By Lena Milton
Carbon sequestration is the process of capturing and storing carbon in order to reduce the amount of carbon dioxide in the atmosphere and overall carbon cycle. Carbon sequestration and negative emissions technologies are crucial methods of slowing climate change, as higher amounts of atmospheric carbon dioxide lead to increased warming. Essentially, by removing carbon from the ‘short-term’ carbon cycle and storing it in a long term way, greenhouse gas emissions are reversed. Carbon sequestration has an important role to play in stopping and reversing global climate change.
CO₂ is a greenhouse gas made up of a carbon molecule and two oxygen molecules. It can be converted into carbon by removing the oxygen, which can be done in several ways. The most common example we think about is a tree that uses photosynthesis to convert the CO₂ into carbon that it uses to grow and releases the oxygen which we then breathe. Seaweed, microalgae, and other biomass does the same thing. Other greenhouse gases include methane and water vapour.
Carbon dioxide is most often stored in “carbon sinks,” anything natural or manmade that absorbs carbon. The most common types of carbon sinks include natural features like trees and plants, the ocean, rocks, and soil. Most carbon sequestration methods work to capture carbon and store it in these carbon sinks, both through promoting natural processes (like photosynthesis) and initiating artificial processes (like machine-driven chemical reactions).
The oceans also hold CO₂, currently an excess amount – which is leading to ocean warming. Some processes remove CO₂ from the ocean water and store it. There is an equilibrium of CO₂ between the atmosphere and the oceans, so CO₂ can be removed from either one and it has the same effect of removing excess carbon from the cycle as long as it is stored in a long-term way.
Why Remove Carbon?
The main reason that carbon needs to be removed is that when it gets released as the greenhouse gas CO₂ from the burning of fossil fuels, it stays in the atmosphere for centuries. So all of the CO₂ emissions that have resulted from human activities since the Industrial Revolution are still there, and all of that is causing the current global warming we are experiencing. Increased atmospheric concentrations of carbon dioxide create the greenhouse effect, warming the planet. Even when we reach zero emissions, meaning no additional excess CO₂ is being emitted, all of the existing CO₂ will still be there, causing warming, unless it is removed.
Carbon Sequestration Permanence
One key thing to know about carbon sinks is they all store carbon for different amounts of time. For instance, a tree stores carbon while it grows and lives, but if it dies that carbon gets released back into the atmosphere as a greenhouse gas. Other methods of carbon sequestration, such as burial or seaweed sinking, can store carbon for much longer, keeping it out of the ‘short term’ carbon cycle for centuries or even thousands of years.
It’s valuable to use long-term storage methods so that the method doesn’t need to be repeated again any time soon. Such a large amount of CO₂ needs to be removed from the atmosphere to return to safe levels that it is challenging enough to remove it and store it one time, let alone two or more times.
There are many potential methods of CO₂ sequestration. In this article, we’ll walk through some of the major types of carbon sequestration projects in order to understand how they store carbon and help stop climate change.
Types of Carbon Sequestration Projects
There are two overarching strategies for carbon removal. First, some projects seek to avoid the release of further carbon emissions through capturing carbon before it is emitted and storing it. This is generally called carbon capture and storage, or CCS. The second type of carbon sequestration project seeks to remove carbon dioxide gas that is already in the atmosphere. This is known as carbon removal. While both are important, avoiding the release of further carbon helps prevent climate change, while removing carbon from the atmosphere works to help reverse existing climate change.
The projects discussed in this article all remove carbon from the atmosphere.
Deep Sea Sinks: Sinking Kelp or Plant Biomass
Like trees, seaweed uses photosynthesis to absorb carbon dioxide, thus storing carbon in the macroalgae. When the seaweed is sunk to the bottom of the ocean in deep areas, the carbon gets stored at the sea floor. Storing carbon in seaweed is a promising method of large scale carbon sequestration as seaweed can remove carbon dioxide from the atmosphere much faster than trees, as it grows nearly 30 times more quickly. Storing algae and plant materials in the deep sea can sequester carbon for centuries and even thousands of years, depending on the location and depth where it is sunk.
At Pull to Refresh, we gather invasive seaweed and then sink it into the deep sea using unmanned, solar-powered vessels. This effectively mimics the natural cycle, but removes carbon dioxide emissions from the atmosphere at a much faster rate than would occur naturally.
Afforestation and Reforestation
In one year, a single tree can absorb nearly 50 pounds of carbon dioxide. In fact, forests absorb around 13% of U.S. carbon emissions. This carbon is then stored in the tree’s branches, trunk, leaves and roots until the tree decays, or until the carbon is released in some other way such as the tree burning or being cut down (deforestation).
Planting trees is one of the simplest ways to remove carbon from the atmosphere either through reforestation (the planting of trees where there was recently a forest) or afforestation (the planting of trees in an area that was not recently a forest). Reforestation and afforestation projects are most successful when performed on a large-scale. While a single maple tree can absorb around 400 pounds of carbon dioxide only 25 years after planting, the average U.S. citizen emits 16 metric tonnes of carbon each year. In order to make a real dent in carbon emissions, many more trees would need to be planted.
