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Enhanced weathering is a carbon capture technology in which ocean alkalinity is increased through depositing rock particles into the ocean. It may sound simple, but there is still much to be examined as the risks are weighed against the benefits.

Currently, a diverse range of carbon capture methods are being used in an attempt to reach negative carbon emissions. Most of these are land-based methods and some are more controversial than others; Bioenergy with Carbon Capture and Storage (BECCS) technologies for example, require crops to be grown that will then get burned to release energy and store carbon underground. 

Enhanced weathering is a method that involves storing carbon in the ocean through a chemical reaction that removes CO2 from the atmosphere. In an effort to accelerate oceanic uptake of carbon in the least intrusive, yet most cost-effective way, scientists have concerns about the impact it will have on marine ecosystems. 

What is Enhanced Mineral Weathering?

Weathering is a natural process whereby rocks are broken down by rainwater, extreme temperatures or human activity. It is a process that takes place over millions of years, constituting an important carbon sink. 

The process begins when CO2 dissolves in droplets of water to form carbonic acid, a weak acid: rainwater has a pH of around 5 to 5.5, but because there is a lot of it available in the environment, it does a lot of weathering over time. Rocks that contain carbonates, like limestone, react quickly because the minerals they are largely made from, such as calcium carbonate, are more reactive than silicates. The dissolved calcium and bicarbonate ions formed from the carbonic acid travel in groundwater to rivers and the sea. Through the calcium carbonate-generating part of the chemical process, there is a net loss of readily-made carbon- half the amount of carbon at the end as there is at the beginning. As sediments continue to accumulate, the carbonate-rich layer will be buried under new layers of sediment that will in time turn to solid rock, like limestone. 

The carbonates that form increase the alkalinity of oceans, leading to a further uptake of atmospheric CO2. The rate of weathering is dependent on temperature, runoff (the availability of water to remove reaction products), grain size of rock or mineral and biological activity, like volcanoes. The annual potential of CO2 consumption is defined by the grain size and the weathering rate of the rocks used.

Enhanced mineral weathering is the speeding up of this natural process, whereby rocks are ground into fine particles and spread across large spans of land or the ocean. Overall, the process requires extraction, processing and the dissolution (reaction) of minerals. Through this, more atmospheric carbon dioxide can be sequestered than what would occur naturally. 

The use of enhanced rock weathering to increase ocean carbon uptake was first proposed in 1995, but was put aside due to the high energy costs of creating lime. 

Expected outcomes of enhanced weathering processes are uncertain. Current research is directed towards the function of alkalinity in the existing natural oceanic carbon cycle, what the effects on ocean chemistry imposed by artificial alkalinity could be and whether it could be maintained in a stable manner and technologies that can be used to increase alkalinity.

Enhanced Weathering Pros and Cons

Enhanced weathering may ameliorate ocean acidification. The added alkalinity also increases the saturation state of carbonate minerals which, if too low, negatively impacts carbonate-producing organisms in the ocean, such as shellfish and coral.

Enhanced weathering would not require its own land, nutrients or freshwater, with the latter only needing to be used when dust avoidance measures from rock deposition become necessary. Rock particles could be applied on open ocean regions or combined with agriculture with the additional benefit of enhancing crop yields and preventing soil erosion.

Additionally, while the range of technologies that have been proposed for increasing ocean alkalinity may pose significant engineering challenges, cost analyses suggest that they are still within the range of other negative emission technologies. 

However, speeding up or changing the course of nature can have disastrous effects. Scientists suggest that rapid uncontrolled changes in pH, carbonate saturation state, and dissolved aqueous CO2 can affect ocean ecosystems. While this process mimics a natural one, it is not natural; the substance would be delivered to ecosystems at rates far higher than normal which could create ‘dead zones’, areas where oxygen levels are too low to support life. Additionally, the amount of olivine necessary for these applications is extremely large and is comparable to present-day global coal mining, a counterintuitive proposition as the planet looks to turn away from mining. In the case of basalt, to sequester one billion tonnes of CO2, more than 3 billion tonnes of basalt would have to be spread, an amount equal to almost half of the current global coal production.

Additionally, at such a large scale, enhanced weathering could change the ecology of the water, leading to an increase in the microbial organisms that produce greenhouse gases such as methane and nitrous oxide. 

