Carbon sequestration has been getting a lot of coverage across mainstream media recently. But what exactly does it mean and how does it work? In this article we provide a detailed breakdown of the carbon cycle and the different types of carbon sequestration processes that happen both naturally and with technology.
Let’s start with a short definition – carbon sequestration is the process of storing carbon dioxide either naturally or via artificial processes. A carbon sequestration definition is also taking carbon dioxide out of the atmosphere and storing it in a safe way so it does not enter the environment again.
There are different types of carbon sequestration – biological, geological, and technological. Biological carbon sequestration is when CO2 is stored in vegetation, soil, or the ocean also referred to as natural carbon sinks. Nature has come up with the process of sequestering carbon either through photosynthesis or oceanic sink in order to stabilize the levels of carbon dioxide as excess amounts warm up the atmosphere.
The geological and technological carbon sequestration methods are designed to mimic the process of natural carbon sink. They usually include sequestering CO2 using carbon capture and storage technologies or carbon capture utilization and storage.
The Carbon Cycle
Carbon sequestration is just one part of the carbon cycle equation. The carbon cycle is referred to as nature’s way of reusing carbon atoms as they are released from organisms and non-organic compounds on Earth and then travel from the atmosphere into organisms and back over and over again. They are the foundation of life and can be found in pretty much everything.
On Earth, carbon is stored in rocks and sediments, in the ocean, and in living organisms. Carbon gets released back into the atmosphere when living organisms die, when volcanoes erupt, or when fossil fuels are burned.
The Earth also absorbs CO2 through the photosynthesis of plants. The ocean also sinks CO2 from the atmosphere. There is a perfect balance between the carbon emitted and absorbed in nature.
Every year the Earth cycles around 750 Gigatons of CO2 naturally. Anthropogenic or manmade CO2 emissions account for around 40 Gigatons per year. It is estimated that around 40% of human emissions are absorbed by nature through terrestrial ecosystems. The remaining amount is being accumulated in the atmosphere and consequently, CO2 levels have been rising.
The Effect Of Increase In CO2 Concentration Levels
The global average atmospheric CO2 concentration levels are currently measured at 416 parts per million. Before the Industrial Revolutions, the levels were standing at 280 parts per million which means human activities have caused around a 48% increase of the concentration of CO2 in the Earth’s atmosphere. A natural rise of 100 parts per million normally takes between 5,000 to 20,000 years to occur.
The increase of carbon dioxide levels is causing the Earth to warm up. Scientists are predicting that human activities like burning fossil fuels will lead to a 1.7 to 4.9 degrees Celsius rise in global temperatures by 2100.
The CO2 is also piling up as nature cannot sequester all the additional amount of emissions the human race is emitting. People are able to increase the rate of CO2 sink via three carbon sequestration methods that can store the additional manmade emissions.
Biological Carbon Sequestration
Biological carbon sequestration combines all the natural ways of storing CO2 usually in plants, forests, glassland, oceans or soil. Forests are normally considered one of the most important natural carbon sinks. When trees and vegetation grow, they soak up CO2 that remains stored in their aboveground and underground parts. If leaves and branches fall off plants or if plants die, the CO2 stored either gets released into the atmosphere or is transferred into the soil.
Plant-rich landscapes like forests and grasslands sequester around 25% of global carbon emissions. The trees’ absorption of CO2 from the atmosphere depends primarily on their species, development phase, the type of soil environment, and climatic factors. Trees sequester most of their carbon aboveground while grasslands – underground. When grasslands burn, the carbon stays fixed in the roots and soil. While forests are able to store more CO2, grasslands stand more resilient in unstable conditions.
Soil carbon sequestration is another method of CO2 storage. CO2 is sequestered in soil in the form of soil organic carbon (SOC) and it represents organic compounds highly rich in CO2 that result from a mixture of decaying material from living organisms in different stages of decomposition, bacteria and fungi or fecal material. SOC can typically store CO2 for a couple of decades.
Soil also stores carbon in the form of carbonates. They are created when CO2 dissolves in water, percolates the soil and then combines with calcium and magnesium minerals. Carbonates can sequester carbon for more than 70,000 years.
Peatlands are another natural carbon sink that has accumulated large amounts of CO2 over the years. As bogs and fens lack oxygen, the carbon of dead plant matter remains trapped. A 4 degrees C of global warming, however, can potentially release 40% of soil organic carbon from the shallow peat and 86% from the deep peat which is dangerous in terms of climate change mitigation.
Oceans Are The Biggest Biological Carbon Sink
Oceans are the biggest carbon sink on Earth. Currently, 39,000 gigatons of carbon (GtC) reside in the oceans and only 750 GtC are in the atmosphere. It soaks up over a third of human emissions. The oceans sequester CO2 via the plankton on its surface that uses photosynthesis to convert carbon dioxide into sugars. When marine animals consume the plankton and eventually die, they sink to the bottom of the ocean and lock the CO2 contained in the plankton for millions of years.
The atmospheric CO2 also dissolves into the ocean and reacts with the water to form carbonic acid. The carbonic acid, though, increases the acidity of the oceanic water and decreases its pH which makes the ocean unsuitable for living organisms. The rise of acidity in oceans also makes it harder for corals and oysters to build their shells. Thus, higher absorption of CO2 amount by the ocean can have a negative impact on marine ecosystems.
Geological Carbon Sequestration
The geological carbon sequestration typically involves capturing CO2 from an industrial source and burying it in underground geologic formations. The storage could be in depleted oil and gas reservoirs, saline formations, under the seabed or in unmineable coal beds. To be sequestered in an easier way, the CO2 is normally pressurized until it transforms into a liquid, and then injected into porous rock formations in geologic basins.
