40 billion tonnesPoured in each year by burning and clearing
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22 billion tonnesDrained out each year by oceans and plants
The difference stays in the bath.
Think of the carbon dioxide in the air as water in a bath. Each year we pour in about 40 billion tonnes by burning coal, oil and gas and by clearing forests. The oceans and plants drain about 22 billion tonnes back out. The difference stays in the bath.
0 per million238 per million476 per million
19602024
Carbon dioxide in the air, measured on a mountain in Hawaii since 1958.
So the level rises by about 18 billion tonnes a year. In the air that shows as about 2.5 more bits per million each year. The level today is about 420. It was 280 before we began. The bath does not care how fast the tap ran last year. It only counts what is in it.
Poured in40 billion tonnes
Drained out22 billion tonnes
Next year we pour in the same 40 and the drain takes the same 22. What happens to the level?
It rises by 18 again.
Right. Same tap, same drain, same rise. The level is now 36 higher than two years ago. Nothing about the tap staying the same makes the bath stop filling.
It goes down. The drain has had another year to work.
Not yet. The drain works at the same time as the tap, every year. It only wins if it is bigger than the tap, and 22 is smaller than 40.
It stays where it is. We did not add any more than last year.
Not yet. We added exactly as much as last year, and last year the level rose by 18. A tap that stays open fills the bath every year it stays open.
Poured in30 billion tonnes
Drained out22 billion tonnes
Change in the level8 billion tonnes
Now turn the tap down by a quarter, to 30. The drain still takes 22. The level still rises, by 8 a year instead of 18. Emissions fell and the amount in the air still went up. Both are true at once. The level stops rising only when the tap is no bigger than the drain.
Move the control to set how much we pour in each year. The drain is held at 22 here; in real life it shrinks as the tap does, so the real stopping point is lower.
Poured in this year40 billion tonnes
Drained out by oceans and plants22 billion tonnes
Change in the level18 billion tonnes
40 billion tonnes40 in, 22 out. The level rises by 18 billion tonnes this year.
Poured in this year30 billion tonnes
Drained out by oceans and plants22 billion tonnes
Change in the level8 billion tonnes
30 billion tonnes30 in, 22 out. The level rises by 8 billion tonnes this year.
Poured in this year22 billion tonnes
Drained out by oceans and plants22 billion tonnes
Change in the level0 billion tonnes
22 billion tonnes22 in, 22 out. The level holds. It does not fall.
Poured in this year15 billion tonnes
Drained out by oceans and plants22 billion tonnes
Change in the level−7 billion tonnes
15 billion tonnes15 in, 22 out. For the first time the level falls, by 7.
Poured in this year5 billion tonnes
Drained out by oceans and plants22 billion tonnes
Change in the level−17 billion tonnes
5 billion tonnes5 in, 22 out. The level falls, by 17 a year.
At which setting does the amount in the air stop rising?
Only at zero. Anything poured in adds up.
Not yet. The drain is real. Oceans and plants take out about 22 a year today, so a tap that small holds the level in this bath. The real drain does shrink, which is why the true answer sits between this one and 22.
At 30. A cut of a quarter is a big cut.
Not yet. A big cut slows the rise. It does not stop it: 30 in and 22 out is still 8 more in the bath every year. The bath fills more slowly, and fills.
At 22 or less, when the tap is no bigger than the drain.
Right. In this bath, yes. In the real one the drain weakens as the level stops climbing, so the tap has to end up much nearer zero than 22. Either way, a big cut is not the same as a stop.
Move the control.
Emissions in 2020, change on 2019−5.4%
Carbon dioxide in the air in 2020, change on 20190.6%
This is why two headlines can both be true on one day: emissions fell this year, and carbon dioxide in the air hit a new record. One is about the tap. The other is about the bath. In 2020 the tap closed by 5 in 100 and the bath still rose by the usual amount.
Poured in, this year40 billion tonnes
Poured in, next year34 billion tonnes
Drained out, each year22 billion tonnes
This year the tap is 40 and the drain is 22. Next year the tap is cut to 34 and the drain stays 22. How much does the level rise over the two years together?
billion tonnes
Work out each year's rise, then add the two.
Right. 18 the first year and 12 the second. The cut in the second year still added 12. A cut changes how fast the bath fills. Only the total poured in, minus the drain, sets the level.
If every country stopped all emissions tomorrow, how long before the carbon dioxide level in the air came back to where it was in 1750?
Pick one. Nothing is scored.
About half of what we addedgone within a century or so
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The last partthousands of years
Thousands of years.The drain slows as the level falls. Oceans and plants take out roughly half of what we added within a century or so. The last part lingers for thousands of years while rocks slowly wear it away. Nothing we stop doing tomorrow undoes it quickly.
Every claim about carbon is about the tap or about the bath. Which is each of these?
One card at a time. Tap the pile it belongs to.
Card 1 of 6
Global emissions fell 5% in 2020.
Carbon dioxide passed 420 parts per million.
A country promises to cut its emissions by half by 2030.
The level in the air has risen every single year since 1958.
Airlines put out about 900 million tonnes this year.
The air holds half again as much carbon dioxide as in 1750.
Global emissions fell 5% in 2020. → The tap
Not yet. This is the tap. Emissions in a year is how much was poured in that year, and pouring less is the tap turned down.
Carbon dioxide passed 420 parts per million. → The bath
Not yet. This is the bath. Parts per million is how much is already in the air, the level of the water.
A country promises to cut its emissions by half by 2030. → The tap
Not yet. This is the tap. A promise about emissions is a promise about how much will be poured in each year.
The level in the air has risen every single year since 1958. → The bath
Not yet. This is the bath. A level rising every year is the water climbing, even in years the tap was turned down.
Airlines put out about 900 million tonnes this year. → The tap
Not yet. This is the tap. Tonnes put out this year is a flow into the bath, not the amount already in it.
The air holds half again as much carbon dioxide as in 1750. → The bath
Not yet. This is the bath. Comparing today's level with 1750's is comparing two water lines, not two flows.
The tapwhat we pour in this year
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The bathwhat is already in the air
Right. A rate for a year is the tap. A level, or a comparison of levels, is the bath. The tap can fall while the bath keeps rising, and in 2020 it did.
Lesson complete
Carbon dioxide in the air is a level. Emissions are the tap. A smaller tap still fills the bath.