Wait One Minute Before Deploying Enhanced Weathering in Tropical Peatlands
Rewind: Role of CDR in Climate Portfolio
From early IPCC reports, carbon dioxide removal (CDR) techniques have gone from fringe asset to a major pillar in achieving net-zero emission and, eventually, net-negative emissions. The recent IPCC AR6 report reflects that consensus — even under most optimistic decarbonization routes significant negative emissions are required.
It’s a growing view that CDR can achieve the emission abatement in hard-to-decarbonize sectors like airlines, heavy manufacturing, and oil & gas (Oxy’s big bet). Deemed a distraction by some taking away focus (and money) from renewables, electrification, and energy efficiency — the unsexy but best ways to mitigate climate change —while extending the life of polluting assets. Whichever your view, there is no question that significant money and political will are making CDR an important tool to mitigate humanity’s impact on the earth.
Role of EW in CDR Portfolio
Enhanced Weathering (EW) is one technique under the guise of geochemical Negative Emission Technologies (NETs) and already companies like Project Vesta, Heirloom, and Lithos Carbon are popping up worldwide.
EW leverages the natural carbon cycle of the earth, a process that naturally captures ~1 billion tons of CO₂ per year (context: anthropogenic emissions were 36.3 billion tons in 2021). It accomplishes this by processing silicate and carbonate rocks to increase their surface area, increasing their rate of capture of CO₂. The captured carbon is either stored in soils or leached to the ocean where the bicarbonates (HCO₃^-) and carbonates (CO₃^-) increase ocean alkalinity, allowing for more carbon uptake by the ocean.
Ignoring problems such as costs for grinding minerals, increased mining activity, impacts on poor communities, and land-use debate, it is a promising technology. I’m not making light of it, all CDR technologies have significant challenges, and EW can make a significant impact, if done correctly. Tropical peatlands have come into the forefront of potential sites for EW due to its increased efficacy in humid tropical regions and could add increase the carbon sink potential of peatlands. Sounds great!
Luckily, before anyone manifested this idea, Klemme et al. (2022) conducted research on the effects of EW in tropical peatlands. Their results are not great; essentially environmental responses to EW could negate some, all, or more than all carbon captured by EW. We’ll get into the nitty gritty below; the key question answered was how are peat soils, rivers, and coastal waters affected by the changing soil acidity induced by EW.
Research Results
Peat soil decomposition (read CO₂ emissions) is controlled by the enzyme phenol oxidase which is strongly controlled by pH and oxygen levels. EW induces a pH increase which favors the decomposition of peat soil (again CO₂ emissions). Currently, uptake estimates for EW do not account for this mechanism and overestimate the carbon uptake of EW in tropical peatlands.
Klemme et al. went further and produced estimates for how much CO₂ could be emitted due to this mechanism. The first case is theoretical if EW is deployed in a 100% tropical peatland environment where the second case evaluates deployment in Sumatra which has ~15.6% peatland coverage. Quick numbers: evaluated EW technique applied 1 kg/m²-yr, inducing pH increase of 0.2–1.3, and carbon uptake of 25–50 gC/m²-yr.
Case 1 resulted in emissions between 25–190 gC/m²-yr which clearly is a disastrous outcome. Luckily it is theoretical, hopefully Sumatra fairs better.
Case 2 results in a more manageable emissions of 4–30 gC/m²-yr which could reduce efficacy of EW by a little or by a lot (unfortunately a lot of uncertainty which makes it even scarier to me at least). Scary enough, but the authors went further and evaluated other effects too.
Although peat soils make up 75–91% of estimated emissions, there is still the issue of river and coastal water emissions. Klemme et al. used real-world data from Southeast Asian rivers to validate a dynamic model and inputted EW. Biggest takeaway was that for rivers in peatlands, oxygen levels decrease into lethal anoxic conditions. Even if we don’t care about the fish and other aquatic life, those anoxic conditions limit the river’s ability to absorb the CO₂ leached from EW. In other words, the entire mechanism of EW carbon storage is limited plus aquatic life is negatively impacted and, worse of all, local people will bear the weight of that impact.
Another factor, not that we needed one, is the transformation of the bicarbonate back into CO₂ in acidic conditions — this outcome has been observed for liming experiments on plantations and could result in a direct remission of 15% of carbon the EW captures.
Conclusion
The positive news is that this research has been published and more is likely on the way. It should dissuade any deployment before the dynamics are better understood.
What does this mean for EW as a whole? Well it eliminates a potential location but overall it will continue to be developed. The difficulty in combatting climate change is balancing quick deployment with positive outcomes. The challenge is that many feedback mechanisms are not well understood.
Companies, universities, and research groups like the Working Lands Innovation Center and the Carbon Mitigation Initiative are pursuing the necessary steps to understand those mechanisms and meaningfully deploy EW. The future is bright for carbon dioxide removal but we must properly evaluate where and how the technique is deployed even as the clock ticks closer to tipping points.
