FAQs: Enhanced Rock Weathering as a Carbon Dioxide Removal Strategy

The Leverhulme Centre for Climate Change Mitigation (LC3M) is investigating a carbon dioxide removal strategy called enhanced rock weathering. Weathering is the natural chemical breakdown of rocks that removes carbon dioxide (CO2) from the atmosphere. These processes can be accelerated (or enhanced) by milling rocks to fine grains with a high surface area helping them react faster. The most pragmatic approach is to spread rock dust on the soils of managed croplands such as arable farms, which is the focus of the Leverhulme Centre’s research activities. This technique could also be used alongside other carbon removal options such as planting trees to re-establish forests.

These FAQs were developed in response to our Leverhulme Centre international public engagement events. They supplement our detailed 2018 scientific synthesis published in the international peer-reviewed journal Nature Plants. Free online access to the paper is available here: http://rdcu.be/HlCp

 

Our research focuses on the abundant natural volcanic rock, basalt.  Basalt is a fine-grained rock produced with the rapid cooling of lava generated by volcanic eruptions.  Basalt is composed of a mixture of natural minerals and is compatible with Soil Association organic fertilizer standards.  When crushed and introduced onto cropland soils, the basaltic minerals dissolve in the soil.  This sets off natural chemical reactions that help capture and store CO2 in the soil and in soil drainage waters. The reactions also release nutrients that support crop growth and restore soils.

A general ‘rule of thumb’ is that you need 4 to 5 tonnes of rock dust per tonne of carbon dioxide removed.  But this number is derived from theory and a central goal of our research is to understand what the number looks like in practice.  We are aiming to understand the carbon dioxide removal potential of crushed rocks applied to croplands under field conditions.  In this agricultural context, factors such as application of fertilizers, soil properties, climate and crop type, could change the efficiency of carbon capture.

At all of our research sites, we are assessing the CO2 emissions associated with mining, grinding, transporting and spreading the rock dust to enable us to calculate net CO2 removal after accounting for these costs.  All of our field sites use basalt rock dust as a by-product of working quarries that is too fine to have traditional commercial use.  This means there are no additional CO2 emissions from mining and grinding at our field sites.

Human societies have long known that volcanic soils are fertile, ideal places for growing crops without adverse human health effects.  Basalt contains at least six plant-essential nutrients (including potassium, phosphorus and calcium).  When basalt rock dust breaks down in soils, it can help to make soils less acidic in a process similar to the current practice of spreading lime on fields, boosting crop yields and forest growth.  This suggests amending soils with basalt rock dust provides a natural fertilizer and soil restorer.  Our early results confirm that enhanced weathering is compatible with conventional crop production methods, that it makes soils less acidic, and that there is no accumulation of toxic elements in the edible parts of the crops.

The increasing concentration of carbon dioxide in the atmosphere, mainly resulting from the burning of fossil fuels, is gradually making the oceans more acidic, which can have damaging effects on marine life including coral reefs. This is the ‘other’ carbon dioxide problem, i.e., besides climate change.  Modelling and field experiments suggest that, if undertaken at large scale, enhanced weathering might provide an antidote to this problem by balancing out the increased acidity. By reversing the effects of human-caused acidity in the oceans, enhanced weathering could support the growth of certain types of marine life, such as corals and shellfish. It is also possible that dissolved silica from the rock dust, reaching the oceans via river drainage systems, boosts ocean productivity by enabling other groups of organisms (e.g., diatoms) to flourish.

Scientists in the future will need to assess these potential positive effects and critically examine any possible negative effects.  These include environmental consequences of rock particles washing into rivers, where they may affect sediments and water transparency (turbidity), with unknown impacts on microbes, plants and animals.  We emphasize that such effects are only likely to apply at a very large-scale roll out of enhanced weathering.  Our research trials are being conducted on a small scale (a few fields); at this scale, any changes in soil drainage water chemistry will be greatly diluted on reaching rivers and the ocean.

The Leverhulme Centre for Climate Change Mitigation is undertaking carefully monitored field trials in the USA, Australia, the UK and Malaysian Borneo, on plots ranging from 1 to 4 hectares (1 hectare is the size of an international rugby field).  These sites encompass a range of climates, soils and crops, which allows us to assess the broad suitability of enhanced weathering for major agricultural ecosystems worldwide.  Across all our sites, we use basalt rock dust as a by-product of existing basalt mines. We spread approximately 4-5 kg per square metre, equivalent to about 40-50 tonnes per hectare.  These rates are similar to annual organic fertilizer application rates, but higher than for agricultural liming practices, which are generally undertaken every 4-5 years.  All aspects of our field trial sites, including soil drainage water chemistry and local stream water chemistry, are being intensively studied and monitored by dedicated teams of scientists.

