The UN Intergovernmental Panel on Climate Change (IPCC) warns catastrophic consequences of global warming cannot be avoided simply by reducing emissions of carbon dioxide (CO2) to the atmosphere.
Its estimates that limiting global warming to 1.5 degrees over pre-industrial levels cannot be achieved unless we remove an additional 100 billion to one trillion tons of CO2 from the atmosphere by 2100.
If this removal is started by 2050, 2 billion-20 billion tons of CO2 must be removed per subsequent year and permanently sequestered on Earth. It warrants examination of how this might be done – expanded details are available in an article by Douglas Fox in Scientific American, July 2021.
The main options for actively removing CO2 from the atmosphere include forestation; direct air capture (DAC) of CO2, bioenergy with carbon capture and storage (BECCS), CO2 storage in land and ocean and mineral carbonation.
CO2 is the primary food of green plants that suck CO2 from the air and chemically combines it with H2O (photosynthesis) to form the substance of the plants. World forests currently absorb 2.4 billion tons of CO2 annually, about half of which is lost to deforestation, fires etc. Ending deforestation and restoring degraded forest is an effective, low-technology method of reducing atmospheric CO2. Efforts are afoot to end tropical deforestation and restore degraded forests by 2030.
Chemical filters
DAC sucks large volumes of air through chemical filters that absorb CO2. The trapped CO2 can now be pumped underground for storage. In Iceland the Orca plant, opened in 2021, employs DAC to draw down 4,000 tons of CO2 annually. DAC is expensive and costs must be reduced greatly before it can be deployed on a really effective scale.
One brilliant solution in principle is mineral carbonation, that is converting captured CO2 into solid rock
BECCS is the process of burning crops and agricultural residue to generate electricity, or to produce biofuels, and to bury the related CO2 emissions underground. Crops suck carbon from the atmosphere as they grow. CO2 is released when they are subsequently burned to generate energy but this CO2 is sequestered and never re-released to the atmosphere. In principle, BECCS could sequester 5.2 billion tons of CO2 annually. However, difficulties remain to be overcome before this attractive technology can be effectively deployed globally.
The oceans and soils of the world store massive amounts of carbon. Could this be increased? Studies are assessing the effects of growing seaweed in the open ocean to suck large amounts of CO2 from the atmosphere through photosynthesis. Various agricultural practices such as reduced tillage to increase the amount of carbon dioxide stored in soil are also under development.
Basalt rocks
Sucking carbon out of the air is one thing, but preventing it from ever re-entering the atmosphere is another. One brilliant solution in principle is mineral carbonation, that is converting captured CO2 into solid rock. In some parts of the world such as Oman, Alaska, Canada, California, New Zealand and Japan, basalt rocks originally formed in Earth's mantle dozens of kilometres below the surface have been shoved to the surface by plate tectonic activity. These peridotite rocks are rich in elements like calcium and magnesium that can react with CO2 to form salts.
The plan is to build a global infrastructure of machines to suck CO2 from the air and inject it into wells drilled several kilometres deep where peridotite rocks are much hotter than at the surface. CO2 reacts avidly at these higher temperatures with calcium, magnesium etc in the rocks to form solid carbonate salts that will remain permanently sealed in the rocks. Testing is under way in Oman to investigate the capacity of this technology to sequester CO2.
Oman alone has enough rock to sequester a billion tons of CO2 a year but this would require a massive infrastructure, including 5000 injection wells. History tells us that such development is feasible. When the oil industry was in its infancy in the mid-1800s, oil wells were only a few metres deep. This was all scaled up to meet demand and the US now has over a million oil and gas wells.
Storage capacity
The potential storage capacity of peridotite rocks is massive, estimated at 60 trillio-600 trillion tons of CO2 – roughly 25 to 250 times the amount of CO2 humans have added to the atmosphere since 1850. Undoubtedly human ingenuity has the potential to get us past global warming. Of course, human ingenuity didn’t prevent us from causing global warming. Lack of wisdom is our real problem, not lack of ingenuity.
William Reville is an emeritus professor of biochemistry at UCC