A Breakthrough in Verifying Carbon Dioxide Removal by Rock Dust Weathering

The weathering of basalt bedrock (cracked material at the base of the picture) to form laterite soils (dark brown material) in Madagascar. This process of soil formation removes carbon dioxide from the atmosphere (image credit Werner Schellmann).

For over a billion years, rock weathering has played a central role in regulating Earth’s climate. So what’s behind the rock-climate connection? It turns out that the chemical breakdown of rocks naturally removes carbon dioxide (CO₂) from the atmosphere by converting it into carbonate molecules. 

Carbonates are “stored” by nature as dissolved ions such as bicarbonate (HCO₃^–) in the ocean and as calcium and magnesium carbonate minerals in limestones. The remarkable thing about this process is that carbon naturally stored as carbonate is kept stable (out of the atmosphere) for hundreds of thousands to millions of years – essentially permanently from a human perspective. 

The removal of carbon dioxide from the atmosphere by rock weathering decreases Earth’s greenhouse effect, leading to lower global temperatures. Over geologic time, this process acts as a balance to the natural sources of carbon dioxide, such as volcanic outgassing and other oceanic and terrestrial processes. And since the rate of rock weathering increases with increasing temperature, it acts as a sort of global thermostat. When volcanic activity causes an increase in CO₂ levels, the global temperature increases (due to the greenhouse effect); this, in turn, increases the rate of rock weathering, which consumes excess CO₂, thus moderating Earth’s heating over geologic time. 

The problem is our natural global thermostat has been overridden by human activity. The rapid emission of large amounts of carbon dioxide and other greenhouse gasses through the burning of fossil fuels is swamping the effects of natural rock weathering.  In other words, natural rock weathering – the major mechanism that sequesters CO₂ over geologic time – can’t keep up with the human output of carbon dioxide. 

To put this in perspective, it is estimated that natural rock weathering removes approximately one billion tons of carbon dioxide from the atmosphere every year. While global carbon dioxide emissions from human activities are on the order of 40 billion tons per year. This imbalance has led to a sharp increase in global CO₂ levels and a corresponding increase in temperature. It is estimated that the concentration of CO₂ in the atmosphere has risen from 280 parts per million (ppm) before the industrial revolution to 420 ppm today. This corresponds to a global temperature increase of 1.1 degrees Celsius and has led to major environmental and ecological stress on every continent. 

Lowering carbon dioxide emissions and actively removing carbon dioxide from the atmosphere is vital for the health and well-being of the global biosphere, which of course, includes us humans. The United Nations Intergovernmental Panel on Climate Change (IPCC) states that both aggressive CO₂ emission reductions and active removal of carbon dioxide from the atmosphere will be necessary to keep the Earth from reaching catastrophically high temperatures.     

Therefore, one of the key questions in the climate change discussion is how to remove carbon dioxide from the atmosphere, on a global scale, without disturbing ecosystems or causing other unintended negative consequences. This brings us back to rocks.  

Enhanced Rock Weathering and Remineralization

As discussed above, we know that rock weathering is a major natural force that removes carbon dioxide from the atmosphere and stores it for geologic time periods. The problem is natural rock weathering is too slow to keep up with anthropogenic CO₂ output. But what if we could accelerate or enhance the rate of rock weathering and use it as a natural means of atmospheric carbon dioxide removal? This is where enhanced rock weathering (ERW) comes in. 

Over the past decade or so, there have been hundreds of scientific studies showing that enhanced rock weathering is a technically feasible global carbon dioxide removal (CDR) strategy. The enhanced weathering process involves grinding rocks into fine powder, exposing them to natural weathering environments, and letting nature take its course. 

Fine-grained rock powders dissolve much faster than the rock outcrops, boulders, and cobbles that occur naturally. This is because the fine rock particles have more surface area, which makes them more reactive. For this reason, when fine-grained powders undergo natural weathering processes, the rate of weathering is much faster, which enhances the rate at which they convert carbon dioxide to carbonate molecules. 

A particularly promising strategy for deploying ERW is to mix the rock dust into agricultural soils. This terrestrial ERW approach is beneficial because it not only sequesters carbon dioxide but also improves soil quality.  Many natural rock types, such as the volcanic rock basalt, contain elements that are essential nutrients for plants: magnesium, calcium, and phosphorus, as well as trace elements that boost plant health and resilience. 

