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Everything you need to know about Enhanced Rock Weathering (ERW)
Over the past few months I have created a number of deep dives into the most important engineered carbon removal technologies: biochar, bio energy with carbon capture and storage (BECCS) and direct air capture (DAC) - see here, here and here if you missed them or want to remind yourself how they compare.
In the final post in this series I’m going to focus on enhanced rock weathering (ERW). In short, ERW involves accelerating the natural geological process whereby carbon dioxide is mineralised into rock.
Before we get into the detail though here's a quick recap on why carbon removal technologies are so important, their relationship with emission trading schemes, and the key characteristics to look out for.
Almost all of the 2 Gt of CO2 currently sequestered each year comes from ‘conventional’ methods such as afforestation, reforestation and management of existing forests. In contrast, only 0.002 Gt CO2 per year is currently sequestered using engineered carbon removal.
The latter may only account for a tiny fraction of overall carbon removal today, but that will need to change significantly over the next three decades. For we cannot simply rely on conventional, nature-based carbon removal if we are to meet net zero targets. The Global Warming of 1.5 ºC report from the Intergovernmental Panel on Climate Change (IPCC) states that “all pathways that limit global warming to 1.5°C with limited or no overshoot project the use of carbon dioxide removal.” The IPCC report estimates that the world will need cumulative carbon removal in the order of 100–1,000 Gt CO2 over the 21st century.1
It’s important to keep an eye on how carbon removal technology is developing. Not only because it is in demand from corporations wanting to invest in removing their current (and even their historical emissions), but also because regulated carbon compliance schemes such as the EU and UK ETS’ are in the process of considering how best to incorporate such carbon removals. If approved, some of these carbon removal methods will be used alongside cuts to emission allowances in meeting regulated compliance targets, providing a strong government backed tailwind to their adoption (see Reflexivity and the EU ETS 'Endgame').
Demand for engineered carbon sequestration solutions is high because of their long-term durability relative to nature-based (‘conventional’) solutions and the lower requirement for ongoing monitoring. Nature based carbon removal benefits accrue over several decades or more, but risk being undone due to fire or illegal logging. On the other hand, engineered carbon removal projects typically sequester carbon much more rapidly and with much less risk of subsequent release (see Technology-based carbon removal credits crucial if net-zero targets are to be met).
That being said, it would be unfair to characterise engineered carbon removal technologies as all offering very similar attributes. Each technology delivers carbon removal and sequestration across a spectrum of durability, requirement for ongoing monitoring and challenge involved, overlap with natural processes, risk of subsequent release, current cost, potential scalability, knock-on impacts on energy or land use, degree of co-benefits, and potential unintended consequences.
With that recap complete, let’s get into the detail on ERW.
When CO2 combines with rainwater it forms carbonic acid. As it falls from the sky and interacts with soil and rock it mineralises into a stable carbonate form. Naturally occurring rock weathering typically takes thousands of years to sequester carbon, but once completed the carbon is locked away for hundreds of thousands of years, with a corresponding very low risk of the carbon being re-released back into the atmosphere. Natural rock weathering absorbs around 0.3% of global greenhouse gas emissions.
Using a variety of techniques, including spreading crushed silicate rocks on farmland, ERW speeds up this natural biological process, reducing the time it takes to sequester the carbon to a matter of years. ERW relies on the most reactive rocks (e.g. silicate rocks such as basalt that are rich in calcium and magnesium) and the quarrying industries for rock mining, grinding and spreading the crushed mineral. Since rocks such as basalt and the industries that depend upon it are well-established across the globe, ERW has the potential to be highly scalable.
How much does ERW cost? Recent estimates from academics led by the University of Sheffield put the global cost of ERW at around $75-250 per tonne CO2. That puts ERW firmly in the same cost bracket as BECCS and DACCS, but slightly more expensive than biochar (see BECCS - the carbon removal chimera).
