Carbon dioxide (CO2) is a naturally occurring gas that plays a significant role in Earth’s climate system. It is considered the most important long-lived greenhouse gas, absorbing and radiating heat, which contributes to planetary warming. Since the Industrial Revolution, human activities have increased atmospheric CO2 levels by approximately 50%, primarily due to fossil fuel burning. This rise is directly linked to global temperature increases, ocean acidification, and various extreme weather events. Understanding processes that decrease atmospheric CO2 is important for mitigating these environmental changes.
Natural Biological Removal
Living organisms naturally remove carbon dioxide from the atmosphere, primarily through photosynthesis. Plants, algae, and some bacteria utilize sunlight to convert atmospheric CO2 and water into sugars for energy and to build their biomass, releasing oxygen as a byproduct. This fundamental biological process is a major component of the global carbon cycle, continuously drawing CO2 out of the air.
Expanding forest cover through reforestation and afforestation enhances this natural CO2 removal. Reforestation involves replanting trees in areas previously forested but cleared, while afforestation establishes new forests in areas without trees for a long time. These practices increase plant biomass, storing carbon in trunks, branches, leaves, and roots. Efficient forest carbon sequestration programs could account for a quarter of desired global CO2 mitigation this century.
Beyond terrestrial plants, microscopic marine organisms, particularly oceanic phytoplankton, also absorb substantial CO2. These tiny organisms perform photosynthesis in the upper layers of the ocean, converting dissolved CO2 into organic matter. When they die, some carbon-rich material sinks to the deep ocean, effectively removing carbon from surface waters and the atmosphere.
Natural Physical and Chemical Removal
The Earth’s oceans serve as a vast natural carbon sink, absorbing a significant portion of atmospheric CO2 through physical and chemical processes. One such process is the oceanic solubility pump, where CO2 dissolves directly into surface waters due to differences in partial pressure between the atmosphere and the ocean. This dissolved CO2 then circulates through ocean currents, eventually being transported to deeper waters.
While the ocean acts as a massive absorber of CO2, its capacity is not limitless. As ocean waters warm, their ability to absorb CO2 decreases, potentially leading to less CO2 uptake from the atmosphere. Additionally, CO2 absorption leads to ocean acidification, which can negatively impact marine ecosystems.
Another natural process that removes CO2 from the atmosphere is enhanced weathering, also known as enhanced rock weathering. This slow geological process involves the reaction of atmospheric CO2 with silicate rocks. Rainwater, which contains dissolved CO2 forming carbonic acid, comes into contact with these rocks, and a chemical reaction occurs that converts the CO2 into bicarbonates. These bicarbonates are then transported by rivers to the ocean, where they can be stored in sediments for thousands to tens of thousands of years. While naturally occurring, human intervention can accelerate this process by crushing specific rock types, like basalt or olivine, and spreading them over land, increasing their surface area.
Engineered Carbon Removal Technologies
Human-designed technological approaches are being developed to actively remove CO2 from the atmosphere, complementing natural processes. One such technology is Direct Air Capture (DAC), which uses chemical or physical processes to extract CO2 directly from ambient air. Large fan-like machines pull air into specialized facilities where chemical solvents or solid sorbents capture CO2 molecules. The captured CO2 is then concentrated and can be permanently stored deep underground in geological formations, such as depleted oil and gas reservoirs or saline aquifers.
Bioenergy with Carbon Capture and Storage (BECCS) is another engineered approach that combines biological processes with technological carbon capture. This method involves growing biomass, such as plants and trees, which absorb CO2 from the atmosphere through photosynthesis. The biomass is then used to generate energy, and the CO2 emissions produced during combustion or conversion are captured before release. The captured CO2 is subsequently transported and stored in secure geological formations, aiming for net-negative emissions.
Carbon sequestration refers to the long-term storage of captured CO2. This often involves injecting compressed CO2 into deep underground rock formations, where it can be physically trapped in pore spaces, dissolve into existing fluids, or react to form stable minerals. These geological storage sites are typically located more than a kilometer underground and are chosen for their ability to securely contain the CO2 for thousands to millions of years. While these technologies hold promise, they are often energy-intensive and currently operate at a smaller scale compared to the vast natural carbon cycles.