Where carbon goes
Carbon moves between air, ocean, plants and rock. Learn why burning fossil fuels overloads that cycle, and why emitting a little less still means CO₂ keeps rising.
THE BIG LOOP
Carbon is never used up, only moved
Carbon sits in four big stores: the air, the ocean, land (plants and soils) and rocks, which include coal, oil and gas. It flows between them all the time. Plants pull CO₂ out of the air to build leaves and wood; animals, microbes and fires put it back. The ocean breathes it in and out at its surface.
The carbon in the wheat of your sangak bread was CO₂ in the air a few months ago. Digest it, and within hours some of it is CO₂ in your breath again.
Two speeds of the same cycle
Fast cycle
Air, plants, soils and the surface ocean. Carbon moves in months to centuries: a season of growth, a fallen leaf, a forest fire. Big flows, but they roughly balance on their own.
Slow cycle
Rocks, deep sediments and fossil fuels. Carbon moves in millions of years, through volcanoes and the slow weathering of rock. Fossil fuels are ancient plant carbon, locked away there.
Check yourself
Sina looks at CO₂ measurements taken since 1958 on Mauna Loa in Hawaii. On top of the steady rise, CO₂ dips every northern summer and climbs again every winter. What causes the yearly wiggle?
- Northern plants grow in summer, taking up CO₂, and release it as leaves rot
- Factories slow down over the summer holidays and speed up again when winter arrives
- The instrument drifts with the seasons and has to be recalibrated
- Warm summer seas absorb more CO₂ than cold winter seas, then give it back in winter
Show the answer
Northern plants grow in summer, taking up CO₂, and release it as leaves rot
Right. It's the planet breathing: the fast cycle in action. Most land, and so most plants, is in the north, so their growing season shows up in the global air.
THE SHORTCUT
Fossil fuels move slow carbon into the fast cycle
Burning coal, oil and gas takes carbon that the slow cycle locked away over millions of years and releases it into the air in decades. Together with clearing forests, humans now add roughly 40 billion tonnes of CO₂ a year. The fast cycle can't absorb it all, so it builds up: from about 280 ppm before industrialisation to above 420 ppm by the mid-2020s, still rising.
Ice cores from Antarctica trap tiny bubbles of old air. They show CO₂ is now higher than at any time in the 800,000 years those cores cover. And all the world's volcanoes together emit roughly a hundredth of what humans do.
A bathtub with the tap on
The CO₂ in the air is the water in a tub. Our emissions are the tap; the ocean and land soaking up carbon are the drain. Right now the tap runs about twice as fast as the drain, so the level rises. Turn the tap down a little and the level still rises, just more slowly. It only stops rising when the tap is turned down to about what the drain can take. Where the analogy breaks: this drain isn't fixed. As the planet warms, the ocean and land may soak up a smaller share (more in lesson 8).
Check yourself
Leila reads that global CO₂ emissions fell by 10% one year. She concludes that the amount of CO₂ in the air must have fallen that year.
Show the answer
False
False. Emissions are the tap, not the water level. A 10% smaller flow into the tub is still far more than the drain removes, so CO₂ in the air keeps rising, only a little more slowly. That's exactly what happened in 2020: emissions dropped during the pandemic, and CO₂ still reached a new high.
Where each year's CO₂ goes (rounded)
| Destination | Share |
|---|---|
| Stays in the air | about half |
| Absorbed by the ocean | about a quarter |
| Taken up by plants and soils | about a quarter |
THE OCEAN'S PRICE
Ocean acidification
The ocean's help isn't free. CO₂ dissolving in seawater forms a weak acid. Since pre-industrial times, the surface ocean's pH has fallen from about 8.2 to about 8.1. The sea is still slightly alkaline, but the pH scale is logarithmic, so that small step means roughly 30% more acidity.
Corals, oysters, mussels and some tiny plankton build shells and skeletons from calcium carbonate. More acidic water makes that harder, which matters for reefs and for the fisheries that depend on them.
Check yourself
Does it move carbon into the air or out of it?
- Heating a house with natural gas
- A young forest growing on abandoned farmland
- A forest fire
- Seawater dissolving CO₂ at the surface
- Clearing and burning rainforest for pasture
- Rock slowly weathering over thousands of years
Show the answer
Into the air: Heating a house with natural gas, A forest fire, Clearing and burning rainforest for pasture
Out of the air: A young forest growing on abandoned farmland, Seawater dissolving CO₂ at the surface, Rock slowly weathering over thousands of years
Check yourself
Nazanin's cousin insists the extra CO₂ comes from volcanoes, not people. Which piece of evidence answers him best?
- CO₂ levels are measurably higher in big crowded cities than out in the countryside
- The last few years have been some of the hottest on record
- Volcanoes erupted less often in the last decade than the one before
- The added carbon has fossil fuels' fingerprint: no carbon-14, extra carbon-12
Show the answer
The added carbon has fossil fuels' fingerprint: no carbon-14, extra carbon-12
Yes. The isotope fingerprint points at old plant carbon, which is what coal, oil and gas are. Add that humans emit roughly 100 times more than all volcanoes, and the case is closed.
What this means for you
- CO₂ in the air is a stock; emissions are a flow. Slowing the flow slows the rise; it takes deep cuts to stop it.
- To stop warming altogether, scientists say CO₂ emissions need to reach net zero: whatever we still emit is matched by what's removed.
- Forests help, but carbon in trees can go back up in a fire or a felling. They can't safely hold all the carbon we dig up.
- Leaving fossil carbon in the ground is the surest way to keep it out of the fast cycle for good.
Check yourself
Match each piece of carbon to roughly how long it stays put
Show the answer
- A leaf that falls in autumn → Back in the air within a year or two
- Wood in a growing forest → Decades to centuries, unless it burns
- CO₂ carried into the deep ocean → Centuries or longer
- Coal under the ground → Millions of years, unless we burn it
Lesson recap
- Carbon moves between air, ocean, land and rock: a fast cycle of months to centuries and a slow one of millions of years.
- Fossil fuels shortcut slow carbon into the fast cycle; CO₂ has risen from about 280 ppm to above 420 ppm.
- About half of each year's CO₂ stays in the air; the ocean and land take up the rest, and the ocean grows more acidic.
- CO₂ in the air is a stock: small cuts only slow its rise, and halting warming needs net zero CO₂ emissions.
- The isotope fingerprint shows the extra carbon comes from fossil fuels, not volcanoes.