Afforestation projects must also be done in a conscientious manner so as not to disrupt fragile ecosystems with non-native trees or reduce biodiversity by planting only the same type of tree. Allowing existing forests to regrow, known as natural regeneration, is another effective method to ensure carbon remains stored in trees that is often cheaper and more successful than planting trees. While there are a number of tree-planting organizations out there, it’s important to do your research before choosing one to support in order to ensure they carry out reforestation without causing further damage, using responsible land management practices.
Although trees can be an effective method of reversing emissions, they don’t provide long-term storage of carbon and they require a lot of land and water to grow. As forest fires increase over time, this also reverses the efforts of planting trees. This is one of the reasons why, though a lot of projects that seek to plant trees via drone are amazing, they don’t follow-through on the land & water resource management as effectively as other carbon sequestration methods.
Direct Air Capture and Storage
Direct air capture and storage works to remove carbon dioxide from the air and then store that carbon underground, in a product, or reuse it as fuel. For example, direct air-capture companies Climeworks and Carbon Engineering use machines that filter ambient air and remove carbon dioxide. This carbon is then mixed with water and compressed to be reused as fuel or pumped underground, where the carbon is turned into stone. As with any carbon sequestration project, this helps to remove carbon dioxide from the atmosphere and store it (fairly) permanently underground.
While this is an effective method of CO₂ sequestration, it should be noted that filtering the air does require energy and thus actually emits carbon to operate. However, Climeworks’ air capture machines are powered by renewable energy, making their carbon footprint relatively low. In fact, overall, out of every 100 metric tons of carbon dioxide captured from the air, at least 90 tons are permanently removed.
Carbon Farming: Soil Sequestration
Soil is a major global carbon sink, with the Earth’s soils containing around 2,500 gigatons of carbon – more than three times the amount of carbon in the atmosphere. In particular, healthy soil with the right amount of organic matter and thriving microbiota store more carbon than degraded soil. Human activities that increase the amount of carbon dioxide soil can absorb help remove carbon from the atmosphere. These practices are frequently referred to as carbon farming, which describes a number of agricultural activities that increase the amount of soil carbon sequestration while also pursuing agricultural goals like raising livestock or producing crops.
One of the main types of carbon farming is known as regenerative agriculture, a set of agricultural practices that work to restore soil fertility. For example, no-till farming is farming that does not rely on disturbing the soil to plant crops. This not only can increase crop yields, but actually reduces soil erosion and increases soil health, allowing the soil to store more carbon. In fact, a 2015 paper found that farms that use regenerative farming increase soil carbon storage over time, and have soil that more closely resembles the carbon levels of a natural forest.
Mineralization and Enhanced Weathering
Both mineralization and weathering store carbon use naturally-occuring chemical processes to store carbon in rock or other materials. Carbon mineralization is the process of turning carbon dioxide, a gas, into a solid mineral through chemical reactions. This process stores carbon permanently in rocks, thus removing it from the atmosphere. While these chemical mineralization reactions occur naturally, they can be sped up artificially in order to sequester carbon dioxide at a faster rate.
Weathering is the process by which silicate minerals are exposed to the atmosphere and break down over time. This disintegration allows for a chemical reaction that absorbs carbon. As silicate is ground down, absorbing carbon, it gradually turns into rock (mineralization) or is used for marine sea creatures’ shells. While this process removes carbon from the atmosphere, it occurs on the scale of millions of years. Enhanced weathering speeds up this natural process by spreading large amounts of finely ground silicate or carbonate rocks over a large surface area such as a field, beach or the ocean surface. This speeds up the natural process of weathering in order to remove carbon dioxide from the atmosphere faster.
For example, Vesta is one company that uses weatherization to increase carbon storage. Their process dissolves olivine-containing (silicate) rock into sand, which is then spread on coastlines, where it reacts with seawater in order to store carbon dioxide permanently in seawater as shells and carbonate rocks.
Ocean Alkalinity Enhancement
The ocean is another major carbon sink, as carbon dioxide is absorbed and reacts with water to create carbonic acid. In fact, the ocean already contains 50 times more carbon dioxide than the atmosphere, absorbing around 22 million tons of carbon dioxide every single day. While this process successfully stores carbon in the ocean, it also makes the ocean more acidic. Ocean acidification, caused by the ocean’s absorption of excess carbon dioxide in the atmosphere, is a major problem for marine species and ocean ecosystems.
Ocean alkalinity enhancement seeks to increase the ocean’s alkalinity, which works to fight acidity. By reducing the ocean’s acidity, it can store more carbon with fewer consequences. Ocean alkalinity works to reduce acidity as it increases the number of ions in the ocean that react with carbonic acid, and turn the acid into carbonate (carbon-storing) minerals.
