Carbon dioxide (CO2) is a naturally occurring gas that is part of Earth’s carbon cycle, moving between the atmosphere, oceans, land, and living organisms. While essential for life, human activities have significantly increased atmospheric CO2, raising climate change concerns. Nature has inherent mechanisms to remove CO2, acting as carbon sinks. Understanding these natural processes is important for climate regulation and mitigating atmospheric CO2 levels. These systems include terrestrial ecosystems, ocean processes, and geological formations, each playing a distinct role in carbon sequestration.
Terrestrial Ecosystems
Land-based ecosystems remove atmospheric CO2 through biological processes. Plants, trees, and forests are key to this removal, absorbing CO2 via photosynthesis. During photosynthesis, plants use sunlight, water, and atmospheric CO2 to produce glucose and oxygen. The carbon from CO2 incorporates into the plant’s biomass. Forests function as major carbon sinks, with reforestation and afforestation enhancing their capacity to sequester carbon.
Soil plays a significant role in carbon sequestration by storing organic matter. When plants and animals die, their organic material decomposes and integrates into the soil. Soil organic carbon is influenced by land management practices. Practices like no-till farming and cover cropping help retain and increase soil carbon. Applying organic amendments like compost and integrating agroforestry systems can also enhance soil carbon storage.
Wetlands, including peatlands, have a strong capacity for carbon storage. These ecosystems feature waterlogged, oxygen-poor soils, which slow organic matter decomposition. This anaerobic environment allows dead plant material to accumulate as peat, storing carbon for thousands of years. Peatlands are the largest natural terrestrial carbon store globally, holding more carbon than all other vegetation types combined. Protecting and restoring these wetlands prevents stored carbon release.
Ocean Processes
The world’s oceans are major carbon reservoirs, absorbing a large portion of atmospheric CO2 through various processes. A primary mechanism is direct CO2 absorption into seawater. The ocean surface acts as an interface where atmospheric CO2 dissolves, driven by concentration differences. Once dissolved, CO2 reacts with water to form carbonic acid, which converts into bicarbonate and carbonate ions. These dissolved forms allow the ocean to store carbon, contributing to its role as a major carbon sink.
Marine life plays an important role in carbon removal through the biological pump. This process begins with microscopic marine plants called phytoplankton. Phytoplankton absorb CO2 through photosynthesis, forming the base of the marine food web. When these organisms die or are consumed, carbon-rich organic matter sinks to deeper ocean layers. This sinking material, often called “marine snow,” sequesters carbon for decades to millennia.
Coastal ecosystems, known as “blue carbon” habitats, are effective carbon sinks. Mangroves, seagrass beds, and salt marshes are examples. These ecosystems capture and store carbon in their plant biomass and waterlogged soils and sediments. Anaerobic conditions in these submerged soils prevent rapid decomposition, allowing carbon to accumulate over hundreds to thousands of years. Mangroves and salt marshes can sequester carbon at rates higher than tropical forests, storing a substantial portion of the world’s blue carbon in their sediments.
Geological Carbon Sinks
Natural geological processes contribute to the long-term removal of atmospheric CO2. A primary mechanism is rock weathering, where atmospheric CO2 dissolves in rainwater to form a weak carbonic acid. This acidic rain reacts with rocks, especially silicate rocks, causing them to break down. Carbon from the carbonic acid converts into bicarbonate ions during this reaction.
These bicarbonate ions are transported by rivers to the oceans. Once in the marine environment, carbon can be incorporated into marine organisms to form shells and skeletons, or precipitate as carbonate minerals, forming sedimentary rocks like limestone on the ocean floor. This process locks away carbon for millions of years, acting as a natural thermostat for Earth’s climate. While natural weathering is slow, “enhanced weathering” explores accelerating this phenomenon by grinding silicate rocks into fine powders and spreading them, increasing surface area for reaction with atmospheric CO2.