Releasing divalent cations to sequester carbon on land and sea

The natural response of the carbon cycle to the warming induced by increased atmospheric CO2 features two negative feedbacks that remove CO2 from the atmosphere. One, caused by the greater acidity of the oceans, is for carbonate minerals to be dissolved, which causes an increase in the ability of seawater to contain carbon (as the bicarbonate ion). The other is for warmer conditions to increase the rate at which silicate minerals dissolve, with the products either precipitated as carbonate minerals, or flowing to the oceans. This silicate weathering also removes CO2 from the atmosphere.

Intentional acceleration of these two weathering feedbacks is a potential approach to remove the CO2 added to the atmosphere by burning of fossil fuels, and therefore alleviate extreme climate change. Such an approach is challenging, however, because to be useful at a significant scale (i.e. 1-10 GtC pa removal), requires a dramatic increase in weathering relative to natural rates. Whether such accelerated weathering is a feasible route to remove significant atmospheric CO2 is unknown. This proposal will address this unknown, and provide a comprehensive assessment of the feasibility of CO2 removal by accelerated weathering, including consideration of the technical, economic, environmental, and societal aspects of the approach.

The core of our work will be a life-cycle assessment of the enhanced-weathering approaches that might lead to 1-10Gt removal of CO2 per year. This modelling will start from the availability of minerals for weathering, paying particular but not exclusive attention to waste materials from industries such as mining. It will consider how the weathering of these minerals might be enhanced, either through treatment in mining waste piles or, in collaboration with project partners, by addition to soils. It will also consider the fate of the weathered materials, either as carbonate on land or in the sea, or as alkalinity in the sea. It will assess the economic cost of such approaches, the energy requirements, the environmental damage they would cause, and the societal limitations on such approaches (e.g. social acceptability, political, legal, governance).

In some key areas, understanding is not yet sufficient to allow this life-cycle assessment. We will address these gaps in knowledge by five specific pieces of research. These will:
1. Characterise how much waste material is available for enhanced weathering, including its location, its grain size, and its chemistry and mineralogy. This is critical information to underpin the life-cycle assessment.
2. Measure how quickly typical minerals weather and how this weathering rate changes with temperature and, particularly, through addition of microbes that are known to cause accelerated weathering of silicates.
3. Assess how best to scale up weathering to the 1-10GtC pa level. This will be done by both modelling of possible engineered approached to weathering, and by experiments on piles of silicate and carbonate minerals (each of 10 cubic meters), in which the conditions are altered and responses measured.
4. Assess the response of the ocean to increased alkalinity resulting from enhanced weathering. If more carbonate is produced in the ocean, it reduces the effectiveness of enhanced weathering; we will measure the rates of both inorganic and biological carbonate formation and their impact in the C cycle globally.
5. Consider how society will response to possible scenarios for accelerated weathering, and whether this may limit such an approach. Will enhanced weathering be socially acceptable? Will there be the political will to pursue it? Are their legal or governance barriers?

This project brings together a trans-disciplinary team to determine the efficacy of enhanced weathering and ocean alkalinity enhancement for greenhouse gas removal, as well as its technical and socio-economic feasibility, its wider governance, ethical and societal issues, and environmental impact. The project is expected to benefit a range of communities, in different ways and over different timescales:

1. Government policymakers: who are tasked to achieve the goal of returning global temperatures to 1.5 oC above pre-industrial level. In support of this task, this project will provide a comprehensive assessment of the quantity of CO2 that could be removed due to enhanced weathering and ocean alkalinity enhancement, and over what timescale. The project will also provide predictions for CO2 uptake for a range of different technologies and technological approaches.

2. Industry, who are committed to reducing levels of greenhouse gas emissions and to developing techniques to manage atmospheric CO2. In this project we will work directly with the mining industry and provide a comprehensive assessment of how they could turn their 'waste' products into a commodity for CO2 reduction thereby reducing the carbon footprint of their activities. Managing levels of CO2 in this way will require industrial capabilities on a large scale that represents an opportunity to develop new income streams to the UK economy.

3. Intergovernmental Panel on Climate Change: Our research on the efficacy of enhanced weathering and ocean alkalinity enhancement is expected to feed in to the 6th IPCC Assessment Report that will have a strong focus on climate change mitigation strategies.

4. Civil Society: Global warming is a global issue that will significantly impact the lives of our children and grandchildren. Civil society is keenly interested in science that works to mitigate these impacts, but is rightly concerned that geoengineering approaches may have unintended consequences that do more harm than good. This project will undertake research to assess the potential costs and benefits of enhanced weathering and ocean alkalinity enhancement approaches. Communicating the results of this work, to both NGOs and the wider public, will be critical for driving this approach to full-scale deployment.