Sequestration methods and global warming
Sequestration methods are strategies aimed at capturing and storing carbon dioxide (CO₂) and other greenhouse gases (GHGs) from the atmosphere to mitigate global warming. Since the Industrial Revolution, atmospheric CO₂ levels have escalated significantly, primarily due to fossil fuel consumption, leading to heightened concerns about climate change. Various sequestration techniques exist, categorized into geological, biological, and chemical processes. Geological methods include injecting CO₂ into depleted oil and gas reservoirs or saline aquifers, while biological methods utilize organisms like microalgae, which can absorb CO₂ during photosynthesis.
Ocean sequestration is another critical approach, leveraging the ocean's capacity to dissolve CO₂ or support the growth of phytoplankton. Chemical methods also exist, employing substances that chemically bind to CO₂. Despite the potential of these diverse methods, challenges remain, including high costs and environmental risks, such as the potential for CO₂ leaks from underground storage facilities. Overall, while sequestration methods present opportunities for reducing atmospheric GHG concentrations, a holistic approach that includes reducing emissions is vital for addressing the global warming crisis effectively.
Sequestration methods and global warming
Sequestering CO2 and other GHGs from the atmosphere by geological, biological, or chemical processes and storing them in forms that will not affect Earth’s climate is a primary mechanism for reducing atmospheric GHG concentrations and limiting global warming.
Background
Since the Industrial Revolution began, atmospheric levels of carbon dioxide (CO2) have risen from 275 parts per million to more than 420 parts per million by 2024. Evidence suggests that this observed rise in atmospheric CO2 levels is due primarily to the use of fossil fuels. There is a worldwide awareness that global warming is a result of the rising levels of CO2 and other greenhouse gases (GHGs). Earth’s atmospheric CO2 level could easily exceed 500 parts per million by 2050 if no corrective actions are taken. A variety of methods of CO2 have been suggested, including the use of oceans, underground caves, chemical reactions, or living organisms for CO2 removal. Several pilot projects for CO2 sequestration are under way in different countries.
Ocean Sequestration
Oceans serve as Earth’s largest CO2 sink—and a natural one as well. CO2 simply dissolves in the ocean or is taken up by resident microscopic (phytoplankton). Based on these ocean features, scientists are considering two approaches for CO2 sequestration. One option envisions direct injection of a stream of CO2 waste into the ocean depths. An immediate problem with this approach is that direct injection may create acidic water that would kill a majority of oceanic life forms.
A second idea is to enhance by algae. Algae are the most important on Earth, consuming CO2 as they photosynthesize and converting it into organic matter, a process known as carbon fixation. For instance, oceanic algae annually sequester 2 gigatons of CO2, compared to the 1.5 gigatons sequestered by all terrestrial ecosystems together. Some of this CO2 permanently remains in the ocean in the form of or silicates. One proposal for algal sequestration suggests fertilizing the ocean with iron in order to stimulate photosynthesis by algae. However, because water circulates very slowly throughout the oceans, this method might require hundreds of years before significant amounts of CO2 are sequestered.
Underground Sequestration
CO2 can also be pumped into depleted underground oil and gas reservoirs. There is enough underground capacity to store the gigatons of excess atmospheric CO2 accumulated over hundreds years. These reservoirs would be sealed and capped to avoid the escape of CO2. Safety is a concern for CO2 underground storage, as the CO2 may escape, suffocating people and animals in its path. This happened in Cameroon in 1986, when 1 million cubic meters of CO2 erupted from the Lake Nyos crater, killing eighteen hundred people. This CO2, however, was not placed underground by humans. Another possibility is to inject CO2 into saline aquifers, geological formations that contain salt water in their pore spaces. These aquifers have the capacity to store up to 10,000 gigatons of CO2.
Microalgal Sequestration
One proposed solution for would be to use microalgae (microscopic algae) to capture CO2 that is discharged into the atmosphere by power stations and other industrial plants. Several US start-up companies (GreenFuel Technologies, GreenShift, Solix, and Valcent Products) are pursuing this idea by using pilot-scale algal photobioreactors to remove CO2 from power plant waste gases. In 2022, Danish green-tech start-up Algiecel unveiled a photobioreactor to absorb carbon dioxide emissions. are photosynthetic microorganisms similar to plants, but they grow quickly and can greatly increase their total within hours. Photobioreactors are various types of closed systems made of an array of transparent tubes in which microalgae are cultivated and monitored.
The design of a photobioreactor is an important factor governing algal productivity for CO2 sequestration. Photobioreactors should be simple, inexpensive, and energy efficient and should allow a high-cell density of algal growth. For the efficient removal of CO2 from power plant effluent gases, it would also be advantageous to use algal species that can tolerate and assimilate high concentrations of CO2 (more than one hundred times atmospheric levels).
Large-scale cultivation of microalgae in photobioreactors as a technology for CO2 sequestration will have an impact on global climate only if the algal biomass is used to substitute for fossil fuel. Algae in photobioreactors, for example, can be used to generate environmentally friendly such as and hydrogen. Although the use of algal biomass as an energy source releases CO2, the process as a whole can be considered carbon neutral in that the CO2 released during conversion to fuel or through the use of fuels had been assimilated during the original growth of microalgae and can be recycled again through uptake by microalgae.
Other Biological Sequestration
Other biological methods of CO2 sequestration have also been investigated. One is the conversion of CO2 into methane by microbes called archaea. The resulting methane could be used as fuel. Another biological method of carbon sequestration would be to enhance natural terrestrial ecosystems. Terrestrial ecosystems absorb CO2 mainly through plant photosynthesis. These ecosystems can help remove CO2 from the atmosphere by storing it in plant biomass and soils. Certain woody plants, such as poplars, are being considered for CO2 sequestration. These plants consume CO2 during vegetative growth and convert it into lignin, a biological compound that is very resistant to decomposition. However, even when woody plants are used, the CO2 will ultimately return to the atmosphere by fermentation or through respiration by microbes and animals. A potential solution would be similar to microalgal sequestration technologies, involving the use of plant biomass to generate biofuels.
Chemical Sequestration
In addition to the geological and biological methods mentioned above, there are also chemical methods for CO2 sequestration. These methods use chemicals such as magnesium silicate, amines, or ammonia to capture CO2. All known CO2 sequestration technologies are very expensive. Cheaper carbon dioxide sequestration methods are becoming a reality, as recent research has promised to drive the high costs down. It may, however, still be more cost-effective to adopt alternative fuels and avoid CO2 emissions to the extent possible rather than continuing to emit CO2 and relying on sequestration to mitigate its greenhouse effect.
Key Concepts
- carbon neutral: the quality of neither subtracting nor adding a net amount of carbon to the atmosphere
- photosynthesis: process whereby carbon is taken from the atmosphere and incorporated into organic compounds by plants in order to store solar energy by converting it into chemical energy
- sink: a source of storage of a substance that removes it from environmental circulation
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