Ground, powdery basalt at a quarry in Dunedin, occurring here as a byproduct of producing aggregate for the construction industry. Basalt is a rock type targeted for application in ERW. (Photo: Gavin Hartley, Blackhead Quarries Ltd)
Silicate and carbonate weathering by carbonic acid is a key component of the natural, mid- to long-term global carbon cycle. This process plays out in two key environments. On land it occurs in the soil, where silicate mineral weathering transforms carbon from atmospheric carbon dioxide (CO2) into solute bicarbonate (HCO3). The marine environment acts as the main storage ‘facility’ for this carbon at time scales of tens of thousands to millions of years, storing it as a solute at shorter time scales, or facilitating the formation of carbon-containing rocks such as limestone (CaCO3) at longer time scales.
This transformation of carbon as a natural process depends on a range of environmental factors and feedbacks on land and in the ocean. Natural changes to this transformation process, and the resulting effects on the climate, play out on long geological time scales. However, by intentionally speeding up the process on a large scale, silicate weathering has been suggested as a negative emissions technology to reduce anthropogenic CO2 in the atmosphere and counter current anthropogenic warming (Taylor et al. 2016).
One way to accelerate the relevant processes in the soil is by applying crushed silicate rock (Figure 1) to the most weathering-active part of the soil (i.e. the topsoil) on agricultural land, similar to the application of common fertilisers. The first generation of such large-scale field trials is now operating in many parts of the world, aimed at adapting the natural weathering process into a widely applicable, negative emissions technology. It was therefore timely for the Ministry for Primary Industries to commission a report on enhanced rock weathering (ERW), particularly with the focus on an accounting method for ERW as a negative emissions technology in New Zealand’s greenhouse gas inventory.
That report, written by Peter Almond from Lincoln University, Stephan Heubeck from Best Practical Option, and Manaaki Whenua – Landcare Research’s Andre Eger, has been published online to provide impetus for further research and policy development. Alongside suggesting a method for including ERW in the national greenhouse gas inventory, the report gives a first assessment of the practical applicability of ERW from a New Zealand perspective, taking into consideration the suitability and distribution of likely rock sources relative to the areas of application, and assessing the risks and knowledge gaps of using ERW.
As with most new technologies (even when based on a solid geochemical foundation), there are still some uncertainties, mainly relating to what rates of carbon removal from the atmosphere are realistically achievable in different agricultural settings and climates. There are also uncertainties about the side-effects on soils, agricultural products, and the environment overall – some likely to be beneficial, others less so. In addition, there are social, cultural, legal, and economic considerations, requiring a transdisciplinary perspective.
Broad adoption of ERW will hinge on its recognition as a viable negative emissions technology by governments. This requires the development of a robust accounting method for carbon removed from the atmosphere via ERW, along with the appropriate activity data, and integration into national greenhouse gas inventories recognised by international treaties. Arguably, such a method needs to represent the complexity of the weathering process, but must also be practically applicable. For example, the method should account for the potential formation of particular new minerals in the soil, since this would reduce the efficiency of CO2 removal from the atmosphere.
The report concluded that New Zealand has a high potential for ERW, but more targeted work is needed before wider adoption can be recommended. Ultimately, we should bear in mind what Taylor et al. wrote so aptly back in 2016: enhanced rock weathering should not be treated ‘as a panacea for erasing impacts of fossil fuel burning, but as a sobering indication of actions that may be required if fossil fuel emissions are not phased down rapidly.’
