Carbon is the tank everyone talks about. It is not the only one. The planet has several: fresh water, forests, the fish, the ozone layer, the nitrogen that farms put on fields. Each has its own tap and its own drain, and each can be run too fast.
6 of 9
of nine lines crossed, 2023
In 2009 a group of scientists drew nine lines, one for each tank, and called them planetary boundaries. A boundary is their estimate of how far a tank can be drawn down and still keep going. In 2023 they re-counted and found six of the nine crossed.
6 of 9
crossed
How many of the nine lines were crossed in the 2023 count?
Six.
Right. Climate is one. The others are the living world, land turned from forest to farms, fresh water, fertiliser (nitrogen and phosphorus) washing off fields, and new man-made chemicals and plastics.
All nine.
Not yet. Three were still inside their line in 2023: the ozone layer, dust and haze in the air, and the acid in the sea. The last of those crossed in the 2025 count.
Three, the ones in the news.
Not yet. The news reports the carbon tank. Most of the crossed lines are tanks that rarely make a headline, like fertiliser runoff and fresh water.
Four of the nine tanks. Tap the one that was still inside its line in 2023.
Tap the part of the picture that answers it.
The ozone layer
++
Fresh water
++
Fertiliser runoff
++
Forests
The ozone layer
Right. Inside the line. The hole over Antarctica still crosses it each spring, but the layer as a whole is refilling, which lesson 8 explains. A reading can move back.
Fresh water
Not yet. Crossed. Rivers run lower and soils dry where people take more than the rain returns, on enough of the land to cross the 2023 line.
Fertiliser runoff
Not yet. Crossed. Nitrogen is at about three times its line, phosphorus about twice. Both go on to fields faster than the land can use them.
Forests
Not yet. Crossed. The 2023 count says too much forest has been turned into farmland for the land tank to hold.
Fertiliser on the field→Runs off into the river→Algae bloom→Oxygen gone
Take the fertiliser tank. Nitrogen feeds crops. The share the plant does not take washes into rivers and the sea, where it feeds algae instead. The algae die and rot, and the rotting uses up the water's oxygen. Fish leave or die. The boundary is a count of nitrogen put on fields in a year.
Picture one field. Move how much fertiliser goes on and watch two things: the crop, and the nitrogen that washes off. The numbers are made up to show the shape.
Crop10 tonnes, say
Washed off1 tonne, say
Fertiliser on the field: Low
Low. The crop is small. Almost nothing washes off; the plants take nearly all of it.
Crop16 tonnes, say
Washed off3 tonnes, say
Fertiliser on the field: Medium
Medium. The crop has grown a lot. A little more washes off.
Crop18 tonnes, say
Washed off8 tonnes, say
Fertiliser on the field: High
High. The crop has barely grown since medium. The runoff has nearly tripled.
Crop18.5 tonnes, say
Washed off15 tonnes, say
Fertiliser on the field: Very high
Very high. The crop has stopped growing. The field cannot use any more, and nearly all the extra goes to the river.
What happens past the medium setting?
Runoff only begins at very high.
Not yet. Some washed off at every setting. What climbs is the share: at low nearly all is used, at very high nearly all is lost.
The crop barely grows and the runoff keeps climbing.
Right. A plant takes what it can use. Past that, every extra bag is a bag for the river. The gain flattens and the cost does not.
Both keep rising together.
Not yet. The crop rose from low to medium and then stopped. The runoff did not stop. They move together only at the start.
Move the control.
1987: the drain shut→Thinning to the mid-1990s→Since: slowly refilling→About 2040: back
The ozone tank is the one refilling. A rule in 1987 began shutting the drain. The layer kept thinning until the mid-1990s and has slowly refilled since. Over Antarctica the hole still crosses the line every spring; the layer as a whole has stayed inside its line. A line is not a verdict; it is a reading, and readings move.
Each card is one of the nine tanks. In the 2023 count, was it past its line, or inside it?
One card at a time. Tap the pile it belongs to.
Card 1 of 6
Carbon in the air
The ozone layer
Fresh water taken from rivers and soil
Acid in the sea
Dust and haze in the air
Forest turned into farmland
Carbon in the air → Past the line
Not yet. Past. The air holds more carbon dioxide than the boundary allows, and the count has risen every year since the lines were first drawn.
The ozone layer → Inside the line
Not yet. Inside. The one tank that is refilling, after the 1987 rule shut its drain; only the hole over Antarctica still crosses the line, each spring.
Fresh water taken from rivers and soil → Past the line
Not yet. Past. Added to the crossed list in 2023, when the count was widened to include water in the soil, not only in rivers.
Acid in the sea → Inside the line
Not yet. Inside, in 2023, but close to the line. The 2025 re-count put it across. The sea takes up carbon from the air and turns slightly more acid as it does.
Dust and haze in the air → Inside the line
Not yet. Inside. Smoke and soot are bad for lungs and change the rain, but at the world scale the 2023 count put this tank inside its line.
Forest turned into farmland → Past the line
Not yet. Past. The line is a share of the world's original forest still standing, and the count says too much has gone.
3Past the line
vsvs
3Inside
Right. Six tanks on the cards, three past and three inside. The ozone layer is inside and refilling; the sea's acid was inside and has since crossed. The lines are readings, taken again every few years.
Carbonzero
vsvs
Forestzero
Picture a country that cuts its carbon to zero and its forests to zero. Under the nine lines, is it sustainable?
Yes. Carbon is the one that matters.
Not yet. Carbon is one of nine. The forest tank has its own line, and a country with no forest has crossed it as far as it can be crossed.
Yes, because trees grow back.
Not yet. They do, at the forest's speed, which is decades. Lesson 2: the tap exists and has a size. A chainsaw is faster than the tap.
No. It fixed one tank and emptied another.
Right. Nine lines mean nine tanks. Filling one by draining another moves the problem, and the forest line is one of the six already crossed.
Lesson complete
The planet has nine tanks with nine lines. Six were counted crossed in 2023, a seventh in 2025.