Studies of the carbon cycle are aimed at understanding how the atmospheric carbon content is regulated by geological and biological processes. Over the past century, fossil fuel burning has overwhelmed natural processes, quickly transferring a large amount of buried carbon (in the form of organic matter, petroleum, coal, and natural gas in sedimentary rock formations) into the atmosphere as CO2. On longer timescales, natural processes (e.g., volcanism, subduction, chemical weathering, sedimentation, metamorphism, glaciation, wildfires) also shift carbon between the atmosphere, oceans, sedimentary formations, soils, plants, and deep interior. These processes produce cycles of increasing and decreasing atmospheric CO2 that occur over timescales of thousands, millions, and billions of years. At each timescale, different processes are primarily responsible for the changes.
Over the past century and through the next, changes in the greenhouse gas content of the atmosphere are the most important factor affecting climate, although changes in atmospheric particulates and clouds are also important. Burning coal, oil, and natural gas continues to add greenhouse gases and aerosols to the atmosphere, reducing emissions of infrared radiation to space and causing Earth’s global mean surface temperature to rise. The amount of increase depends on feedbacks in the climate system, especially the (poorly known) feedback from clouds. On even shorter timescales (years to decades), changes in atmospheric particle loading, notably sulfate aerosols, can affect climate, in