Lesson 06 · 3 min · 5 things to do
Carbon is a stock, not a flow
Explain why falling emissions still means a rising total.
- Flowing in each year40 unitsDraining out each year10 unitsAdded to the total30 units
The bath fills whenever the tap beats the drain. Turning the tap down is not turning it off. A country cuts its annual emissions by 30%. What happens to the amount of carbon dioxide in the atmosphere?
- Yes.Emissions are a flow into a bath that barely drains. Cutting the flow slows the filling; only stopping it stops the level rising.
- Not quite.That would need the atmosphere to empty at the rate we stopped adding. Most of what is up there stays for a very long time.
- Not quite.That needs emissions to reach roughly zero, not 30% lower. Anything above the drain rate keeps adding.
Emissions are a flow. The atmosphere holds a stock. Almost every confused argument about climate is one of these mistaken for the other.
Slide the annual emissions and watch the total in the atmosphere over thirty years.
UnchangedHalved by year 30Net zero by year 30UnchangedThe total climbs steeply and steadily. Nothing about the flow is falling.
Halved by year 30The climb slows and does not stop. The total is still higher every year.
Net zero by year 30The line flattens at the end. It flattens high, because everything added on the way is still there.
Even in the best frame the line never comes back down. Why?
- Yes.This is why the target is called net zero rather than 'reduce emissions'. Stabilising the level requires the flow to reach roughly nothing — and lowering it requires taking carbon back out.
- Not quite.The shape follows from arithmetic. A bath with the tap off and a very slow drain empties very slowly.
- Not quite.The third frame reaches zero by year 30, and the line still only flattens.
Move the control to see what changes.
Which of these statements are about the flow, and which about the stock?
"Emissions fell 4% last year."
"Concentration passed 420 parts per million."
"This country emits 300 million tonnes a year."
"Warming is driven by the total accumulated so far."
"New car sales are now half electric."
Yes.News reports flows because flows change every year and make headlines. Temperature responds to the stock — which is why a year of good news on emissions changes the physical situation so little.A bath receives 40 units a year and drains 10. Emissions are cut to 20 units a year. How many units are still added each year?
unitsYes.20 in, 10 out. A halving of emissions still fills the bath — at a third of the old speed, and in the same direction.Why does this matter for reading energy news?
- Yes.A record year for renewables, a fall in coal, a country's emissions dropping — all real, all flows. The bath level is what the physics responds to, and it only appears in stories about concentration.
- Not quite.The figures are usually accurate. What goes missing is which of the two quantities they describe.
- Not quite.Flows are the only thing anyone can change. They matter enormously — as the tap, not as the level.
Lesson complete
Emissions are the tap. The atmosphere is the bath.
Next: The last tenth is the hard part →