cement production. The other ~20% originate from land use change and deforestation. Because gaseous carbon dioxide does not react quickly with other chemicals, the main processes that change the carbon dioxide content of the atmosphere involve exchanges with the earth's other carbon reservoirs, as explained in the following sections. Interactions with other systems Atmospheric carbon is exchanged quickly between the oceans and the terrestrial biosphere. This means that at times the atmosphere acts as a sink, and at other times as a source of carbon. The following section introduces exchanges between the atmospheric and other components of the global carbon cycle. Terrestrial biosphere Carbon is exchanged with varying speed with the terrestrial biosphere. It is absorbed in the form of carbon dioxide by autotrophs and converted into organic compounds. Carbon is also released from the biosphere into the atmosphere in the course of biological processes. Aerobic respiration converts organic carbon into carbon dioxide and a particular type of anaerobic respiration converts it into methane. After respiration, both carbon dioxide and methane are typically emitted into the atmosphere. Organic carbon is also released into the atmosphere during burning. The residence time of carbon in the terrestrial biosphere varies and is dependent on a large number of factors. The uptake of carbon into the biosphere occurs on various time scales. Carbon is absorbed primarily during plant growth. A pattern of increased carbon uptake is observable both over the course of the day (less carbon is absorbed at night) and over the course of the year (less carbon is absorbed in winter). While organic matter in animals generally decays quickly, releasing much of its carbon into the atmosphere through respiration, carbon stored as dead plant matter can stay in the biosphere for as much as a decade or more. Different plant types of plant matter decay at different rates - for example, woody substances retain their carbon longer than soft, leafy material. Active carbon in soils can stay sequestered for up to a thousand years, while inert carbon in soils can stay sequestered for more than a millennium. Oceans Large amounts of carbon are exchanged each year between the ocean and the atmosphere. A major controlling factor in oceanic-atmospheric carbon exchange is thermohaline circulation. In regions of ocean upwelling, carbon-rich water from the deep ocean comes to the surface and releases carbon into the atmosphere as carbon dioxide. Large amounts of carbon dioxide