On a regional basis the difference in costs reflects the relative difference in the price of labour, diesel and electricity. ERW is estimated to cost $160-190 per tonne CO2 in North America and Europe, versus $55-120 per tonne CO2 in less developed but no less large emitters such as China, India, Mexico, Indonesia, and Brazil.2
Although basalt is one of the most widely available minerals on the planet, and is applicable to most agricultural conditions, the main limiting factor in terms of developing its economies of scale is transporting the basalt from the quarry and then spreading it on the field. The longer the distance the less likely the economics stack up, and the more likely that the emissions associated with transportation overwhelm the carbon removed from the atmosphere.
Unlike BECCS, ERW does not rely on sourcing significant quantities of biomass, and placing a potentially unsustainable burden on land and agricultural resources. There is an energy cost involved with pulverising the rocks down to the required level (thought to be around 10-30% of the amount of carbon sequestered), but this could be reduced using renewable energy generation. Besides, the energy demands pale into insignificance compared with those required to suck carbon out of the atmosphere using direct air capture (see Scrubbing the skies: Direct Air Capture (DAC) offers a scalable route to net zero).
In contrast, ERW also provides certain co-benefits that only biochar can begin to compete with. Silicate rocks such as basalt act as soil enhancers when spread on agricultural land: reducing the need for fertiliser, reversing soil acidification, and cutting nitrogen oxide emitted from the land. The benefits from ERW don’t stop there. The bicarbonate ions created by ERW eventually wash out into the ocean where they promote ocean de-acidification. Crustaceans use the calcium carbonate to construct their shells which eventually fall to the seabed to form carbonate rocks like limestone. Once people begin to account for the value of those co-benefits, the net cost of ERW is likely to decline even further.
The same academic research also estimates that ERW has the potential to sequester between 25-100 Gt CO2 over a period of 50 years. This means that ERW could remove up to 10% of the cumulative 1,000 Gt of CO2 which the IPCC estimates is at the high end of that required to stay within the carbon budget. At ~0.5-2 Gt of CO2 per year, ERW has a similar potential for carbon removal as DAC, BECCS or biochar (see Char grilled: Why biochar is the most promising carbon removal technology). Meanwhile, the countries with the greatest potential to utilise ERW as a form of carbon removal are those that have extensive land area devoted to crops, as such, China, the USA and India have the largest opportunity to use ERW for carbon removal.
The ERW process is nothing new, except in the past farmers have only focused on the soil improvement attributes. There is a long history of applying crushed limestone to arable land, reversing the acidification of the soil that can result from the intensive use of fertiliser. For example, Brazil, Malaysia and parts of Africa have been spreading crushed basalt on highly weathered agricultural land for over 100 years. The fact that ERW has been used in the past, and continues to be used today does give some confidence that the risk of unintended consequences on the environment, particularly marine ecosystems, can be minimised.
One of the main challenges involved with scaling ERW is accurately monitoring, reporting and verifying (MRV) the carbon sequestered. It really is only DAC that can currently offer a precise quantity of carbon captured from the atmosphere. Balancing the competing demands for cost, frequency and accuracy, academics have proposed a combination of expensive yet accurate radioactive measurements plus inexpensive yet convenient alkaline water measurements. As the value of the carbon removal method begins to be realised, expect to see a lot more investment into improving the MRV process (see The carbon tracking opportunity: Real time tracking of GHG emissions and carbon sinks is a huge growth market).3
ERW is beginning to be recognised by investors and corporates wanting to meet their climate commitments. In December 2022, Finnish verification body Puro.earth published the world’s first carbon removal methodology based on ERW. It means that ERW projects can now be rewarded financially for each tonne of carbon sequestered, selling ‘CO2 Removal Credits (CORCs)’ to corporate buyers wanting to compensate for their carbon emissions while also contributing to scaling up this powerful form of carbon removal. In mid-2023 ERW credits were reportedly offered at over $500 per tonne.4
BECCS and DACS get all of the headlines when it comes to carbon removal. In contrast, ERW and biochar tend to be forgotten or misunderstood. Yet the latter provide some of the best opportunities to scale carbon removal without any adverse impacts on energy consumption or land and resource use.
When it comes to carbon removal, all four engineered carbon removal solutions will be required to meet net zero. However, it makes sense to focus on those methods that work in tandem with existing natural processes, rather than placing even higher demands on that which is already under pressure.