In the fight against climate change, researchers are looking for ways to remove greenhouse gases released from the atmosphere. Above all, CO2 is to be removed from the air via a carbon storage project and stored in a climate-friendly form. Now environmental engineers have developed a novel method to store atmospheric CO2 for hundreds of years or longer. Trees or wood residues are enclosed in air-free underground caverns where the biomass cannot rot. Initial analyzes suggest that this approach would be both effective and cost-effective. But there are also concerns.
Forests absorb CO2 from the atmosphere, convert it via photosynthesis and store it in the form of biomass. They are therefore considered an important greenhouse gas sink and a key factor in the fight against climate change. Reforestation and renaturation projects are a central component of climate protection. However, when the trees burn or die and rot, the stored CO2 is released again. In the long term, forests therefore contribute little to climate protection.
Inspired by ancient tree trunks
A team led by Ning Zeng from the University of Maryland has now investigated whether wooden biomass can be preserved in such a way that the CO2 bound in it remains removed from the global carbon cycle for hundreds of years or more. The environmental engineers were inspired by a 3,775-year-old Virginian juniper tree trunk that was buried two meters deep in clay soil near Montreal and was therefore surprisingly well preserved. After their discovery, Zeng and his colleagues compared the structure and chemical composition of this ancient tree trunk with samples of today’s trees of the same species. They also buried these pieces of wood in an artificial “wooden vault” – a clay-covered chamber in the ground that is intended to prevent decay and decomposition.
The comparison showed that the old juniper trunk had only lost around five percent of the carbon stored during its lifetime – before or during the almost 4,000 years in the soil. Structurally it still closely resembled today’s trees. “The fact that no decomposition can be observed is probably due to the low permeability of the compact clay soil at the grave site,” write the researchers. This meant that the wood did not come into contact with oxygen, which most wood-degrading creatures depend on. Zeng and his colleagues replicated these conditions in their experimental “wooden grave.”
Cheap method for CO2 storage?
By trapping wood in such an underground device, up to ten gigatons of CO2 could be removed from the global carbon cycle each year, computer modeling calculations have shown. This would only require a small proportion of the world’s trees or wood residues from forestry or after storm damage. According to the team, this approach would only cost around $100 per ton of CO2. That would be significantly less than other carbon storage approaches. For example, storing CO2 in the sea costs around $1,400 per ton of CO2. “Thus, within the spectrum of CO2 removal methods, wood burial offers a good balance between cost and effectiveness,” write Zeng and colleagues. In addition, the method would remove CO2 from the atmosphere for significantly longer than natural forests can.
However, further tests still have to show how sustainable and economical this method actually is. “A complete life cycle assessment is needed to quantify net emissions and impacts on ecosystems, supply chains, and timber vaults, and to understand how these impacts vary by location and timber source,” writes Yale University environmental scientist Yuan Yao in an accompanying commentary to the study. “These findings will be crucial for the development of biomass burial projects on a global scale.”
Source: Ning Zeng (University of Maryland) et al.; Science, doi: 10.1126/science.adm8133