The process of adding rock dust to soils is an ancient practice commonly referred to as soil remineralization. Remineralization restores and optimizes soil nutrient levels and promotes the flourishing of beneficial microbial communities (including Mycorrhizal fungi that are essential for plant nutrient uptake). This process of soil restoration reduces or replaces the need for synthetic N-P-K fertilizers. 

Synthetic fertilizers have been shown to cause soil degradation in many ways, including soil acidification. They also cause environmental damage. Nitrogen run-off from these fertilizers ends up in our waterways, leading to ocean acidification and to the dying of coral reefs.   

By adding rock powders to soils, carbon dioxide is removed from the atmosphere not only by the enhanced weathering process but also by enhanced biological activity. Plant growth increases, which accelerates the formation of stabilized forms of organic carbon in the soil, another natural, long-term method of carbon storage. The carbon dioxide removal potential of soils can be further accentuated by the addition of biochar (plant material burned in the absence of oxygen) with the rock dust. 

The Scientific Challenges of Enhanced Rock Weathering Verification

The large-scale implementation of ERW over large areas of cropland (which is what will be needed to make a significant dent in atmospheric carbon dioxide levels) still requires scientific study. Questions remain about ERW’s effectiveness as a carbon dioxide removal process for different environmental conditions, soil types, and rock amendments.  This verification of ERW’s effectiveness is also a key aspect of commercialization. 

There are several startup companies that are implementing field-scale ERW projects and selling carbon removal credits. The entrance of ERW into the carbon credit economy is driving innovation in the field but has also brought up questions about how well-understood the process is. As stated by Dirk Paessler and others in the Carbon Drawdown Initiative May 17, 2023 working paper:

“[F]or ERW to become a viable CDR business and to make a significant contribution to climate change mitigation, we need to know the actual speed of this process at a scale of years in order to create certificates that can be sold economically.”

Certifying the amount of carbon dioxide that is actually removed by a particular ERW project involves monitoring, reporting, and verification of the actual field rates of rock weathering and the corresponding conversion of carbon dioxide to carbonate molecules. As Paessler and his Carbon Drawdown Initiative (CDI) colleagues point out, part of what makes verifying carbon dioxide removal by ERW challenging is that it is ultimately an indirect CDR process. 

All agricultural soils cycle carbon through various processes, such as photosynthesis, which converts CO₂ to carbohydrates in plants, and respiration, which emits CO₂ from biological activity and the decomposition of organic material. When rock powder is added to soils, it effectively lowers the amount of CO₂ released back into the atmosphere by soil respiration processes. This is because rock weathering converts some of the carbon dioxide gas trapped in soil pore spaces into carbonate molecules. 

As Paessler and others point out in their working paper, around 750 grams of carbon cycle through one square meter of cropland over one year on a sample field. Depending on soil type, plants, and location, however, this value can vary significantly. They estimate that ERW on this sample field will reduce this amount by around 50 grams per year. These values will usually be higher for tropical regions due to higher temperature and moisture levels. Therefore, measuring the carbon dioxide removal effect of ERW involves identifying a relatively small decrease in the total carbon cycle. 

This is made particularly difficult by variations in soils and the multitude of interconnected “parameters, processes and players.” These include the range of grain sizes, the types of minerals present, water content and flow, soil porosity, soil chemistry, plant roots, microbiological communities, nematodes, arthropods, chemical additives like fertilizers, organic additives like manure, secondary minerals, and human activity such as plowing and tilling. All of these variables and players influence each other, resulting in the fascinatingly dynamic and life-giving system we call soil.  

The complexity and messiness (from a scientific standpoint) of agricultural soils make it incredibly challenging – but not impossible – to single out and quantify one particular set of processes, such as the weathering reactions that consume CO₂. This is what Paessler and his team at the Carbon Drawdown Initiative have been focusing on for the past few years. The challenge at hand is summed up in the following statement (Paessler et al., 2023):

“For the [monitoring, reporting and verification] of CDR via enhanced rock weathering we need to know WHEN the actual permanent sequestration of CO₂ happens, i.e. when a CO₂ molecule is securely [and] permanently removed from the field’s carbon cycle and cannot get back to the ambient air. Only then have we achieved true CDR that can be sold on the carbon markets.”  

