Types of Sequestration
Biological Carbon Sequestration
What is a Carbon Sink?
A carbon sink is a resevoir (natural or artificial) of carbon dioxide that was created by a process with the ability to sequester carbon dioxide out of the atmosphere, and store it. Our most important carbon sinks are our forests and oceans [5]. One artificial example is the storage of carbon dioxide underground [4]
Biological sequestration involves storing “carbon dioxide in vegetation such as grasslands or forests, as well as in soils and oceans [3]”. Our oceans act as a large carbon sink - dissolving about 25% of carbon emissions from human sources. While impressive, this alters the pH of the ocean - making it more acidic and hazardous to marine life [3].
An additional 25% of total emissions are caught by our forests and grasslands. Carbon dioxide absorbed through photosynthesis is put back into the ground as plants decay, and leaves fall. Grasslands, in particular, are better carbon sinks than forests – “unlike trees, grasslands sequester most of their carbon underground” [3].
Geological Carbon Sequestration
Geological sequestration is where things get interesting scientifically.
In extremely long time scales geologically, the biosphere has built up fuel deposits like coal and oil deep underground [2]. By creating artificial methods to do this much faster, we can now store “carbon dioxide in underground geologic formations, or rocks”, typically capturing the carbon from industrial sources, and sequestering it into porous rocks for the long term [3]. This is a process known as carbon capture and storage (CCS).
A newer innovation is direct air capture (DAC), which captures carbon directly from the atmosphere rather than an industrial source – but has yet to be scaled up to a cost-effective level [3].
Regardless of how we collect the carbon, geological sequestration is a safer solution. When forests burn down or die naturally, their trees release all of the carbon they’ve captured back into the atmosphere – and reforestation takes long periods of time and lots of water [4]. For this reason, biomass is a dangerous carbon sink. In order to prevent the release of carbon dioxide, “they can be burned in a power plant so that the released CO2 is captured and then buried. This process is known as BECCS”. However, BECCS requires too much land on the large scale, with high use of water and fertilizer [4]. Underground, carbon dioxide can mineralize in geological formations, and stays safely out of the atmosphere for over 10,000 years [4].
Using Them Both Together
All in all, however, a combination of these carbon sequestration techniques is the best solution to this problem. No one carbon removal technology is going to solve this on its own. We are on a good start to this end goal - “global storage capacity increased 32% from 2020, with 27 fully operational CCS facilities worldwide and 108 more facilities in development [4].”
On the DAC side of things, startup companies like ClimeWorks[4] are building scaled up direct air capture plants. Just a couple of months ago, ClimeWorks announced Mammoth - their largest site yet. Additionally, with new policies all over the world to protect our forests, and even grow them, carbon sequestration seems more feasible every year.