Published: 4 June 2024
Contributors: Alexandra Jonker, Amanda McGrath
Carbon sequestration is the process of capturing and storing atmospheric carbon dioxide (CO2), one of the greenhouse gases that contributes to global warming.
Sequestering carbon happens naturally through biological and geological processes and by using human-engineered technologies. Both methods help remove CO2 from the atmosphere and can help mitigate the impacts of climate change. Therefore, interest in carbon sequestration is likely to grow as countries pursue net zero emissions goals and companies seek more sustainable practices.
The Earth's atmosphere is experiencing unprecedented levels of carbon dioxide concentrations as greenhouse gas emissions rise—largely due to human activities such as burning fossil fuels and deforestation. This increase in CO2 from pollutants traps heat, leading to global warming and its consequences, including rising sea levels, extreme weather events and ecosystem disruption.
According to the Intergovernmental Panel on Climate Change (IPCC), immediate steps must be taken to limit global warming to 1.5°C (34.7°F) above preindustrial levels.1 Carbon sequestration can play a crucial role in achieving this target. Capturing and storing CO2 directly reduces its presence in the atmosphere, which slows down the rate of global warming and reduces the severity and frequency of climate change-related events.
In addition to climate change mitigation, carbon sequestration offers other benefits. Natural carbon sequestration processes, such as photosynthesis, play a vital role in maintaining healthy ecosystems. Sequestration efforts can also improve soil health, enhance agricultural productivity and protect biodiversity by promoting the growth of carbon sinks (such as forests and wetlands).
Methods for sequestering carbon generally fall into these four categories:
In biological carbon sequestration, natural processes capture and store carbon in ecosystems. For example, through photosynthesis, plants absorb CO2 from the atmosphere and convert it into organic matter. Natural areas such as forests, grasslands and wetlands act as significant carbon sinks and can store large amounts of carbon for extended periods. And soil can naturally sequester carbon, but this ability is impaired when agricultural practices disturb and overturn it, releasing the trapped organic carbon.
Several land management practices aid biological carbon sequestration. Reforestation and afforestation significantly increase carbon storage capacity. Sustainable agriculture—such as reduced or no-till farming, which minimizes disruption of soil carbon, or agroforestry, which integrates trees and shrubs alongside crops and livestock—also aid sequestration. Restoring degraded wetlands, such as bogs and mangroves, improves their carbon storage in sediment and biomass. However, these ecosystems can also emit methane.
Ocean carbon sequestration involves storing carbon dioxide in the oceans. The oceans naturally absorb about a quarter of the CO2 humans produce.
There are two main strategies for aiding ocean carbon sequestration:
- The direct injection method captures CO2 from emission sources and injects it directly into the deep ocean. The ocean's high pressure and low temperature cause the CO2 to dissolve or form solid structures known as hydrates.
- The ocean fertilization method aims to enhance the ocean's natural absorption by adding nutrients, usually iron, to the surface of the ocean. This method stimulates the growth of microscopic plants and microbes that absorb CO2 from the atmosphere through photosynthesis. When they die, they sink to the bottom of the ocean, taking the absorbed carbon with them. However, this method is still experimental and its long-term effects on marine ecosystems are not fully understood.
Geological carbon sequestration captures CO2 and injects it into deep, underground porous rock formations for long-term storage—that is, millennia. The CO2 is compressed, transported by pipelines and stored in sites such as depleted oil and natural gas reservoirs, unusable coal seams or deep saline aquifers. Injected at depths of at least 800 meters, high pressure and temperature conditions ensure efficient storage in the natural pore spaces of the geological formations.
Technological carbon sequestration is a range of human-engineered methods that are designed to capture and store carbon dioxide from large-point sources or directly from the atmosphere. These technologies aim to remove carbon emissions from the carbon cycle and prevent their release into the atmosphere.
The best known technological approach to sequester carbon is carbon capture and storage (CCS). It involves capturing CO2 emissions from major contributors, such as power plants, factories and industrial facilities. The captured CO2 is compressed and transported (usually through pipelines) to suitable geological storage sites. Geological carbon sequestration is a part of the process, but CCS is a broader term that encompasses the entire process of capturing, transporting and storing carbon.
Other technological methods include:
- Direct air capture (DAC), which extracts CO2 directly from the ambient air, offering a way to remove historical emissions that are already in the atmosphere.
- Bioenergy with carbon capture and storage (BECCS), which involves burning biomass (plants or agricultural elements) to generate energy, then capturing and storing the resulting CO2 emissions. This method might offer the potential for "negative emissions" because the CO2 absorbed by the biomass during growth is removed from the atmosphere and stored.
- Mineral carbonation, which is a method of converting CO2 into stable carbonate minerals (like limestone) through reactions with metal oxides or silicates. This conversion prevents them from affecting the atmosphere.
While the terms "carbon storage" and "carbon sequestration" are sometimes used interchangeably, there are key differences between the two.
Carbon storage is the static state of carbon being held within a system, known as a carbon pool or reservoir. It's a snapshot of the carbon content at a specific point in time.
Carbon sequestration is the active process of capturing and transferring atmospheric carbon dioxide into long-term storage reservoirs. It's a dynamic action that removes CO2 from the atmosphere.
While carbon sequestration offers a promising way to reduce the amount of carbon dioxide contributing to climate change, it also presents several challenges and limitations:
Large-scale sequestration projects have high upfront capital costs and ongoing operational expenses, which makes them expensive to implement and maintain.
There are concerns about the safety and long-term effectiveness of geological storage, and the potential for leakage.
There are some negative environmental effects associated with sequestration, such as land use changes, ocean acidity, forest fires and other ecosystem disruption.
Despite these challenges, advancements in technology and policy are expected to make carbon sequestration a key tool in the fight against climate change.
Carbon sequestration is expected to play an increasingly important role in the fight against climate change. One study found that various CCS methods could sequester at least 550 million tons of carbon dioxide per year—the equivalent of removing over 1 billion passenger vehicles from roads.2
Demand for effective carbon sequestration solutions only grows as countries and companies set ambitious emissions reduction targets. Ongoing research and development in areas such as DAC and BECCS are expected to expand the scope and impact of carbon sequestration in the coming years. And, enhancing natural carbon sinks through reforestation, improved land management and wetland restoration is gaining traction.
Governments are also implementing carbon pricing mechanisms, tax credits and other incentives to encourage investment in carbon sequestration projects at home and through cross-border partnerships and collaboration. For example, in the United States, companies that capture and store CO2 are eligible for a tax credit per metric ton of carbon dioxide sequestered.3
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CCS is the process of capturing and sequestering carbon dioxide emissions before they enter the Earth’s atmosphere and contribute to global warming.
Footnotes
¹ "IPCC Sixth Assessment Report" (link resides outside ibm.com), Intergovernmental Panel on Climate Change (IPCC), April 2022.
² "The technological and economic prospects for CO2 utilization and removal" (link resides outside ibm.com), Nature, 6 November 2019.
³ "Carbon capture and storage in the United States" (link resides outside ibm.com), United States Congressional Budget Office, December 2023.