Science

How do we measure Carbon Dioxide Removal?

10

minute read

Quantifying Carbon Dioxide Removal Through Enhanced Weathering: A manuscript

Enhanced weathering (EW) is emerging as an exciting new carbon dioxide removal (CDR) technique. Enhanced weathering is scalable now and has a staggering potential to remove gigatonnes of CO2 annually. However, monitoring, reporting, and verification (MRV) of CDR with EW – how we measure its effectiveness – is widely viewed as the biggest barrier to scaling of the technique. Robust MRV is crucial to the widespread operationalization of EW. Ultimately, trust in the EW industry depends on these three letters.  

To bring greater transparency to the topic, InPlanet’s research team has documented the current state of MRV science across industry and academia, focusing on the measurement or quantification of CDR. Matthew Clarkson, Christina Larkin, and Philipp Swoboda, together with leading scientists from across the world, gathered all the knowledge available from existing literature and summarized decades of research into one manuscript that is now publicly accessible. This manuscript is intended to help develop the industry as a whole and will undoubtedly become a useful resource for anyone interested in EW.

Our Head of Carbon, Matthew, comments on the process: “Whilst developing our internal methodology, we decided this knowledge needed to be shared rather than locked away in our drive until we completed the crediting process. We prioritized creating a resource that can accelerate the entire industry, offering the much-needed clarity required by the wider CDR community.”

The first publication of the manuscript as a pre-print in November 2023 was very timely, helping to contribute to the formation of emerging standards. The original pre-print received some excellent feedback and clocked up over 1800 views. Now the manuscript has undergone external third-party review by academics and is accessible as an open-source publication in Frontiers in Climate.

Here, we summarize the manuscript, which focuses on the science of quantifying CDR with EW, the most critical cornerstone of MRV.

Rock Solid Science

The challenge in calculating CDR with EW is that it is, by nature, an open-system CDR pathway. Open-system pathways for CDR are conducted in the real world, in our oceans or fields, rather than in a lab or under controlled conditions. As you might expect, real life is more complex and noisy.

Fortunately, we are not developing these methods out of thin air. The science applied to MRV in EW is grounded in well-established research, combining geochemistry, geology, agronomy, and soil science. We have gathered what is known about rocks, soils, plants, and weathering and are applying this knowledge to establish robust quantification of EW. 

“There was a lot of perceived mystery around measurement approaches for EW. But people didn’t appreciate that the fundamental science behind these methods is well established,” says Matthew Clarkson.  

Christina, our Head of Science and Research, having previously undertaken EW field trials as a Research Fellow at the University of Southampton, highlights that “while there is complexity in open-system pathways, there are also many options to trace reactions and robustly estimate CDR.”

Mariane Chiapini in the laboratory pipetting water sample

MRV in a cup of tea

At InPlanet we use an analogy to better illustrate the two main measurement approaches: solids and liquids. 

Picture yourself relaxing in the morning, and a loved one brings you a cup of hot beverage. Because you have a sweet tooth, you want to confirm if they added sugar to it. Of course, you can ask them, but maybe you want to verify this, too! 

One way to find out is to pour out the drink and check the bottom of the cup for any remaining undissolved sugar. This gives you a rough idea of the sugar content, but you miss out on savoring the drink. 

Alternatively, you can taste the beverage directly to estimate how much sugar is in it. When tasting the beverage, you may have more insights such as what the beverage is, if it is coffee or tea; what kind of tea it is; and if it is a blend; where it is from; etc. You could even take a sample to measure the caffeine content if you wanted to.

Similarly, in our weathering process, we analyze both solids (the mixture of soil and rock powder) and liquids (soil waters) to fully understand how much carbon dioxide is being removed. With solid measurements, we estimate how much rock powder has dissolved based on what remains in the soil. If we know how much was added, we can then infer how much CO2 was removed during the dissolution. This is an indirect measurement approach.

By contrast, by looking at the soil waters, we measure the removed carbon dioxide directly as dissolved inorganic carbon. We also learn so much more, including, for example, the pH or temperature of the water and other important parameters. This approach is the most direct approach to CDR quantification.

It is the combination of these two methods that gives us the most complete picture of EW and its carbon removal efficiency.

You can learn more about the coffee analogy by listening to it directly from its ideator, Matthew Clarkson, who was featured on Carbon Removal Newsroom, a podcast by Nori. Start listening from minute 5:00 to for deeper insight on “How InPlanet is Growing ERW.”   

What are we looking out for?

To understand these different types of analysis, we first need to identify the main things we want to quantify to track how rocks are weathering, namely bicarbonate and cations. Taking a step back, we can look into the chemical reactions that occur when rocks react with carbon.

