#1. Enhanced rock weathering carbon capture: Measuring it right
An essay on challenges & breakthroughs of carbon capture measurement in Enhanced Rock Weathering (ERW).
Before going into the deep end, let’s knock off some basics about weathering from our charts.
Rock weathering is a natural process where carbon dioxide in the atmosphere reacts with rain water to form a weak acid called Carbonic acid (H2CO3).
When this weakly acidic rain water (carbonic acid) comes in contact with silicate rich rocks like basalt, they form something called ‘carbonates’.
Carbonates are chemically stable compounds that keeps CO2 locked in.(Read this detailed explanation by Lithos Carbon)
These carbonates either exist as solid, permanently locked in stones such as calcium carbonate or they get flushed down to oceans in dissolved form as bicarbonates (HCO3–) where they are locked for 100,000+ years.
Enhanced rock weathering (ERW) just accelerates this natural process.
Silicate rich rock dust such as crushed basalt is applied on huge farmlands in a scientifically modelled way, such that it transforms the crop fields into carbon capture centers (credit: Lithos Carbon) while improving soil fertility.
As rock dust has far greater exposed surface area than solid rocks, it accelerates the weathering reaction to capture CO2.
While Enhanced Rock Weathering (ERW) has the potential to remove gigaton scale carbon while boosting crop yields, the big question is -
How can we reliably measure the amount of carbon captured ?
In other words, how can we be certain that we actually captured as much carbon as we estimated ?
Measuring ERW carbon capture: Difficult, not impossible
For ERW to be seen as a trustworthy carbon removal pathway, we must find a credible & rigorous way to verify the quantity of carbon removed over time.
This is where it gets incredibly complex.
This is because farmland soil (to which rock dust is applied) is an open system, unlike a DAC (Direct Air Capture) where carbon removal happens within specific equipment. Soil freely interacts with its surrounding environmental factors such as rainfall, moisture evaporation, flora, fauna, microbes etc. most of which are hard to measure and control.
Here’s how -
The expected amount of carbon collected by 1 cubic meter of soil sample over a year is ~120g/year. This is just 5-8% of the annual carbon flux through the soil.
Now add to that factors like - 500-1500 liters of annual rainfall, 200-800 liters of annual evaporation, that keep altering the concentration of whatever little carbon is collected.
Then there are factors like varying soil bio-chemistry, sub-soil flora/fauna, alkalinity, topography etc. that keeps changing between fields and even within the same field.
With so many variables, it’s nearly impossible to directly measure the amount of carbon collected in a farmland due to enhanced weathering.
Crossing the MRV chasm
In order for ERW to transition from a potential pathway to a promising pathway for gigaton carbon drawdown, it needs to cross the MRV chasm.
MRV stands for Measurement, Reporting and Verification - a system designed to prove & report that a specific method has actually removed CO2 from the atmosphere as projected.
It is a multistep process - begins with establishing a baseline level, collecting data over time, and submitting results to a third party to verify accuracy.
Setting up a MRV method for enhanced weathering is complicated, as it is nearly impossible to directly measure carbon collected in open systems (such as croplands) as against closed systems (like Direct Air Capture)
So how do we cross the MRV chasm ?
The first step is to measure it right.
This starts at building robust models based on rigorous “in the field” sampling as against lab experiments that help measure proxies to make an estimation of capture rates.
There are different approaches to measure enhanced weathering ‘in the field’.
One approach is to monitor the amount of reaction products (bicarbonates) due to weathering, dissolved in drainage waters over time. In other words, a water based alkalinity analysis. The disadvantage here is - it gives us a point in time direct measurement of weathering rate, not a cumulative one.
Another method to estimate weathering rates is by measuring the change in relative abundance of certain isotopes in the soil.
This is where companies like Lithos Carbon has pioneered a method of estimating carbon capture rates from declining abundance of cations relative to Titanium isotopes in the soil.
To elaborate a bit -
It deploys a concept called a two component mixing model - where basalt mixed with soil results in certain relative abundance of carbon removing elements (such as Magnesium) present in basalt dust.
As weathering progresses, this relative abundance goes down - as Magnesium locks carbon in form of carbonates and leaves the soil as drainage run-off.
However, the implementation is a bit more nuanced. They use isotope dilution mass spectrometry to measure the amount of mobile elements ( Sodium, Magnesium, Calcium ions) lost from the soil relative to immobile Titanium ions in the parent soil.
This gives us an indirect measurement of how much carbon gets removed via weathering.
Over the next few years, companies like Lithos Carbon & UNDO, plan to collect lots of data points from such field trials & empirical analysis and use it to train their machine learning models.
Lithos has gone one step ahead & deployed a cradle to grave measurement framework.
It calculates carbon capture not only at the field level but also extrapolates it to estimate carbon carried from field through river run off & sequestered permanently in oceans.
Emerging 3rd party MRV protocols for ERW
While ERW project developers are doing their bit to build robust measurement systems, platforms like Puro and Verra are working to establish a 3rd party method to verify these removals and tie it to carbon credits.
Puro came out with Enhanced Rock Weathering in Soil Methodology, with information on eligibility requirements and verification, risk management measures, and requirements for simulation-based quantification approaches. Whereas, Verra is setting up an advisory group to create verification standards for ERW CDR credits.
Wrap up
ERW has the potential to offer a gigaton CDR pathway at sub $100/tCO2e cost point. However, establishing robust empirical methods for estimating weathering driven capture rates and translating it into a industry accepted MRV protocol is critical.
It will be one giant leap in the effort to build credibility & trustworthiness of ERW as a source of high quality, long permanence CDR.
In effect, this could unlock carbon removal purchases from second generation corporate buyers that move the needle towards mass market adoption of CDR.
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