Carbon cycle

The carbon cycle is a complex system of movement and storage. It describes how carbon is recycled and reused throughout Earth's various systems. This cycle is a vital biogeochemical process. It moves carbon between the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere. Without this constant recycling, Earth could not sustain life.

Scientists divide the carbon cycle into two distinct speeds. The fast carbon cycle, or biological carbon cycle, moves carbon within a few years. This involves moving substances from the atmosphere to the biosphere and back. The slow carbon cycle, also called the deep carbon cycle, takes millions of years. It moves carbon through the Earth's crust between rocks, soil, the ocean, and the atmosphere.

The atmosphere contains carbon in two main forms: carbon dioxide and methane. Both gases are part of the greenhouse effect because they absorb and retain heat. While methane produces a larger greenhouse effect per volume, carbon dioxide is more significant. This is because carbon dioxide exists in much higher concentrations. Carbon dioxide is removed from the air primarily through photosynthesis. It also dissolves directly into bodies of water like oceans and lakes.

When carbon dioxide dissolves in water, it reacts to form carbonic acid. This process contributes to ocean acidification, which changes marine chemistry. The ocean is a massive carbon reservoir. The surface layer exchanges carbon rapidly with the atmosphere to maintain equilibrium. However, the deep ocean contains the largest active pool of carbon near the surface. It holds 50 times more carbon than the atmosphere. This deep layer exchanges carbon through thermohaline circulation, a process that takes hundreds of years.

The terrestrial biosphere stores carbon in living organisms and the soil. About 500 gigatons of carbon are stored in living plants and organisms. The soil holds approximately 1,500 gigatons of carbon. Autotrophs, such as plants, extract carbon dioxide from the air to create organic carbon. Heterotrophs receive carbon by consuming other organisms. Carbon leaves the land through respiration or combustion. It can also be washed into rivers or stay sequestered in the soil for thousands of years.

Most of Earth's carbon is stored in the lithosphere, or the Earth's crust. About 80% of this geological carbon is limestone and its derivatives. These form from the calcium carbonate in the shells of marine organisms. The other 20% is stored as kerogens. These form when terrestrial organisms are buried under high heat and pressure. Carbon can return to the atmosphere through volcanic activity. This happens when carbonate rocks are subducted into the mantle and undergo metamorphism.

Humans have significantly disturbed these natural cycles over the last few centuries. We modify land use and mine ancient organic remains like coal, petroleum, and gas. Burning these fossil fuels releases stored carbon into the atmosphere. By 2020, atmospheric carbon dioxide had increased nearly 52% over pre-industrial levels. This increase contributes to global warming and changes the ocean's pH. These human activities alter the balance that once kept carbon levels stable.

In the very distant future, the carbon cycle will change again. As the Sun ages, its increased luminosity will speed up surface weathering. This will likely push more carbon dioxide into the Earth's crust as carbonate. Scientists predict that in about 600 million years, carbon dioxide levels may fall too low for C3 photosynthesis. Eventually, the cycle may end entirely. If oceans evaporate in 1.1 billion years, plate tectonics may stop. Without volcanoes to pump out carbon, the cycle could cease between 1 and 2 billion years from now.