Picture two numbers on one page. What Heathrow put into the air in 2025: 19.4 million tonnes. What was avoided on its behalf: a SAF saving of 621,000 tonnes, printed beside the count but not inside it. Net zero means the first number minus everything taken out equals nothing. Heathrow's date for that is 2050.
Gross: all you put in→Minus what you take out→Equals net→Net zero: the two match
Net is the word that does the work. Gross is everything you put in. Net is gross minus what you take out. A plan can reach net zero two ways: put in less, or take out more. The second way is where the word offset comes in: paying someone else to take out carbon, or to not put it in.
Gross100 tonnes
Taken out100 tonnes
Net0 tonnes
A company puts in 100 tonnes and pays for 100 tonnes of tree planting. What is its net, and what is its gross?
Net zero, gross 100. The smoke still went up; the trees are on the other side of the sum.
Right. Gross is what went up the chimney, unchanged at 100. Net is after the subtraction. Both are true, and a report that only prints net has left out the chimney.
Both 100. Trees do not count.
Not yet. Under the counting rules they can, as an offset, which is what makes net differ from gross. The lesson is about watching which number is which.
Both zero. The trees cancel the smoke.
Not yet. They cancel in the sum, not in the sky. The 100 tonnes went up; gross records that. Net is the number after the trees are counted against it.
Buildings and vanscarbon neutral, 2020
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The flightsnot included
Heathrow has used offsets before. In 2020 it called its buildings and infrastructure carbon neutral: 93 in 100 cut since 1990 through efficiency and green power, the last 7 offset with certified tree planting in Indonesia and Mexico. True, and about its own boilers and lights: a tenth of one in 100 of the footprint. Not the flights.
Move the control through what is put in, with the same 1 million tonnes taken out each year. The figures are made up and round.
Put in1 million tonnes
Taken out1 million tonnes
Net0 million tonnes
Put in, million tonnes: 1
Put in 1 million, take out 1 million: net zero.
Put in2 million tonnes
Taken out1 million tonnes
Net1 million tonnes
Put in, million tonnes: 2
2 in, 1 out: net 1. Doubling the input doubled nothing on the other side.
Put in5 million tonnes
Taken out1 million tonnes
Net4 million tonnes
Put in, million tonnes: 5
5 in, 1 out: net 4.
Put in10 million tonnes
Taken out1 million tonnes
Net9 million tonnes
Put in, million tonnes: 10
10 in, 1 out: net 9.
Put in19 million tonnes
Taken out1 million tonnes
Net18 million tonnes
Put in, million tonnes: 19
19 million in, about Heathrow's gross: net 18. Taking out one million tonnes a year is an enormous project, and it barely dents it.
At Heathrow's gross of 19 million tonnes, how much would have to be taken out each year for net zero, if nothing else changed?
Nineteen million once, then it is done.
Not yet. The gross arrives again next year. Each year's input needs its own removal, forever, unless the input falls.
All 19 million, every year. Taking out is a yearly job, not a one-off.
Right. Net zero needs the two sides to match every year. Nineteen million tonnes removed a year is bigger than any removal project that exists, which is why the plan leans on putting in less.
One million. That is the offset in the frames.
Not yet. One million out against nineteen in is net eighteen. The frames show the offset barely moving the net at Heathrow's size.
Move the control.
Put in lesscounted at the pump
++
SAFcounted over a lifecycle
++
Trees and bogsa model over 30 years
So the sum has three parts, and each has a different kind of proof. Put in less: counted at the fuel pump, firm. SAF: counted over a lifecycle, claimed carefully, kept beside the total until the rules settle. Taken out by trees or bogs: a model of what might grow over thirty years, the least certain of the three.
Each is a number in Heathrow's report. Is it inside the footprint total, or printed beside it?
One card at a time. Tap the pile it belongs to.
Card 1 of 6
The cruise of every departing flight, 17.4 million tonnes
The SAF saving, 621,000 tonnes
Passengers' taxi and train journeys, 398,000 tonnes
The carbon of drilling and refining the jet fuel, 3.9 million tonnes
The supply chain's concrete and steel, 266,000 tonnes
Electricity counted by the grid's real mix, 48,368 tonnes
The cruise of every departing flight, 17.4 million tonnes → Inside the total
Not yet. Inside. Heathrow counts the whole flight out, which is why its total is so large.
The SAF saving, 621,000 tonnes → Printed beside it
Not yet. Beside. The rules on who may claim a SAF tonne are unsettled, so Heathrow prints the saving next to the total, not in it.
Passengers' taxi and train journeys, 398,000 tonnes → Inside the total
Not yet. Inside. Caused by the airport, owned by nobody at it, counted as scope 3.
The carbon of drilling and refining the jet fuel, 3.9 million tonnes → Printed beside it
Not yet. Beside. Making the fuel before it is burned is tracked and printed separately, because the rules have not settled who reports it.
The supply chain's concrete and steel, 266,000 tonnes → Inside the total
Not yet. Inside. What the airport buys to build is scope 3 and has always been in the ground count.
Electricity counted by the grid's real mix, 48,368 tonnes → Printed beside it
Not yet. Beside. The total uses the certificate figure of 3 tonnes; the grid-mix figure is printed alongside so a reader can see both.
3Inside the total
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3Beside it
Right. Three in, three beside. The ones beside are not hidden; they are printed on the same page. Reading a footprint means reading the page, not only the total line.
PwC: four totals, a look→The regulator: the spending→Campaigners: the targets→2050: nobody can check yet
Three kinds of people check. An accountant, PwC, checks four of the totals each year, in what it calls limited assurance: a look, not a full audit. A regulator checks Heathrow's spending on carbon. Campaign groups call the targets small. Nobody checks whether 2050 will arrive, because nobody can. That is the honest state of the count.
Diesel lorriesput in: unchanged
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Taken outbought from whom?
A delivery firm says it is net zero from this year. Its lorries still run on diesel. Which question tells you most about the claim?
Are the lorries electric yet?
Not yet. They are not, and the firm did not claim they were. Net zero can be reached on paper with offsets, which is why the question has to be about both sides of the sum.
Does the firm have a certificate?
Not yet. A certificate is one kind of proof on the taking-out side. It says the firm bought something; it does not say how firm that something is, which is the thing to ask.
What was put in, and what was taken out, and how firm is the proof on each side?
Right. Net zero is a subtraction. With diesel lorries, the input is unchanged, so the whole claim rests on the other side: what was bought, from whom, and how certain the removal is.
Lesson complete
Net zero is a subtraction: what you put in, minus what you take out. Watch both sides.