What is the carbon cycle?
Carbon is the backbone of every molecule of life: sugars, fats, proteins and DNA. The carbon cycle is the movement of carbon between the atmosphere, oceans, living things, soils and rocks. Over thousands of years the flows in and out of each store have roughly balanced, keeping the amount of carbon dioxide in the air, and so Earth’s climate, fairly steady.
Carbon stores
| Store | Approximate carbon (GtC) |
|---|---|
| Atmosphere (pre-industrial / today) | 590 / about 890 |
| Plants | 450 |
| Soils | 1,500–2,400 |
| Surface ocean | 900 |
| Deep ocean | 37,000 |
| Fossil fuel reserves | roughly 1,000–2,000 |
| Rocks (limestone and other sediments) | over 60,000,000 |
The processes
- Photosynthesis takes about 120 GtC a year out of the air into plants.
- Respiration by plants, animals and microbes returns CO₂ to the air.
- Decomposition of dead material by fungi and bacteria releases carbon from soils.
- Feeding passes carbon along food chains.
- Diffusion moves CO₂ between the air and the sea surface; ocean currents and sinking plankton carry it to the deep ocean.
- Combustion of fossil fuels and forests releases carbon to the air.
The fast and slow cycles
The fast carbon cycle moves carbon between air, life, soils and the surface ocean over years to centuries. The slow carbon cycle works over millions of years: carbon is locked into limestone and fossil fuels, and returned by volcanoes and the weathering of rocks. Burning fossil fuels short-circuits the slow cycle, moving carbon that took millions of years to bury back into the air within a few human lifetimes.
How humans are changing it
Since 1850, burning fossil fuels and clearing forests has added about 700 GtC to the atmosphere. Atmospheric CO₂ has risen from about 280 parts per million (ppm) to over 420 ppm, higher than at any time in at least 800,000 years. Carbon dioxide is a greenhouse gas: it absorbs heat radiated by Earth’s surface, so more CO₂ warms the planet. Each doubling of CO₂ is expected to warm Earth by roughly 3 °C in the long run.
Ocean and land sinks
Fortunately, not all emitted carbon stays in the air. The ocean absorbs about a quarter, and land plants another quarter or so, partly because extra CO₂ boosts photosynthesis. But the ocean becomes more acidic as it absorbs CO₂, and its ability to take up carbon weakens as the surface water fills up: chemists call this the Revelle buffer factor. The deep ocean could store much more, but mixing down takes centuries.
Bringing it back into balance
To stop CO₂ from rising, human emissions must fall to the point where they no longer exceed what the ocean and land absorb: net zero. Ways to get there include renewable energy, energy efficiency, protecting and restoring forests, peatlands and wetlands, and technologies that capture carbon. Try the scenarios to see how much difference the timing makes.
Key terms
- Carbon sink / source
- A store that takes in more carbon than it releases / one that releases more than it takes in.
- GtC
- Gigatonnes of carbon: a billion tonnes. 1 GtC in the air is about 0.47 ppm of CO₂.
- Ocean acidification
- The fall in seawater pH caused by absorbing CO₂.
Common misconceptions
- “Plants make carbon.” Carbon is never created; plants take it from CO₂ in the air.
- “CO₂ disappears quickly once emitted.” Much of it stays in the air-ocean system for centuries.
- “Planting trees alone can solve climate change.” Forests help, but cannot absorb carbon as fast as fossil fuels release it.
Key takeaways
- Carbon moves between air, life, soils, oceans and rocks through photosynthesis, respiration, decomposition, diffusion and combustion.
- Burning fossil fuels moves ancient carbon into the air far faster than natural processes remove it.
- About half of human emissions stay in the atmosphere; the rest goes into the ocean and land.
- Reaching net zero emissions stops atmospheric CO₂ from rising.
Model note: a simplified five-box model calibrated to reproduce the observed rise in CO₂. It leaves out climate feedbacks such as warmer oceans absorbing less carbon, so future values are indicative rather than forecasts. pH and warming are approximate formulas based on CO₂ only.