Woody biomass burial

Introduction

As the Intergovernmental Panel on Climate Change concluded in its Sixth Assessment Report, atmospheric removal of carbon dioxide is “an essential element” of strategies to meet the Paris Agreement’s objectives of holding temperatures to 1.5°C or below 2°C above pre-industrial levels. In many climate scenarios, the scale of requisite atmospheric carbon dioxide removal in the latter half of this century is as much as 10–20 gigatons annually. Cumulatively, the world community may need to remove a whopping 700–1,000 GtCO2 from the atmosphere between 2011–2100. 

However, some of the approaches expected to bear much of the load in the carbon removal sector face major challenges. Direct air capture technologies can extract carbon dioxide from the atmosphere, with many approaches using chemicals or physical filters. However, while some analysts have projected that direct air capture could remove up to 310 gigatons of carbon dioxide by 2100, the technology faces some serious headwinds. For example, a recent report indicates that the Icelandic direct air capture facilities of the leading company in the sector, Climeworks, have underperformed to date. Moreover, the cost of capturing a ton of carbon dioxide with this approach still hovers around a thousand dollars per ton, and it’s far from clear how quickly costs will decline. 

Another widely touted approach is bioenergy with carbon capture and storage (BECCS), which involves the generation of energy with biomass feedstocks, paired with systems to capture carbon dioxide at the point of combustion or fermentation. in scenarios that hold temperatures to 2°C in 2100 BECCS has been projected to sequester between 170-650 GtCO2, with one study finding that the approach could be responsible for half of carbon dioxide removal by 2100. However, serious questions abound about the sustainability of this approach, including the potential impact on food prices of diverting large amounts of land for biomass production, as well as huge water demands. 

The need for relatively inexpensive, sustainable, and easily deployable carbon removal approaches has given rise in recent years to discussion of a CDR approach termed “woody biomass burial” (WBB). WBB weds biology and geology to effectuate carbon removal. It has been characterized as “the simplest, lowest tech way to hide carbon from our atmospheric cycle.” This article will seek to assess the potential prospects for large-scale deployment of WBB, as well as potential barriers and risks. 

Overview of woody biomass burial 

Forests are an extremely important terrestrial carbon sink, taking up nearly 16 gigatons of carbon dioxide annually through photosynthesis, and holding approximately 861 gigatons of carbon in branches, leaves, roots, and soils. However, huge amounts of carbon are released back into the atmosphere through: 1. burning, including fires and fire risk management practices, such as prescribed burns, and 2. decomposition of deadwood, which usually occurs in a few decades at most, due to degradation by fungi, insects and microorganisms. Deadwood holds approximately 8% of forest carbon pools. It’s been estimated that this results in the release of almost 11 gigatons of carbon into the atmosphere annually, equivalent to approximately 115% of the emissions associated with fossil fuel combustion. Moreover, climate change is projected to substantially accelerate annual carbon fluxes from deadwood in boreal regions, with one recent study suggesting a 27% increase at one research site under projected levels of warming. 

Woody biomass burial involves burying woody biomass underground in an environment that sustains anaerobic conditions. This can limit the decomposition of the wood over time, halting the release of carbon stored in the biomass back into the atmosphere for protracted periods of time.

The most widely discussed option is the “Wood Vault” concept developed by Ning Zeng, a professor at the University of Maryland, and colleagues. The Wood Vault approach contemplates the collection of both “opportunistic sources” of woody biomass, such as logging residue, thinning for fuel treatment in forests, wood trimmings from backyards, and wood processing residue, as well as wood harvested from forests developed for the purpose of effectuating sequestration of carbon dioxide.

The “poster child” Wood Vault is a burial mound that can accommodate approximately 100,000 tons of green biomass in a space of one hectare (equivalent to two football fields). The wood would be stored in a trench five meters deep, covered by a mound 20 meters above ground, designed to shed rainwater and ward off surface pond water. About a third of the filled space in the trench would be occupied by a clay sealant, water and soil backfill. Surrounding the biomass with clay would help to starve it of oxygen and water, which can ward against decomposition. Segregation of topsoil, which contains bacteria with enzymes that also promote decomposition, is also a critical aspect of the construction of the vaults. Current cost estimates for the process are $100-200 per ton of carbon dioxide sequestration, and proponents contend that optimization could reduce costs to $30-100 per ton over the next 10-20 years. 

