Picture three things taken from the ground on the same day: a tree, a bucket of groundwater, a barrel of oil. Come back in fifty years. A new tree stands where the old one was. Some water has soaked back in. The oil has not moved at all.
Sunlighttomorrow
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A treedecades
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Coal300 million years
A thing that comes back on a human clock is called renewable: sunlight tomorrow, a tree in a few decades, a fish stock in a few years. Coal and oil come back too, but so slowly that nobody alive will see it. Much of the world's coal formed about 300 million years ago.
In, a year0 barrels, say
Out, a year1 barrel, say
Is there a rate at which using oil is sustainable?
No. Its tap is shut, so any drain empties it in the end.
Right. With nothing coming in, the only question is when. A slow drain gives a later date, not a different answer.
Yes. New oil forms all the time.
Not yet. It does, over millions of years. A tap that drips once in a million years is no tap on any clock a person lives by.
Yes, if we use it slowly enough.
Not yet. Slow is still a drain on a tank with no tap. The level drops by less each year and never stops dropping.
Groundwateras fast as rain soaks in
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A fish stockat the fish's pace
Most real tanks sit in between. Groundwater refills, but only as fast as rain soaks through rock. Soil forms, but a few millimetres a century. A fish stock breeds, at the fish's own pace. Renewable does not mean endless. It means there is a tap, and the tap has a size.
Each card is something people take. Does it come back on a human clock, or not?
One card at a time. Tap the pile it belongs to.
Card 1 of 6
Sunlight on a solar panel
Timber from a planted forest
Coal from a mine
Wind turning a turbine
Water from a deep desert aquifer, filled by rain that fell thousands of years ago
Natural gas from under the sea
Sunlight on a solar panel → Comes back
Not yet. Comes back. The Sun fills that tank again every morning, and nothing a panel takes makes tomorrow dimmer.
Timber from a planted forest → Comes back
Not yet. Comes back. Trees grow, which is a tap. It runs at decades per tree, which is a size.
Coal from a mine → Never comes back
Not yet. Never. The tap that made coal ran about 300 million years ago. The seam has whatever it has.
Wind turning a turbine → Comes back
Not yet. Comes back. Wind is the air moving because the Sun heated it unevenly, and the Sun does that daily.
Water from a deep desert aquifer, filled by rain that fell thousands of years ago → Never comes back
Not yet. Never, on a human clock. There is a tap, but it is so slow that the water counts as a stock, not a flow. Pump it and it is gone.
Natural gas from under the sea → Never comes back
Not yet. Never. Gas formed the way coal and oil did, from life buried for millions of years. Nothing is refilling the field.
3Comes back
vsvs
3No tap
Right. Three have a tap that runs on a human clock and three do not. The desert aquifer is the trap: it is water, and water feels renewable, but the tap filling it is slower than a human life.
Grows, a year200 trees
Cut, a year300 trees
A renewable tank can still be emptied. Cut a forest faster than it grows and it goes the way oil goes, only slower. The question is the same for every tank: how big is the drain against the tap.
Picture a forest of 10,000 trees where 200 grow to full size each year. Move how many are cut and watch what stands after fifty years. The numbers are arithmetic on that rule.
Start10,000 trees
After 50 years15,000 trees
Trees cut a year: 100
Cut 100, grows 200. A hundred more grow than are cut. After fifty years the forest is 15,000 trees, bigger than it started.
Start10,000 trees
After 50 years10,000 trees
Trees cut a year: 200
Cut 200, grows 200. In equals out. Fifty years on, the forest is exactly the size it was.
Start10,000 trees
After 50 years5,000 trees
Trees cut a year: 300
Cut 300, grows 200. A hundred more are cut than grow. After fifty years, 5,000 trees stand.
Start10,000 trees
After 50 years0 trees
Trees cut a year: 400
Cut 400, grows 200. Two hundred more are cut than grow. In year fifty the last tree goes.
At 300 trees cut a year, is this timber renewable?
No. Once a forest shrinks, it has become non-renewable.
Not yet. Shrinking is a tank being drained. Non-renewable is a tank with no tap. The forest still has its tap; the drain is just bigger.
The tree is renewable. This forest is being used past its tap, so it shrinks.
Right. Renewable says there is a tap. It does not say the tap is bigger than the drain. At 300 the drain wins, and a renewable thing is being used up.
Yes, so the number cut does not matter.
Not yet. The number cut is the drain, and the drain is the whole question. Renewable is permission to use the tap, not the tank.
Move the control.
19005 in 100 of the land
202413.5 in 100 of the land
Britain did this to itself. By 1900 about 5 in 100 of its land was woodland, after centuries of cutting for ships, fuel and fields. Then the cutting slowed and planting started. By 2024 woodland covered about 13 or 14 in 100 of the UK. A renewable tank refills if the drain is allowed to fall below the tap.
Slide the marker to the share of the UK's land that is woodland today.
Slide the marker to where you think the answer is, then press Lock. Nothing is scored.
123
your guess · 0%
the answer · 13.5%
0%25%50%
1None0%
2A quarter25%
3Half50%
Your guess: —
About 13 or 14 in 100. Guesses land higher, because a country with a word for its forests feels forested. It is one of the least wooded countries in Europe, and well over twice as wooded as it was in 1900.
A phonecopper inside
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A minethe only tap?
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Recyclingback round
A phone has copper in it. Copper is dug from the ground. A friend says copper is renewable because it can be recycled. Who is right?
Recycling is a tap, so copper is renewable.
Not yet. A tap adds new copper to the world. Recycling moves copper that was already taken out. The ground has the same amount either way.
Copper is renewable because mines keep finding more.
Not yet. Finding more is measuring the tank. It tells you the tank was bigger than you thought, not that anything is filling it.
Copper never comes back. Recycling slows the drain; it is not a tap.
Right. Recycling sends the same copper round again, so less new copper is dug. The tank in the ground is still only draining. Nothing is putting copper back into it.
Lesson complete
Renewable means there is a tap, and the tap has a size. No tap on a human clock means never.