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 values1, and it continues to rise.
Limiting global warming to a damage limiting 1.5°C (we are currently at 1.1°C2) 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 21003. There are many ways to remove carbon dioxide from the atmosphere, but our focus is on a natural and permanent removal pathway - weathering.
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 years4, 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 emissions5.
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.
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.
Our process can be explained in four steps:
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
Our measurement protocol involves three independent geochemical measurements to constrain weathering rates and carbon removal in the fields where we work.
By tracking carbon through the soil, soil-water, and gas phases, we gain a comprehensive understanding of how our material performs across a range of different environments. The data we gather from these detailed studies allow us both to quantify our carbon drawdown and optimise for carbon removal at scale.
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By tracking soil chemistry over time, we can monitor the breakdown of our concrete amendment. We can determine the initial amount of calcium added to the soil and measure how much is lost due to weathering, which indicates the speed of carbon drawdown. The faster the weathering, the quicker the carbon drawdown.
We also monitor changes in soil pH over time, which is a major benefit of enhanced weathering and an important indicator of how weathering is progressing.
We monitor soil waters to better understand the weathering process. Changes in soil-water chemistry are the most reliable indicator of carbon removal because the bicarbonate ion, the main carbon storage molecule, is soluble in water and is the product of the carbon removal reaction.
Higher bicarbonate concentrations in the soil waters at our amended sites relative to our control plots provide a clear signal that we are removing carbon dioxide from the atmosphere.
We can validate our carbon removal estimates from soil and soil water measurements by measuring carbon dioxide gas fluxing from the soil to the atmosphere. Our expectation is that amended sites will show a decrease in soil-air carbon dioxide fluxes compared to control sites, consistent with increased dissolved bicarbonate levels in the soil-water.
By increasing the pH level of the soil, it is possible to decrease the emission of nitrous oxide, which is a potent greenhouse gas, to the atmosphere. Tracking nitrous oxide emissions gives us a more comprehensive understanding of how our material impacts other important trace gas fluxes.