One option for achieving this is to take regular soil samples and use super-sensitive analytical techniques to determine the loss of rock dust over time. However, this technique does not account for processes that could dissolve the rock without consuming carbon dioxide. A more direct measure of ERW success would be to analyze the amount of carbonates produced by the weathering process. 

The presence of elevated bicarbonate concentrations in soil solutions following rock dust application is a tell-tale sign of active enhanced rock weathering and can be used to quantify how much CO₂ has been consumed by the process. The problem with this approach is that natural variations in soil chemistry make it difficult to determine when bicarbonate levels are elevated relative to natural background fluctuations. Soil solutions are constantly reacting and flushing downward through the various soil horizons. The heterogeneous nature of this process can lead to significant variations in bicarbonate concentrations, even for nearby samples.   

These rather gloomy perspectives on quantifying carbon dioxide removal via enhanced rock weathering are not simply pessimistic speculations. In their working paper, Paessler and others present results from several scientific studies showing how difficult it is even to detect, much less quantify, carbon dioxide removal from enhanced rock weathering. But that’s not the end of the story. 

It turns out that measuring carbon dioxide directly using simple, over-the-counter CO₂ gas sensors may be the key to overcoming the measurement challenges posed by soil heterogeneity and complexity. But to better understand how and why this works, we need to know more about the actual experiments that are being performed.

Enhanced Rock Weathering Verification Experiments

The Carbon Drawdown Initiative (CDI) started its experimental work in March 2021 with an open-field test in Fürth, Germany. For this experiment, researchers treated one large field with 40 tons per hectare of basalt powder and another large field with 240 tons per hectare of basalt powder. Biochar was also added along with the basalt powder. They monitored an adjacent untreated plot as a control. 

Initially, they could not get clear results. There wasn’t a reliable way to sort out the effect of weathering from all the other noise. By adding much larger amounts of rock dust (400 tons per hectare), they were at least able to demonstrate that weathering was occurring. While this is more rock dust than would be used in farming, the results could help give us an idea of what happens with more realistic application rates. Eventually, however, they concluded that instead they needed a new method of experimentation to get more precise measurements with lower amounts of rock dust. 

In 2022, CDI started their “XXL lysimeter” experiments, for which they installed 20 large (300 liters) lysimeters in soils adjacent to the open field tests. A lysimeter is a device that collects soil moisture so that it can be measured and chemically analyzed. There is a video that provides more information on the importance of lysimeters in soil research. In addition, CDI has written a blog post with specific information about CDI’s lysimeters. 

The basalt amendment rates for the large lysimeter experiments were 100, 200, and 400 tons per hectare. These application rates are considered excessive from a soil remineralization standpoint (the usual recommendation for soil enrichment is between 1 – 40 tons per hectare) but were employed in the experiments in an effort to generate a clear and measurable basalt weathering and carbon dioxide removal signal. The soil pore solution from these tests was sampled regularly and analyzed for the byproducts of CO₂ removal, which are released during basalt weathering. 

Also, in May 2022, soils from the Fürth experiments were used to set up a series of column experiments at the University of Hamburg. These more carefully controlled and instrumented tests were performed using the same basalt type and application rates as the XXL lysimeter tests. They provided data that complemented and confirmed observations made in the field. 

Then in January 2023, Carbon Drawdown Initiative started a suite of greenhouse experiments involving 350 smaller lysimeter pots containing different soil/rock dust mixtures. They are using combinations of 15 types of German soils and 11 different types of rock dust with various application rates. The greenhouse pots were planted with perennial grass and regularly irrigated. 

Ultimately, the point of all of this experimental work is to answer two questions: (1) how fast do the rock dust amendments weather under different conditions? and more importantly, (2) how much carbon dioxide is removed during this weathering process? As mentioned above, these questions proved difficult to answer with the chemical analyses of soil solutions from CDI’s experiments. 

