen since the 1970s while its noise footprint shrank, because planes got quieter faster than they got more numerous. For carbon, the planes have been getting cleaner at about 1 in 100 a year. A runway adds 58 in 100 at once.
each 20 in 100 cleanerTwice the buses
↓→
up 60 in 100Total
A city doubles its bus fleet with buses that burn 20 in 100 less fuel each. A councillor says bus carbon will fall. What does the multiplication say?
It rises. Twice the buses at 80 in 100 each is 160 against 100.
Right. Two times 0.8 is 1.6. Each bus is cleaner and there are twice as many; the total is 60 in 100 higher. Whether that is worth it depends on the cars they replace, which is a different count.
It falls 20 in 100, as the councillor says.
Not yet. That is the per-bus figure. The fleet doubled, so the per-bus cut is applied to twice as many buses, and the total rises.
It stays level. Doubling and cutting cancel.
Not yet. Doubling is plus 100 in 100; the cut is minus 20. They would cancel only if each bus burned half as much, not a fifth less.
Lesson complete
Total carbon is flights times carbon per flight. Grow one faster than you cut the other, and it rises.