Carbon moves constantly between the atmosphere, oceans, land, and living things. That movement is the carbon cycle. Plants pull carbon dioxide from the air through photosynthesis. Animals eat the plants and release carbon through respiration. When organisms die, decomposers return carbon to the soil and air. Oceans absorb carbon and store it in deep water and sediments.

The cycle has kept atmospheric carbon roughly stable for thousands of years. That balance is now broken. Burning fossil fuels releases carbon that was locked underground for millions of years. Deforestation removes trees that would have absorbed carbon. The result is a rapid buildup of CO2 in the atmosphere, from about 280 parts per million before industrialization to over 420 today.

Some carbon sinks absorb part of what humans emit. Oceans take up roughly a quarter. Forests and soils take up another quarter. The rest stays in the atmosphere and drives warming. As oceans warm, they absorb less. As forests burn, they release more. These feedbacks could accelerate climate change.

Key reservoirs

• Atmosphere
• Oceans
• Soils and permafrost
• Fossil fuel deposits
• Living biomass