So far, the race to Net Zero has mainly focused on reducing how much CO₂ we add to the atmosphere through cutting fossil fuel use and increasing renewable energy. However, even if emissions fall to zero tomorrow, temperature will still rise due to the carbon already in the atmosphere.
Reaching Net Zero and keeping global warming below 2°c cannot be achieved by reducing emissions alone; it will require carbon to be removed from the atmosphere and securely stored. This is known as carbon sequestration and there are three types: biological, geological, and technological.
Biological carbon sequestration
This is the storage of atmospheric carbon in vegetation, soils, woody products, and aquatic environments. As vegetation grows, it absorbs carbon dioxide from the air via photosynthesis, which is sequestered (stored) by the plant throughout its lifetime. Woody plants are a great carbon sink, as up to 50% of their mass is carbon. Oceans also store carbon dioxide; so does soil. This is known as an ‘indirect’ or passive form of sequestration. By encouraging the growth of plants – particularly larger plants like trees – advocates of biologic sequestration hope to help remove CO₂ from the atmosphere.
During the past three decades, agroforestry (combining agriculture and trees) has become recognised the world over as an integrated approach to sustainable land use because of its production and environmental benefits.
It is important to note that carbon sequestered in soils and aboveground vegetation could be released again to the atmosphere through land-use or climatic changes. Decomposition can cause the release of carbon stored in forests back to the atmosphere. So can wildfires. Both processes join oxygen in the air with carbon stored in plant tissues to produce carbon dioxide gas.
Geologic carbon sequestration
Geologic carbon sequestration is the process of storing CO₂ in underground geologic formations to prevent its release to the atmosphere. The CO₂ is usually pressurized until it becomes a liquid, and then it is injected into porous rock formations in geologic basins. There are two types of reservoirs used: Injection into a sandstone or other porous reservoir that currently contains saltwater, termed ‘saline aquifer’ storage, and injection into a hydrocarbon-bearing strata such as an oil or gas reservoir, a gas shale, or a coal seam.
In this way, carbon dioxide can be captured from power plants and other large industrial facilities, and injected deep underground into strata in which it will remain isolated for long periods of time. The geologic formation in which the gas is stored must be overlain by another layer of impermeable rock to seal in the injected CO₂. The technology for sequestering CO₂ is still being developed, although a few industrial-sized projects are operating worldwide.
Technological carbon sequestration
Scientists are exploring new ways to remove and store carbon from the atmosphere using innovative technologies, looking beyond simply removing CO₂ to ways it can be used as a resource, for example as a raw material to produce graphene, used to create screens for smart phones and other tech devices. Graphene production is limited to specific industries but is an example of how carbon dioxide can be used as a resource and a solution to reducing atmospheric carbon.
Also under development is Direct Air Capture, by which carbon is captured directly from the air and scientists are investigating engineering molecules that can change shape by creating new kinds of compounds capable of singling out and capturing carbon dioxide from the air. The engineered molecules act as a filter, only attracting the element it was engineered to seek.
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Pauline Meechan, Sherwood Forest Friends of the Earth