Carbon dioxide (CO2) is a naturally occurring atmospheric gas that traps heat, maintaining Earth’s habitable temperature. While CO2 levels have varied throughout history, the planet possesses natural mechanisms that actively remove this gas from the atmosphere. These processes operate over different timescales, from rapid biological cycles to slow geological transformations, regulating atmospheric CO2 concentrations and preventing unchecked accumulation.
The Ocean’s Role in Carbon Removal
The oceans are a major carbon reservoir, absorbing atmospheric carbon dioxide through physical and biological processes. The physical pump involves CO2 dissolving directly into surface waters, a process more efficient in colder regions. Cooler surface waters absorb more CO2.
These cooler, CO2-rich waters become denser and sink, initiating the thermohaline circulation. This deep ocean current system transports dissolved carbon into abyssal depths, sequestering it from atmospheric exchange for hundreds to thousands of years.
The biological pump, driven by marine organisms, complements this physical process. Microscopic phytoplankton absorb CO2 from surface waters for photosynthesis. When these organisms are consumed or die, their organic remains sink through the water column.
Some sinking organic matter decomposes, releasing carbon into deep ocean waters, while a portion reaches the seafloor. Over geological timescales, this material can be buried, forming sedimentary rocks and locking away carbon for millions of years. Marine organisms also form calcium carbonate shells and skeletons, contributing to carbonate sediment formation on the ocean floor, further removing carbon.
Terrestrial Ecosystems as Carbon Sinks
Land-based ecosystems are carbon sinks, mainly through plants and soils. Plants, including forests, absorb atmospheric CO2 during photosynthesis, converting it into organic compounds that form their biomass. This process removes carbon from the air, storing it in trees, shrubs, and grasses.
Forests are significant carbon reservoirs, with mature trees holding carbon in their trunks, branches, and roots. This carbon storage remains as long as ecosystems are healthy and undisturbed, regulating atmospheric CO2 levels. Extensive plant communities have a greater capacity to draw down carbon.
Soils are also a major carbon sink. When plants and other organisms die, their organic matter decomposes into the soil, storing carbon. Healthy soil ecosystems, rich in microbes and organic material, retain carbon for centuries to millennia.
Peatlands and permafrost regions are effective at storing large quantities of carbon due to waterlogged or frozen conditions that slow decomposition. These environments prevent rapid release of stored carbon, making them globally important long-term reservoirs.
Long-Term Geological Carbon Sequestration
Over geological timescales, Earth’s systems sequester atmospheric carbon dioxide through slow processes. Chemical weathering of rocks, particularly silicate rocks, is a primary mechanism. This process begins when atmospheric CO2 dissolves in rainwater, forming carbonic acid.
This acidic rainwater reacts with silicate minerals in rocks, breaking them down. Atmospheric CO2 is consumed and transformed into bicarbonate ions during this reaction. These bicarbonates are transported by rivers to the oceans.
Once in the ocean, marine organisms utilize these bicarbonates to construct their shells and skeletons. Upon their death, these calcium carbonate remains accumulate on the seafloor, forming deposits of limestone and other carbonate rocks. This cycle, from weathering to rock formation, locks carbon away from the atmosphere for millions of years, acting as Earth’s natural thermostat.
Another geological process is the formation of sedimentary rocks from accumulated organic matter. Over millions of years, ancient plant and animal remains can be buried under layers of sediment. Under pressure and heat, this organic material transforms into fossil fuels like coal, oil, and natural gas. This process sequesters carbon that was once atmospheric or living biomass. The formation of these deposits, along with carbonate rocks, has reduced atmospheric CO2 levels over hundreds of millions of years.
Historical Insights into Low CO2 Periods
Earth’s history provides evidence of periods with lower atmospheric CO2 concentrations. Past ice ages, such as the Last Glacial Maximum around 20,000 years ago, had atmospheric CO2 levels of approximately 180-200 parts per million (ppm), a stark contrast to pre-industrial levels of around 280 ppm. During these cold periods, enhanced carbon sequestration occurred, with colder ocean waters absorbing more CO2 and terrestrial vegetation influencing carbon storage.
The expansion of ice sheets and changes in ocean circulation contributed to the efficiency of the ocean’s biological and physical pumps. Shifts in vegetation zones and permafrost formation also locked up more carbon on land. These historical lows show how oceanic and terrestrial processes naturally reduce atmospheric CO2.
Further back, the Carboniferous period (359 to 299 million years ago) experienced very low atmospheric CO2 levels, potentially below 300 ppm. This reduction was largely due to widespread organic matter burial, forming massive coal deposits. The proliferation of land plants, coupled with geological conditions favoring their burial, removed vast quantities of carbon.