The minimum depth of the CO2 that is normally injected is 1km below the surface but some projects involve more than 2 km deep underground sequestration where it stays for hundreds to millions of years. Specialized expertise is involved in determining and examining the impact of injecting CO2 underground. Normally the target zones have a proven record of trapping buoyant fluid securely for millions of years.
CO2 is usually stored in sandstone or limestone medium, capped by a layer of low permeability that acts as a seal. Once it is injected and encounters a seal or also called caprock, it spreads laterally. There is a danger of leakages as the carbon dioxide could find fault planes or the pressure of the injection could cause earthquakes. The data from the number of carbon capture and sequestration projects though have proven the viability and safety of this method. Furthermore, once stored, the carbon is controlled through strict monitoring and reporting processes.
There is another improved technology for geological storage of emissions. It involves dissolving CO2 into water with other elements and then storing it underground where it turns into rock. The process is based on forming carbonates from the CO2 dissolved in the water that binds with the rocks underground. This way, carbonates trap the CO2 from escaping into the atmosphere and can stay stable for thousands of years without the threat of leakages.
Technological Carbon Sequestration
Technological carbon sequestration is referred to the process of removing CO2 from the atmosphere and storing it under the surface or via utilization – processes also known as carbon capture and storage (CCS) and carbon capture utilization and storage (CCUS). Scientists have come up with innovative geoengineering ways to reduce the levels of dangerous emissions in the atmosphere.
Carbon capture and utilization differs from CCS as it doesn’t involve the permanent storage of emissions underground. Instead, the CO2 is seen as a resource and converted by the industry into valuable materials and products. Those end-use products are considered carbon neutral as the CO2 from industrial emitters like power plants is captured and then reused rather than released ambiently into the air.
There are a variety of products that can be manufactured using CO2 as an ingredient. It could be reused to make plastics, concrete, fabrics, biofuels, graphene, meat, fish and poultry food, and many others. Emerging startups and more mature companies are coming up with various technologies that efficiently utilize the waste gas into a resource and thus provide a solution for reducing emissions.
There is a long development road ahead for CCUS as economics need to improve and the market for the raw material needs to increase. Despite that, it offers enormous potential for economies to profit from the investments those technologies provide, especially past the covid pandemic.
Ways To Enhance Carbon Sequestration
The carbon sequestration examples above could be boosted to increase the amounts of CO2 being absorbed. Since natural carbon sinks like forests and grasslands soak up vast amounts of CO2, properly managed forest areas, reducing the rates of deforestation are greatly effective ways of sequestering carbon. Reforestation and afforestation are some of the most important treatments for climate change mitigation.
Afforestation refers to planting trees in areas where there were no forests previously, while reforestation is the replanting of trees in areas usually deforested before. Increasing the amount of forest land is one of the most preferred ways by scientists and the community to remove carbon dioxide. Using appropriate trees like fast-growing types can sequester much more CO2 than slow-growing species.
Paulownia trees for example, are deemed to be the trees of the future as they can be harvested in 5 years and their leaves sequester 10 times more carbon than other species. It is often used for reforestation purposes as it is drought resistant, temperatures tolerant and can easily grow back from the roots after being harvested. A hectare of Paulownia trees soaks up to 100 tons of CO2 and releases 75 tons of oxygen per year.
Properly managed agriculture can also play a major part in increasing soil carbon sequestration. The process of enhancing agricultural methods of sequestering CO2 is also called carbon farming. Increasing the soil organic matter reduces fertilizers and improves the soil’s water retention rates. Organic farming is also examined to increase the amount of CO2 captured in soil by 18% compared to traditional farming. Best management organic farming practices like conservation tillage, cover crops, compost and manure amendments are often used to improve carbon sequestration.
Other Natural Carbon Sinks That Can Reduce Emissions
As peatlands are a great carbon sink method, urbanization and intensive industrial development have led to their degradation and thus the release of extra CO2 into the atmosphere. Limiting the intensive treatments on peat bog environments will lead to a reduction of the anthropogenic factor.
Scientists are also examining ways to increase ocean carbon sequestration capacity. Fertilization of the oceans using iron is an example of geoengineering technique. Iron is a food compound for phytoplankton so this method aims to encourage the growth of microorganisms that absorb CO2. This technique is also controversial as it can lead to a disruption of ocean nutrient balance.
The growing of seaweed and algae in shallow and coastal areas is also being explored. Algae grows fast, captures CO2 in the process and can be used for the production of biofuels with low carbon intensity. Seaweed can be transported to the deep ocean where it can sequester carbon for millennia. It has been estimated that if seaweed farms covered 9% of the ocean they could remove 53 gigatonnes of CO2 per year from the air.
Technologies like carbon capture and underground storage or carbon capture and utilization can also sequester massive amounts of dangerous emissions. According to the Global CCS Institute, 26 carbon capture and storage facilities are currently operational and they sequester around 40 million tons of CO2 emissions per year. The majority of those projects are used for the purpose of enhanced oil recovery. An increasing number of CCS projects solely built for permanent underground sequestration have been scheduled to come online this decade.
Carbon sequestration is a natural process involved in the carbon cycle mechanism of our planet. Thanks to the natural and artificial ways of sequestering CO2 emission, the life balance on Earth could be maintained and additional GHGs emitted by human activities can be partly or entirely offset. The biological carbon sequestration methods offer some of the greatest potential for effective management of CO2 levels in the air, while technologies like carbon capture and storage and CCUS can be used to limit the negative effects of fossil fuel industrial use until carbon-free energy sources become more available.