Carbon is among the most abundant substances in the universe; although severely depleted on Earth, it is the primary structural element in biochemistry. Complex interactions between carbon and climate have stabilized the Earth system over geologic time. Since the modern instrumental CO₂ record began in the 1950s, about half of fossil fuel emissions have been sequestered in the oceans and land ecosystems. Ocean uptake of fossil CO₂ is governed by chemistry and circulation. Net land uptake is surprising because it implies a persistent worldwide excess of growth over decay. Land carbon sinks include (a) CO₂ fertilization, (b) nitrogen fertilization, (c) forest regrowth following agricultural abandonment, and (d) boreal warming. Carbon sinks in both land and oceans are threatened by warming and are likely to weaken or even reverse as emissions fall with the potential for amplification of climate change due to the release of previously stored carbon. Fossil CO₂ will persist for centuries and perhaps many millennia after emissions cease.

碳是宇宙中最丰富的物质之一；尽管在地球上已严重耗尽，但它是生物化学中的主要结构元素。碳与气候之间的复杂相互作用使地球系统在地质时间上保持稳定。自 1950 年代现代仪器 CO ₂记录开始以来，大约一半的化石燃料排放已被隔离在海洋和陆地生态系统中。_{海洋对化石 CO 2}的吸收受化学和循环控制。土地净吸收令人惊讶，因为它意味着全球范围内持续增长超过衰退。土地碳汇包括（a）CO ₂施肥，（b）施​​氮肥，（c) 农业废弃后的森林再生，以及 ( d ) 北方变暖。陆地和海洋中的碳汇都受到变暖的威胁，随着排放量的下降，碳汇可能会减弱甚至逆转，并且由于先前储存的碳的释放，可能会加剧气候变化。化石 CO ₂将持续存在几个世纪，甚至在排放停止后可能存在数千年。