The devil is in the geochemistry: how we are enhancing Earth’s natural weathering process
Prior to the industrial revolution, the concentration of carbon dioxide in the atmosphere was 280 parts per million.
Today it is 420 parts per million, a level not seen in over three million years when the global average temperature was 3oC warmer than pre-industrial values, and it continues to rise.
Limiting global warming to a damage limiting 1.5°C (we are currently at 1.1°C) requires not just urgent decarbonisation of the global economy, but also permanent removal of 20-660 billion tonnes of carbon dioxide from the atmosphere by 2100. There are many ways to remove carbon dioxide from the atmosphere, but our focus is on a natural and permanent removal pathway - weathering.
Weathering is the breakdown of rocks and minerals at the Earth’s surface.
On timescales of millions of years, weathering regulates Earth’s temperature because it consumes carbon dioxide by neutralising the carbonic acid (H2CO3) that forms when rainfall dissolves atmospheric carbon dioxide. This process is a sink for atmospheric CO2 because weathering reactions convert carbonic acid to bicarbonate ions (HCO3-), which are a stable store of carbon. Once the weathering reaction has occurred, bicarbonate ions flow to the ocean via surface and ground waters, where they remain in solution for ~80,000 years, before eventual precipitation in limestones. Crucially, there is a negative feedback loop (thermostat) in the process. This loop arises because the rates of weathering reactions, and therefore rates of atmospheric carbon drawdown, increase whenever there is excess carbon dioxide in the atmosphere (warmer conditions), to eventually restore equilibrium.
Unfortunately, this feedback loop is far too slow to absorb carbon dioxide at a rate that could offset the human-induced carbon dioxide surplus. It is estimated that natural ‘background’ weathering currently captures ~870 million tonnes of carbon dioxide per year - just 3% of annual anthropogenic carbon dioxide emissions.
Enhanced weathering is a strategy that greatly accelerates the natural weathering process to remove atmospheric carbon dioxide at a rate that can mitigate our current climate crisis.
Typically, enhanced weathering means crushing calcium- and magnesium-rich rocks and minerals to give them a larger surface area before applying them to agricultural soils, where the concentration of carbon dioxide is up to 10 times higher than in the atmosphere.
Enhanced weathering materials with high concentrations of calcium or magnesium cations (Ca2+, Mg2+) have the highest carbon removal capacity.
By increasing the amount of these materials that are available to react with carbonic acid (rainfall), significant carbon removal can be achieved in decades as opposed to millennia.
Materials rich in calcium or magnesium are typically well suited for enhanced weathering. We use several materials to achieve the optimum soil conditions to draw down carbon dioxide, but our primary feedstock is returned concrete.
The composition of concrete varies depending on how it is made, but it always contains two principal components: cement and aggregate. Cement, the liquid part of concrete, contains high calcium concentrations that are primarily hosted in fast-weathering minerals such as portlandite and amorphous calcium silicates - perfect for rapid carbon drawdown.
The aggregate (chunks of rock) that makes up the rest of concrete is determined by the local rock sources. Carbonate rocks, such as limestone, and silicate rocks, like basalt, are commonly used. Just like cement, both limestone and the olivine component of basalt will remove atmospheric carbon dioxide when weathered, although their carbon removal capacities and weathering rates differ.
We leverage a natural process, the silicate-carbonate cycle, to permanently remove carbon dioxide from the atmosphere. Our process can be explained in four steps.
We take surplus concrete from the building industry, process it to a fine dust by milling it, and work with farmers to apply it to their fields as a soil pH amendment.
Once the material is applied to agricultural land that has sufficient moisture and is of a suitable pH, it begins to break down, permanently removing carbon dioxide from the atmosphere through the following weathering process:
Carbonic acid (H2CO3) contained in the soil weathers the silicate, hydroxide and carbonate minerals in our material, creating calcium cations (Ca2+) and bicarbonate anions (HCO3-).
This bicarbonate anion is a stable store of carbon. Both the bicarbonate and calcium ions remain in solution in surface and ground waters before flowing to the ocean, where they have a residence time of 80,000 years.
On average, after 80,000 years, the negatively charged bicarbonate ions bond with the positively charged calcium ions to precipitate limestone (CaCO3) on the ocean floor, leading to carbon removal lasting millennia.
Carbonic acid (H2CO3) contained in the soil weathers the silicate, hydroxide and carbonate minerals in our material, creating calcium cations (Ca2+) and bicarbonate anions (HCO3-)
This bicarbonate anion is a stable store of carbon. Both the bicarbonate and calcium ions remain in solution in surface and ground waters before flowing to the ocean, where they have a residence time of ~80,000 years
On average, after 80,000 years, the negatively charged bicarbonate ions bond with the positively charged calcium ions to precipitate limestone (CaCO3) on the ocean floor, leading to carbon removal lasting millennia
We take robust measurements in the fields where we have applied material to measure how much carbon has been removed. Analysing solid (soil), water (soil waters) and gas (CO2 and N2O) fluxes in the field across 10-20% of total land area where we apply material to ground-truth and validate our geochemical models. This approach provides a comprehensive, mass-balance, assessment of the carbon dioxide we remove.
Following this detailed measurement process, we create high-quality carbon removal credits to enable companies, governments and other organisations aligned with a 1.5°C pathway to achieve their climate objectives.
Our measurement approach
Robust in-situ geochemical measurements are essential to demonstrate and quantify carbon removal. Our approach is data-driven and grounded in science.
Our protocol involves three independent measurements to constrain actual in-field weathering rates. By tracking carbon through the soil, soil water and gas phases, we are gaining a comprehensive understanding of the carbon fluxes in the system and how much CO2 we are drawing down.
Want to learn more? Dig into some of the key literature.
Why we need CDR
Concrete as a store of CO2 and carbonation rate estimates