It's been projected that wood burial could be scaled up within a decade or two to sequester 2-10 gigatons of carbon dioxide annually, with the largest potential in South America and Africa, followed by the United States and China. If WBB could be used to lock away 10 GtCO2 annually, it would offset 27% of current annual greenhouse gas emissions, and would be sufficient to meet the IPCC’s net-zero scenario for holding temperatures to 1.5°C by 2060. However, the upper end of these estimates is premised on the utilization of all available coarse woody biomass, and is thus probably unrealistic. Moreover, there could be constraints associated with energy and material requirements to construct vaults, and opportunity costs of foregoing alternative uses of biomass. 

Most models that incorporate CDR into climate policymaking rely heavily in the next few decades on so-called “nature-based solutions,” such as tree-planting and enhancing uptake of carbon dioxide by soil. However, in the longer term, there is a recognition that neutralizing carbon dioxide emissions that remain in the atmosphere for centuries to a millennium or more requires commensurately “durable” storage. These are often referred to as “like-for-like” approaches. 

One of the compelling aspects of WBB is that it could prove highly durable. The Intergovernmental Panel on Climate Change concluded that WBB could retain 99.9% of carbon dioxide over a century, while other recent studies have projected that it could lock in carbon for a thousand years or more. Some of the most compelling evidence to support this proposition is so-called “natural experiments,”  where biomass has been found to be in good condition “for hundreds, thousands, tens of thousands, hundreds of thousands, or even millions of years” in archaeological and geological sites.

Potential risks of WBB

Despite the potentially compelling benefits of WBB in combating climate change, it also could pose substantial risks, especially in scenarios of large-scale deployment. These include the following:

• Methane production in buried biomass: Anaerobic decomposition of biomass can enter a phase in which methane-forming bacteria can produce large amounts of the gas. This could seriously undermine the climate benefits of WBB given the fact that methane has a global warming potential of 27-30x over 100 years. However, given the fact that methane formation is projected to be low in such operations, and spread over a protracted period of time, it’s not likely to have serious implications in terms of radiative forcing.

• Denuding forest growth by burying critical nutrients and other ecosystem impacts: WBB could siphon off nutrients from deadwood that are critical for ongoing forest growth through recycling. While some preliminary analysis indicates that it may not pose a serious risk, more research is dictated, and it may ultimately counsel against WBB in some regions or local ecosystems. Deadwood also often serves as habitat for many flora and fauna species, suggesting that we need to assess whether it makes sense to leave a portion on the ground to maintain the health of ecosystems.

• Diversion of wood from potentially more propitious options to address climate change and other issues. Additional research is necessary to determine if woody biomass burial is the optimal use of wood resources from a climate, or a more holistic, perspective. For example, wood can be used as a feedstock for BECCS, which has the co-benefit of energy production, or biochar, which can enhance crop yields and potentially lock up other greenhouse gases in addition to carbon dioxide. This will require a complex analysis that takes into account factors such as economics and the carbon efficiency of alternatives. 

Conclusions

We are in the early stages of exploring the potential viability of WBB. Puro.earth, a carbon removal registry, has developed a methodology to facilitate the issuance of credits for WBB projects. It recently issued carbon removal credits for a pilot project by a company called Woodcache PBC. The company collected and buried 80 tons of unmerchantable deadwood in Colorado. There are other companies pursuing similar projects focused on forest biomass, including Mast Reforestation, InterEarth, and Carbon Sequestration Inc. There are also other companies casting their net wider, including, Graphyte, which is focused on the burial of timber and agricultural residues. Commentators have emphasized the need for development of a portfolio approach to promote synergies between carbon removal options and minimize economic and sustainability risks. WBB warrants serious consideration to be part of this portfolio.

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