Rocks contain an enormous store of carbon in the ancient remains of plants and animals that lived millions of years ago. This means that the "geological carbon cycle" acts as a thermostat that helps to regulate the Earth's temperature.

For instance, during chemical weathering rocks can suck up CO₂ when certain minerals are attacked by the weak acid found in rainwater. This process helps to counteract the continuous CO₂ released by volcanoes around the world, and forms part of Earth's natural carbon cycle that has helped keep the surface habitable to life for a billion years or more.

However, for the first time this new study measured an additional natural process of CO₂ release from rocks to the atmosphere, finding that it is as significant as the CO₂ released from volcanoes around the world. Currently, this process is not included in most models of the natural carbon cycle.

The process occurs when rocks that formed on ancient seafloors (where plants and animals were buried in sediments) are pushed back up to Earth's surface, for example when mountains like the Himalayas or Andes form. This exposes the organic carbon in the rocks to oxygen in the air and water, which can react and release CO₂. This means that weathering rocks could be a source of CO₂, rather than the commonly assumed sink.