Durability Assessment for Carbon Removal: Analysis

[Geoengineering News]

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https://bezerocarbon.com/insights/durability-assessment-for-carbon-removal-analysis/

04 October 2023

By BeZero Carbon

Executive summary

In any net-zero world, gigatonne-scale removal and storage of carbon dioxide will be essential. Removals have a critical role to play in slowing the pace of climate change, counterbalancing emissions from hard-to-abate sectors, and ultimately helping us to remove more carbon from the atmosphere each year than we emit.

All removals have the ultimate goal of removing carbon from the atmosphere and storing it for a period of time, but understanding how long the carbon remains stored away from the atmosphere is important. There are a range of different carbon storage methods, and there is significant variation between them in terms of the length of time for which they store carbon and the risk of reversal they face.

As the voluntary carbon market continues to mature, buyers are expected to increasingly look towards long-duration removals. As a result, a key emerging consideration for these buyers is durability - a measure of how robustly the carbon is stored. Durability exists on a spectrum, but is a desirable characteristic; the more durably carbon is stored, the less likely it is to ever return to the atmosphere on relevant timescales and contribute to climate change again. 

However, durability is just one metric of quality for carbon credits. It is not an indicator of the likelihood of the credit achieving a tonne of CO2 removed for the duration of the commitment period, which is best assessed by the BeZero Carbon Rating.

This assessment explores the durability of seven leading carbon storage methods: geologic injection, in-situ mineralisation, biochar in soil, enhanced weathering, ocean alkalinity enhancement, carbon in concrete, and forestry projects. 

BeZero Carbon’s opinion on the durability of each of these methods is based on two factors: 

• Physical storage - the potential lifetime of the stored carbon within the medium.

• Risk of reversal - the human or natural factors that could lead to the reversal of CO2 stored and therefore reduce the durability of storage.

However, many storage methods are novel approaches that have not yet been demonstrated at scale. In recognition of this, the analysis also considers the degree of scientific certainty regarding each method, which underpins the confidence in our assessment of durability. A high degree of scientific certainty is also a desirable characteristic for removal methods, alongside durability, as both can help to de-risk purchases and add value to credits.

Between the different carbon storage methods assessed, there are significant variations in durability. The most durable storage methods are likely to be capable of storing carbon for thousands of years or more, with a very low risk of reversal, while the least durable may only store carbon for a number of decades and can face significant reversal risk over that period. 

The most durable storage methods are in-situ mineralisation and carbon in concrete. These methods can both store carbon for many thousands of years, effectively permanently, and with low risk of reversal. 

Geologic injection, enhanced weathering, and mineral ocean alkalinity enhancement are also all likely to store carbon for millennia. However, these methods face more significant risk of reversal, which means they may be less durable than in-situ mineralisation and carbon in concrete. In addition, enhanced weathering and mineral ocean alkalinity are unproven at scale and there remains significant scientific uncertainty regarding these methods in practice.

Biochar in soil is less durable than the above carbon storage methods, with shorter storage timeframes and generally a greater reversal risk. Biochar in soil is likely to store carbon for centuries, but its durability is variable depending on the conditions in which it is produced and those of the soil in which it is spread. Some forms of biochar may therefore be far more durable than others, although this is yet to be fully determined as there are a number of remaining research gaps. 

Forestry is the least durable carbon storage method of those assessed. Forestry projects store carbon over a period of decades to centuries and face a number of significant threats such as wildfires, land-use change, and biotic agents. However, forestry projects have by far the highest level of scientific certainty, with an extensive literature base and a track record of implementation at commercial scale for many years across the globe.

Overall, there is significant variation in the durability of carbon storage between different leading methods. As we move towards a net-zero world, in which large-scale carbon removal is unavoidable, durability should be an important consideration for credit purchasers while developing their portfolios.

Source: BeZero