I can’t resist a good science story involving technology that we can possibly use to stabilize our climate in the face of anthropogenic global warming. This one is a fun story and an interesting, and potentially useful, idea. As we map out potential carbon pathways into the future, focusing on the rest of this century, it is pretty clear that it is going to be extremely difficult to completely decarbonize our civilization. This means we can only slow down, but not stop or reverse global warming. Once carbon is released into the ecosystem, it will remain there for hundreds or even thousands of years. So waiting for natural processes isn’t a great solution.

What we could really use is a way to cost-effectively at scale remove CO2 already in the atmosphere (or from seawater – another huge reservoir) to compensate for whatever carbon release we cannot eliminate from industry, and even to reverse some of the CO2 build up. This is often referred to as carbon capture and sequestration. There is a lot of research in this area, but we do not currently have a technology that fits the bill. Carbon capture is small scale and expensive. The most useful methods are chemical carbon capture done at power plants, to reduce some of the carbon released.

There is, however, a “technology” that cheaply and automatically captures carbon from the air and binds it up in solid form – trees. This is why there is much discussion of planting trees as a climate change mitigation strategy. Trees, however, eventually give up their captured carbon back into the atmosphere. So at best they are a finite carbon reservoir. A 2019 study found that if we restored global forests by planting half a trillion trees, that would capture about 20 years worth of CO2 at the current rate of release, or about half of all the CO2 released since 1960 (at least as of 2019). But once those trees matured we would reach a new steady state and further sequestering would stop. This is at least better than continuing to cut down forests and reducing their store of carbon. Tree planting can still be a useful strategy to help buy time as we further decarbonize technology.

But what if we could keep trees from rotting and releasing their captured CO2 back into the atmosphere? They could then become a longer term sequestration strategy. One way to do this is to build stuff out of the wood, and this also has already been proposed. There is a movement to use more wood for commercial construction, as it has a lower carbon footprint than steel or concrete. Wood in a building that is kept dry can easily last hundreds of years.

A recent study now offers a potential other option – we could just bury trees. But wait, won’t they just rot under ground and still release their CO2? Yes – unless the soil conditions are just right. Ning Zeng and his colleagues set out to study if wood could survive long term in specific kinds of soil, those with lots of clay and low oxygen. Zeng found a location near Quebec with soil conditions he thought would be conducive to preserving wood long term. He dug a trench to place fresh wood in the soil so they could then track it over years and measure its carbon release. But here’s the fun part – when they dug the trench they found a log naturally buried in the soil. They examined the log and discovered that it was 3,775 years old. Not only that, they estimate that the log has lost less than 5% of its carbon over that period of time. Nature has already conducted the experiment Zeng wanted to run, so he published those results.

What this means is that we can potentially just grow trees, find or even create locations with the right conditions (clay seems to be the key), and just bury the logs. Then replace the trees and capture more carbon, without the older trees releasing their carbon back. They analyzed the potential of this method and found:

“We estimate a global sequestration potential of up to 10 gigatonnes CO₂ per year with existing technology at a low cost of $30 to $100 per tonne after optimization.”

That is a lot. The global release of CO2 is now at about 36 gigatonnes per year, so this would be more than a quarter of our current release. So if we can get our global CO2 release to less than 10 gigatonnes per year, and combine it with burying logs in the right conditions, we could get to net zero, and even net negative. Current methods of direct air capture of CO2 cost $100-$300 per tonne, so if we can get this approach closer to the $30 per tonne cost that would be potentially viable. At the low end sequestering 10 gigatons per year of CO2 using this method would cost $300 billion per year. That’s a big number, but not that big if we consider this a global project. Estimates of the cost of global warming range from $1.7 to $38 trillion dollars per year by 2050, which means this could be a cost-effective investment. f

Obviously before scaling up this approach we need more study, including a survey of potential locations. But we can certainly get started planting some trees while we figure where to put them. And a point I frequently make – we should not be putting all our eggs in one basket, or necessarily looking for the one solution to climate change. Reforestation, wood construction, and wood vaulting, combined with other carbon capture technologies, can all work together. We can use trees to capture a lot of carbon over the next 50 and 100 years, altering the path of global climate change significantly.