The ocean’s alkalinity can be increased in several different ways, including depositing alkaline sand on beaches or coastlines or using specialized cells to react seawater with alkaline minerals and then releasing the water back into the ocean. While these methods have the potential to remove billions of tonnes of carbon dioxide from the atmosphere per year without increasing ocean acidification, they may also have unintended consequences, as research on these methods is still in early stages.
Electrolysis Carbon Capture
Electrolysis is a process that uses electric currents to induce chemical decomposition. In the case of electrolysis for carbon capture, an electrolysis machine splits water into hydrogen and oxygen, releasing oxygen into the atmosphere. The hydrogen is then combined with carbon dioxide, which react to create ethanol, methanol and water. These products can then be used to form a variety of goods including fuel and other alcohol-based items. For example, Air Company uses this electrolysis process to remove carbon dioxide from the atmosphere and transform it into products ranging from perfume to alcohol.
Coastal Blue Carbon Capture
Coastal habitats have the potential to store large amounts of carbon, as marshes, mangroves, and seagrass beds can absorb carbon dioxide from the atmosphere and store it in the ground. The carbon captured by coastal organisms like these coastal plants is known as “coastal blue carbon,” and is notably stored in the ground, not in the above-ground plant material.
Studies show that coastal wetlands have the ability to sequester carbon at an annual rate that is ten times higher than mature tropical forests. Thus, protection and restoration of coastal wetlands and mangrove forests is a strong strategy to remove larger amounts of carbon dioxide from the atmosphere. Many organizations focus on this, such as the International Blue Carbon Initiative and many other coastal protection projects.
Bioenergy with Carbon Capture and Storage (BECCS)
BECCS is a method of creating energy from biomass (organic matter like trees, crops or agricultural byproducts) while removing carbon dioxide in the process. BECCS is a technique that burns biomass to create energy, capturing and storing carbon in the process. When biomass is burned, for example in a boiler or furnace, the steam is used to power electricity-generating turbines, thus using biomass to create bioenergy. Carbon dioxide is removed from the gasses that result from burning the biofuels before it can be released, and is then stored by injecting the carbon into geologic formations, including unused natural gas reservoirs, unmined coal beds, or other naturally occurring rocks.
When BECCS is done well, it can remove carbon from the atmosphere and work as a “negative emissions technology.” When biomass grows, it absorbs carbon from the atmosphere. Then, when it is turned into bioenergy, the carbon is captured through the BECCS process and stored permanently. This is only effective when the method of carbon storage is actually able to store the carbon for long timescales.
Biochar (biological charcoal) is a residue made of carbon and ashes produced when biomass like wood is burned. It pulls carbon from the atmosphere, and can then be mixed with compost or soil to store carbon in the ground. While the natural decomposition process of plants is carbon-neutral (dying plants release carbon which is simply absorbed by other plants), biochar takes the carbon out of the cycle, storing it underground for hundreds to thousands of years.
While one study showed that 12% of greenhouse gas emissions could be offset with biochar, representing a great stride forward in the fight against climate change, critics of the strategy contend that more research is needed to overcome the logistical hurdles of actually creating and burying biochar on a large enough scale.
Biochar can also be made from human waste, which can then be used as an agricultural supplement. Biochar has the added benefit of reducing the need for fertilizer and increasing soil moisture levels. Biochar can also absorb water and reduce agricultural runoff into waters and streams, which can contribute to algal blooms or ocean acidification.
Microalgae, microscopic algae organisms, use photosynthesis to produce energy, effectively turning sunlight, water and carbon dioxide into biomass. Many researchers have proposed using microalgae to capture carbon dioxide and store it as biomass. Microalgae are particularly suited to this because they grow faster than many other photosynthetic organisms including trees and terrestrial plants. Microalgae have been proposed as a method of removing carbon dioxide from the atmosphere directly, or as a method of preventing carbon dioxide from being emitted as exhaust (known as flue gas) from carbon-based power plants and other carbon-emitting industrial plants.
While many microalgae carbon-capture projects are in their early stages, the U.S. Department of Energy announced in June 2021 that it would spend 8$ million to fund four projects focused on using algae to decrease emissions.
Carbon Sequestration: A Spark of Hope
While a single one of these projects alone may not be enough to reverse climate change, when taken together, carbon sequestration projects can have a significant impact on reducing atmospheric carbon. The most recent Intergovernmental Panel on Climate Change (IPCC) reports have stated that around one trillion tonnes of CO₂ need to be removed by the year 2100 in order to return to safe levels and avoid catastrophic climate change. Businesses and individuals alike can support a variety of methods in order to push us towards positive change and bring about a livable world for the future. Through voluntary markets, emission reversal can be funded and scaled to reduce the impact of climate change.
At Pull To Refresh, we are developing and deploying seaweed sinking technology to reverse emissions in an efficient and scalable manner.
Start reversing emissions today.
By Lena Milton
Lena Milton is a freelance writer covering sustainability, health and environmental science. She writes to help consumers understand the environmental and ethical challenges in everyday life so we can find viable solutions for both.