Existing basalt mines are widespread around the world and mining technology generates a finely-ground by-product which currently lacks a commercial market.  This product is ideal for enhanced weathering and could be used without releasing any additional CO2 from mining or crushing the rocks.  These materials have been accumulating worldwide for decades.  National inventories of the location, availability and extent of these resources are required to assess the potential contribution of this resource to carbon removal.

Opening new mines for enhanced weathering is expensive and legally complex, so the most pragmatic option to meet a rising demand in the near-term would be to increase production from existing mines.  Any increase in mining activity would require careful management to deal sensitively with noise and pollution resulting from road and rail transport of materials.  It will also require scientists in future to examine potential threats to local ecology, as well as down-stream freshwater and marine ecology.

Addressing the urgent issue of human-made climate change requires drastic reduction in fossil fuel use, plus a portfolio of strategies for removing some of the CO2 which has already been emitted into the atmosphere. Planting trees is an excellent option for CO2 removal but is not sufficient on its own.  Therefore, this activity will need to be supplemented with other CO2 removal strategies.  In fact, enhanced rock weathering is deployable on the same land as tree planting, with the advantage of amplifying carbon capture.  As the rock grains break down, they provide nutrients to support increased tree growth and capture of carbon into biomass.  Furthermore, as the trees grow, their roots and associated microbes accelerate the breakdown of the rock grains capturing carbon in soils.

Our research is funded by the Leverhulme Trust, an independent charitable organization (Leverhulme.ac.uk).  The broader issue of how the world would fund deployment of carbon dioxide removal strategies more generally remains unresolved, although economists and government think-tanks are working hard on a range of options.

Arable farms already apply crushed rock in the form of limestone to reduce the acidity of their soils that results from farming practices, including the use of fertilizers.  Managed croplands, therefore, have the infrastructure such as roads and machinery needed to undertake this approach at scale.  These considerations could make it straightforward to adopt.  Our proposal is that changing the type of rock, and increasing the application rate, would do a similar job to applying crushed limestone but would also help capture CO2 from the atmosphere, storing it in soils and eventually the oceans.

As with all proposed carbon dioxide removal strategies, scale-up would need to start small with rigorous tests to ensure it is effective, safe and socially acceptable, before proceeding to the next stage in coming decades.  Our research indicates that the best places to begin rolling it out would be croplands of the three countries that are currently the highest fossil fuel emitters: China, USA and India.  Croplands of some European countries may also be especially suitable.  These are ideal initial target areas because crop production in warm climates tends to be high and both factors accelerate the breakdown of rock grains.  Scaling up enhanced weathering on existing agricultural land (or with forestry land) means that no land would need to be diverted from other uses. Increasing the productivity on existing land also reduces pressure for converting forested land to agriculture.

We carefully screen the chemistry and mineralogy of any rock materials before adding them to the soils.  If negative effects on crop production occurred, we would know quickly because we are monitoring crop growth and yields continuously and in detail with a range of technologies.  We also have control plots that are not being treated with basalt, so that we can compare crop performance and changes in soil water chemistry between the two.  One long-term consequence of adding basaltic rock dust is that it eventually turns into fine clays, which are beneficial to soils because they help with nutrient retention and water holding capacity.  Effects of long-term usage of basalt rock dust on soils, drainage waters and streams are also being carefully monitored in our field trials.

Basalt rock dust has been successfully used as a fertilizer in developing countries for many years, and some UK farmers have been adding it to soil to improve productivity, but effects on carbon sequestration have not been measured. We don’t yet know if, and under what circumstances, rock dust applications to cropland soils will achieve the anticipated results in terms of carbon capture and improvements to crop and soil health.  There is anecdotal evidence from farmers and forestry organizations about the benefits of adding basalt to soils.

Our provisional results are promising but we are at an early stage of our long-term (10-year) programme of Leverhulme-funded research.  If the evidence base stacks up, we will be able to reach out to a range of land management and farming communities with a view to scaling up. At our field trial site in the USA, for example, we have access through the University of Illinois Urbana-Champaign to major land managers of farms covering tens of millions of hectares in the Corn-Belt.  Through these outreach mechanisms we can organise site visits, with factsheets and economic case studies, to explain the relevance of enhanced rock weathering to farmers.

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Further information

More information on greenhouse gas removal techniques, including enhanced rock weathering, and the role they could play in addressing climate change, is available from the Royal Society; the independent scientific academy of the UK and Commonwealth.
https://royalsociety.org/topics-policy/projects/greenhouse-gas-removal/