The researchers were able to prove that the basalt was weathering by sending soil samples to colleagues at Lithos Carbon for analyses. The Lithos ERW verification method uses ultra-high-precision measurements of elements released by the basalt into the soil during weathering. Some elements, such as sodium, are mobile, that is, they are easily leached from the upper soil horizons. Other basalt-released elements, such as titanium, are immobile, that is, they are not easily leached and therefore accumulate.  By comparing the amounts of the mobile vs. immobile elements, researchers can determine precisely how much basalt has been weathered. From that, they can calculate the amount of carbon dioxide that has been removed by a particular rock dust application.

This method assumes that all of the basalt weathering was caused by the process that converts carbon dioxide to geologically stable carbonate molecules. This assumption is probably valid in the vast majority of cases; however, there are alternative chemical processes that could cause basalt weathering without removing carbon dioxide from the soil (such as weathering by nitrogen-based acids from the over-application of NPK fertilizer). Also, there are semi-permanent pools in the soil where the dissolution products can be “parked” for some time (cation exchange capacity) before the carbon dioxide removal effect actually happens. Thus, the Lithos method provides the maximum possible carbon dioxide removal value.

The samples from CDI’s XXL lysimeter tests that were analyzed using the Lithos method showed definitive signs of basalt weathering. The data indicated that after 8 months, enough basalt had been weathered to remove between 4 and 8 tons of CO₂ per hectare. 

The problem was the chemical water-based samples from these experiments did not show evidence of this carbon removal. The bicarbonate formed by the carbon removal reactions had not yet reached the leachate tank, which can take many months or even years. This led the CDI team to seek another method for verifying the carbon dioxide removal in their experiments. They needed an alternative method that complemented and confirmed the Lithos’ high-precision measurements.  

The Breakthrough

The breakthrough happened when CDI researchers examined data from some simple carbon dioxide gas sensors placed in the XXL lysimeters. Within just days after mixing soils with the basalt, there was an indication of CO₂ removal by basalt weathering. It turns out that carbon dioxide removal is rapidly revealed in CO₂ levels but is more difficult to detect using chemical analyses of soil leachate solutions. This may be due to a delay in the production of carbonate molecules during the process, or it could be that the carbonate molecules precipitate as minerals or get temporarily caught in the soil’s storage pools. Therefore, the bicarbonates don’t yet show up in the solution analyses. 

The cause is still under investigation, but the bottom line is that direct carbon dioxide gas measurements may be a good early indicator for confirming CO₂ removal by basalt weathering. Although Paessler and his CDI colleagues caution that more data is needed to confirm the accuracy of this approach.  

The plot below shows a comparison of the carbon dioxide removal estimates from the Lithos method and the CO₂ gas sensor method. Notice that the Lithos analyses yield significantly higher carbon dioxide removal values relative to the CO₂ gas measurements. This could be due to a lag between basalt weathering and the resulting chemical processes that remove CO₂. Paessler and others, therefore, suggest that the actual carbon dioxide removal rates will likely be somewhere between these high and low values. 

There is still considerable work to be done to refine the various methods of monitoring, reporting, and verification of the carbon dioxide removal process associated with enhanced rock weathering. However, the ongoing work at the Carbon Drawdown Initiative, Lithos Carbon, and many other ERW research groups, at the very least show definitively that they all believe that the process is a viable and technically sound carbon dioxide removal strategy. While there remain questions about potential logistical and environmental challenges associated with large-scale applications, the ongoing scientific work demonstrates that making use of the Earth’s long-term carbon removal process is feasible and may have great agricultural benefits through soil remineralization.  

James Jerden is an environmental scientist and science writer focused on researching and promoting sustainable solutions to urgent environmental problems. He holds a Ph.D. in geochemistry from Virginia Tech and a Master’s degree in geology from Boston College. Over the past 20 years, James has worked as a research geochemist and science educator. He joined Remineralize the Earth because of their effective advocacy, research, and partnership projects that support sustainable solutions to urgent environmental issues such as soil degradation (food security), water pollution from chemical fertilizers (water security), deforestation, and climate change. As a science writer for RTE, his goal is to bring the science and promise of soil remineralization to a broad, non-technical audience. When not writing, he can be found at his drum set.