Enhanced weathering works through a chemical reaction in which CO2 is partially dissolved in rainwater, forming a weak acid (carbonic acid). When rainwater comes into contact with rocks, it reacts with the minerals present in it to form bicarbonates, a stable molecule that is transported via rivers to the ocean, where the carbon is stored securely for thousands of years. Learn more about this fascinating science here.

Cations 

Cations are positively charged atoms. They are released when the carbonic acid in the rain reacts with rock minerals. This means that cations can be found both in the mixture of soil and rock powder, as well as in drainage waters. The most common cations formed in Enhanced Weathering are Ca2+ and Mg2+.

Bicarbonates

Bicarbonate (HCO3-) is a form of dissolved inorganic carbon. It is the product of the chemical reaction between rocks and rainwater. During this reaction, CO2 is converted into dissolved bicarbonate. Dissolved bicarbonate ions can be measured directly in drainage water or soil water samples.

Measurement approaches

The manuscript goes into depth about the different methods for measuring the weathering of rock powder and summarizes all the advantages and challenges. Below, we explain the difference between them and the main benefits of each.

“Tracing geochemical reactions with different measurements is a difficult task, but it can be incredibly powerful when we combine approaches, helping us navigate natural complexity,” points out Matthew.

Picture of pot experiments in a greenhouse

Solid Phase Measurements 

Solid phase measurements involve collecting samples of a mix of solids and rock powder during different stages of weathering. Think of the sugar on the bottom of the cup – this is an indirect analysis, but it still provides interesting and important data about how rock powder is reacting with the soil. 

One of the main advantages of this test is that collecting samples is easier. While liquid sampling would involve traveling to the field of application to collect analysis every time it rains, solid samples can be collected in scheduled routine visits to the application site, perhaps once a year. 

However, one of the main disadvantages is how noisy the signal may be and the number of soil samples needed to capture full variability. Think of a few grains of sugar mixed in with tea leaves at the bottom of your cup. The rock powder is mixed in with soil, and depending on how different that soil is from the rock powder, there may be difficulties in tracing the signal.

Once in the lab, soil samples undergo tests, such as geochemical analysis for cations, to measure the changes in solid-phase minerals and assess the progress of weathering reactions.

Water Phase Measurements

Water analysis is carried out by collecting soil drainage waters, which means the water that goes through the soil when it rains. Think of the water that comes out at the bottom of the plant pot when you water your houseplants.

Water samples need to be collected every time it rains to provide a full picture of how carbon dioxide is reacting with rock powder. Therefore, liquid analyses are more costly and labor-intensive to collect and test.

Once in the lab, water samples go through multiple analyses to measure phases such as dissolved inorganic carbon (DIC), which is an essential test for understanding the flux of carbon dioxide removal through enhanced weathering. Other techniques like ion chromatography, mass spectrometry, and pH measurements are employed to track the concentration of bicarbonate ions and other weathering products in the water phase.

Hand weating blue glove holding  a pH-meter and pipetting water with the other hand

Solids, liquids….and gases?

There is the exciting possibility that measuring gas fluxes into the soil might be able to inform directly on CDR by EW. Gas fluxes can be measured using a chamber placed on top of the soil where rock powder is applied. Using this, CO2 fluxes into and out of the soil can be determined. However, the results of this approach are very difficult to interpret. Unfortunately, the CO2 signal from the inorganic carbon cycle tends to be overwhelmed by the temporary but faster organic carbon cycling (think photosynthesis and respiration). This means that plants and microbes dominate these measurements, and it is hard to resolve the slower and smaller changes resulting from EW. 

Main takeaways

We emphasize that different methods can and should be combined to provide a better estimate of CDR with EW. As an aside, a co-benefit of this robust MRV approach is that the same samples and analytical methods are also used for environmental monitoring. This ensures regular checks and environmental safeguarding of EW deployments.

We highlight that redundant measurements (tackling the same quantification through different methods) will ensure the accuracy and reliability of CDR quantification. It takes more effort and investment, but carrying out multiple measurements is key to guaranteeing reliable and trustworthy MRV in the industry. To manage and analyze all these samples, InPlanet relies on a diverse team of PhD-level scientists committed to going the extra mile to deliver the best and most accurate MRV in the market.

This thorough review concludes by highlighting the need for strong methods to measure, report, and verify EW’s effectiveness in removing carbon dioxide. Using advanced techniques and being transparent about the results will guarantee the effective scaling of EW and allow investors and stakeholders to support innovative climate solutions for a better future.

Picture of the science team in the lab

References

Clarkson, M. O., Larkin, C. S., Swoboda, P., Reershemius, T., Suhrhoff, T. J., Maesano, C. N., & Campbell, J. S. (2024). A review of measurement for quantification of carbon dioxide removal by enhanced weathering in soil. Frontiers in Climate, 6. https://doi.org/10.3389/fclim.2024.1345224

Authored by:

Dr Matthew Clarkson
Dr Christina Larkin
Dr Philipp Swoboda