Biochar & Climate change

Biochar is well known as an option to mitigate climate change. Here, we want to give the big picture & revisit some of the basic science behind it.

Last updated: January 19, 2022

Outline

A. Biochar in the global carbon cycle

To grasp the bigger picture, one must start with the global carbon cycle. Figure 1, reproduced from Friedlingstein & colleagues (2020, available online) describes two features of the global carbon cycle:

The repartition of anthropogenic carbon dioxide emissions in the environment
The magnitude of the fast and slow carbon cycles

Figure 1. Representation of the overall perturbation of the global carbon cycle caused by anthropogenic activities, averaged globally for the decade 2010–2019. ([Friedlingstein & colleagues, 2020](https://doi.org/10.5194/essd-12-3269-2020)) — Figure 1. Representation of the overall perturbation of the global carbon cycle caused by anthropogenic activities, averaged globally for the decade 2010–2019. (Friedlingstein & colleagues, 2020)

1. Repartition of anthropogenic carbon dioxide emissions

Looking at the thick arrows in Figure 1, this figure explains where anthropogenic emissions of carbon dioxide are distributed in various environmental compartments (ocean, atmosphere, terrestrial biosphere).

On the one hand, anthropogenic emissions of carbon dioxide include: 9.4 GtC yr^-1 of fossil carbon and 1.6 GtC of biogenic carbon arising from land use changes.
On the other hand, this carbon is dispersed in the environment: 3.4 GtC yr^-1 is taken up by vegetation, 2.5 GtC yr^-1 is taken up by oceans, and 5.1 GtC yr^-1 is taken up by the atmosphere.

You notice that there is a balance (neglecting here vulcano emissions & smaller uncertainties): (9.4 + 1.6) - (3.4 + 2.5 + 5.1) = 0. In addition, the +5.1 GtC yr^-1 that add up in the atmosphere tells by how much the concentration of carbon dioxide in the atmosphere is increasing, and thereby sets the magnitude of the additional greenhouse effect arising from carbon dioxide.

2. Fast & slow cycles of carbon

Now, looking at the thin arrows in Figure 1, the figures explains the carbon cycling in the ocean, the terrestrial biosphere, rivers, lakes, and coastal areas. The geolocical carbon cycle is not visible, mainly because it is extremly slow (turnovers of millenias to millions of year) leading to small/neglectable natural flows. For the fast carbon cycles (daily to decadal & millenia turovers), the flows are much larger.

One is of particular interest to biobased technologies like biochar: the carbon cycle from vegetation, with its two main mechanisms photosynthesis & respiration. These flows are of about 120 GtC yr^-1 (Ruy, Berry and Baldocchi, 2019). Biomass based removal technologies are basically trying to imbalance the two flows in this cycle: increase photosynthesis (biomass productivity) & slow down respirtation (biomass decomposition).

Anthropogenic carbon removals: how long is long enough?

In short, the longer the better. At minima, to have an effect on the climate system, the removal should last significantly longer than the turover rate of the carbon cycle from which the removed carbon came from.

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Global biochar carbon sequestration potential

There are few studies that estimated the potential for biochar production & related carbon sequestration in biochar. The potential could be up to 3 GtC yr^-1 or, equivalently, 10 GtCO₂ yr^-1

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B. Biochar carbon sink & climate change impact?

Biochar carbon sink, carbon sequestration, carbon removal, biochar decomposition, biochar stability, & climate impact of biochar systems.

All these terms are related. Some are synonyms. Some have different meanings. Let’s clarify them.

First, the largest difference is between the biochar carbon sink & climate change impact of a biochar system. It is illustrated in the Figure 2, below.

Fig 2. The carbon sink of biochar is only a part of the climate change impact of a biochar system.

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Definitions of terms
Reminder: what matters is that biochar systems mitigate climate change, from a life cycle perspective
Insight from LCA research: biochar C sink term is largest contributor to overall climate impact of the systems. Hence, importance to ground assumption on stability on solid evidence

C. Biochar decomposition: how is it measured?

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Basics in soil sciences
Biochar in soils: main processes at play
Incubation experiments, lab & field

D. Biochar stability estimates: how are they built?

Biochar stability estimates have been derived according to two main approaches:

Physical or chemical oxidation & benchmarking against fossil coal
Extrapolation of incubation data to long-time horizons

Here, we only cover the second approach as it relies on actual observations and is becoming the dominant approach in the recent research literature.

The process to extrapolate incubation data to longer times is described in Figure 3, alongside with methodological choices that arise along the way.

Fig 3. From incubation data to stability estimators

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Description of the extrapolation process

long term extrapolation
correlation to properties

Published analyses of biochar incubation data & suggested correlations

E. Financing carbon removals & policy implications

Financing action to mitigate climate change has a long history. For several decades, it took the form of climate compensation through the financing of projects that avoided fossil fuel emissions, deforestation, or encouraged replanting of trees. This was not without debate & distrust. Today, the compensation or offsetting market is moving towards new types of projects: carbon removal projects.

Let’s be clear:

The role of finance is to catalyse change by channelling resources where needed
Climate compensation are a type of financial product, available on voluntary & regulated markets
Climate financial products are built on trust in the seller & feelings created for buyers
Buying climate compensation products can never be a substitute to reducing one's greenhouse gas emissions

When it comes to biochar, in the early 2020s, “biochar C removal certificates” have been defined & are being sold on a non-regulated voluntary market by a handful of organisations.

How do “biochar C removal certificates” work?

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Comparing some methodologies that estimate 100-year stabilities

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Caption of the table
Your biochar data
Pyrolysis temperature (°C)	Carbon content (%, db)	Hydrogen content (%, db)	Oxygen content (%, db)

Calculation methods & returned values
Method	IPCC (2019)	IBI (2013)	Spokas O:C (2010)	EBC C-sink (Agro)	EBC C-sink (Charcrete)
100-year stability (%)	n/a	n/a	n/a	n/a	n/a
100-year C sequestration (kg C/tonne biochar)	n/a	n/a	n/a	n/a	n/a
100-year CO₂ sequestration (kg CO2-eq/tonne biochar)	n/a	n/a	n/a	n/a	n/a
CO₂ sequestration after years (kg CO2-eq/tonne biochar)	n/a	n/a	n/a	n/a	n/a
Notes	Enter your biochar properties in the first row for the calculations to start...

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