Where does the carbon in your breath go next?

Breathe out, and the carbon dioxide you release joins the carbon cycle. That is the endless movement of carbon between the air, the oceans, the soil, the rocks and living things. A plant could pull that carbon into a leaf within a few years. Or it could end up locked in rock for millions of years.

There are really two cycles, running at very different speeds. The fast one is biological. Plants take in carbon dioxide through photosynthesis. Animals eat the plants. Everything breathes or decays, and the carbon returns to the air within years.

The slow one is geological. Rain turns carbon dioxide into a weak acid that slowly dissolves rock. Rivers carry the dissolved minerals to the sea. There, sea creatures build them into shells of calcium carbonate, which can settle on the seabed and stay buried for millions of years. Over such spans, this slow cycle works like Earth's thermostat: it regulates carbon dioxide, and so temperature.

The ocean is the giant most people forget. Its deep water holds fifty times more actively cycled carbon than the air, but it trades with the atmosphere only over centuries. On land, soils are the big store. They hold more carbon than all the plants and the air put together.

Left alone, these flows roughly balance. Burning fossil fuels tips the balance: it returns carbon that the slow cycle buried long ago. Oceans and plants soak up a lot of it, but not all. Over 2015–2024, about half of all the carbon dioxide emitted stayed in the air. And a real share of the extra carbon is expected to linger for centuries to millennia.