Carbon is distributed in Earth's system components: atmosphere, ocean, land, soils, and vegetation. It cycles in the system between various reservoirs where it is stored for different lengths of time (known as the “residence time” of carbon). Carbon exchanges affect atmospheric CO2 concentration: both land and ocean act as carbon sinks. Natural and anthropogenic processes affect the carbon cycle, which is strongly connected to the climate.

Introduction

Since the birth of the solar system nearly five billion years ago, carbon has been cycling in a closed loop on planet Earth. Within this loop, carbon is exchanged among different pools (e.g., atmosphere, oceans, soils, organic matter, sediments, rocks) where it can reside for various periods of time – in some cases, up to millions of years (e.g., hydrocarbons such as oil). Billions of years ago, this natural carbon cycle had a large variability as Earth oscillated between greenhouse eras (when no ice sheets were present on Earth) and icehouse eras (when ice sheets were present). For millions of years, the carbon cycle essentially has depended on the balance between volcanic CO2 input (source) and CO2 removal by chemical weathering (sink), the intensity of the latter serving as a thermostat ensuring that Earth's climate remains within narrow limits and the planet remains habitable. This balance kept the size of the “natural” atmospheric carbon reservoir relatively constant at about 600 gigatons.