Although the addition of alkalinity is common practice in certain constrained marine environments such as in aquaria and shellfish production, scientists believe that more research is needed to understand the wider ecosystem response. 

Current Attitudes Towards Enhanced Weathering

In a 2017 UK survey, over 70% of participants expressed that they’d never heard of enhanced weathering. Further, support for research of the technique was found to be much stronger than support for the technique itself. Lack of public knowledge about this technology could be a factor hindering its large-scale deployment. 

The best suited locations are warm and humid regions, particularly in India, Brazil, South East Asia, and China, where almost three quarters of the global potential could be realised.

Increasing alkalinity in the ocean needs to be assessed more stringently. Some pressing issues regarding ocean alkalinity will have to address whether marine life will thrive or die in a new environment. While much of this work is still in the testing phase, enhanced weathering technologies could become economically and environmentally viable options to realise carbon capture and subsequent negative emissions this century. 

 

Featured image by: Richard Droker

As a carbon mitigation tool, Bioenergy with Carbon Capture and Storage (BECCS) is gaining momentum among scientists and conservationists. But is it effective enough?

Carbon emissions from fossil-fuel use hit a record last year after energy demand grew at its fastest pace in a decade, causing higher oil consumption and more coal-burning across the globe. The International Energy Agency (IEA) recorded 33.1 gigatons of carbon emissions in the global energy sector, up 1.7% from the previous year. While renewable power generation grew last year by about 7%, that was not enough to keep up with the increase in demand.

As global demand for energy continues to surge, emissions from fossil fuel use are expected to further go up unless the world nations start implementing innovative carbon mitigation initiatives.   

Negative Emission Technologies (NETs)–the frontier of climate crisis mitigation–might be an effective solution to reduce the global energy sector’s increasing carbon footprint. One of these proposed solutions, Bioenergy with Carbon Capture and Storage (BECCS), is now gaining momentum among scientists and conservationists. 

How does carbon capture and storage work? 

In a nutshell, carbon capture and storage is a process in which energy is generated from burning biomass. Carbon dioxide (CO2) produced during the process is captured and sequestered in geological storage units. In simplest terms, the BECCS procedure goes like this: plant thousands of trees which remove CO2 from the atmosphere, burn those trees instead of fossil fuels to produce energy, capture the emitted CO2, and then store it underground. It might sound counterintuitive to burn trees to cut emissions. But the ultimate result of the BECCS is the removal of CO2 from the atmosphere.  

You might also like: South Asia Could Be Facing an Energy Crisis

BECCS process

If executed efficiently, BECCS will have a significant impact on the energy sector’s carbon footprint. For instance, if the US could sequester CO2 using BECCS, it could reduce emissions by one gigaton of CO2 equivalent (GtCO2eq) annually by 2050. That’s a significant quantity considering the world emitted 36.2 GtCO2eq from fossil fuel combustion in 2017.

The Fifth Assessment Report from the Intergovernmental Panel on Climate Change (IPCC) projected that BECCS could reduce emissions by around 12 GtCO2eq per year by 2100 globally. 

BECCS is still in its infancy. As an emerging technology, it raises a lot of questions and challenges that are still being debated. Industrial-scale implementation of this solution needs substantial resources: trees need land, water, and even fertilisers; the energy production process needs new transportation facilities and industrial infrastructure. While the usage of a large volume of water may put pressure on the existing irrigation system, a massive amount of fertilisers may cause serious environmental damage.

A recent study on BECCS states that it would cause food shortage in the future.  The world will have to produce 70% extra food by 2050 to keep up with the increasing population, and that means designating more land for agriculture. For a global scale deployment of BECCS, the world needs between 300-600 million hectares of additional land–an area the size of the European Union. If such vast tracts of land are reserved for fast-growing plants as part of BECCS, global agricultural production will be seriously affected.  

Large scale cultivation of trees could also bring problems associated with monoculture and biodiversity loss. This argument, however, is often negated by BECCS advocates who argue that the large-scale cultivation of trees can be carried out on degraded lands that were already used for grazing. 

The scientific community is still debating about the efficiency and side-effects of BECCS. But in desperate times, as a carbon removal technology, BECCS might prove to be a good bet.

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