Crushed olivine was added to a soil core to mimic enhanced weathering, and water was continually dripped through for ~6 months. Our experiments were conducted at 4°C, and are compared to previously ...run identical experiments at 19°C. Olivine dissolution rates in both experiments start out similar, likely due to fines and sharp crystal corners. However, after >100 days of reaction, the dissolution rate at 4°C was two orders of magnitude lower than at 19°C. The accumulation of heavy metals, such as Ni and Cd, was low in both experiments, but soil retention of these elements was proportionally higher at higher temperatures, likely due to enhanced sorption and formation of clays. Overall, this study suggests that olivine dissolution rates in experiments that mimic natural settings are orders of magnitude slower than in normal laboratory experiments, and that enhanced weathering may be a considerably less efficient method of carbon dioxide removal at low climatic temperatures. Both of these conclusions have implications for the application of enhanced weathering as a CO
2
removal method.
While the concept of removing carbon dioxide (CO2) from the atmosphere to help prevent climate change has been around for decades, it is only relatively recently that its importance within climate ...policy has moved into mainstream discussions. As such, conventions for nomenclature are widely debated. The proposed methods of removing CO2 from the atmosphere to restore a level that ensures a stable climate, are diverse and often share little in their form and function beyond their impact on atmospheric CO2. However, for this reason alone, it is useful to refer to these within an umbrella term. In this editorial, we outline why the editorial board has decided to rename this section of Frontiers in Climate to "Carbon Dioxide Removal".
Coastal enhanced weathering (CEW) is a carbon dioxide removal (CDR) approach whereby crushed silicate minerals are spread in coastal zones to be naturally weathered by waves and tidal currents, ...releasing alkalinity and removing atmospheric carbon dioxide (CO
). Olivine has been proposed as a candidate mineral due to its abundance and high CO
uptake potential. A life cycle assessment (LCA) of silt-sized (10 μm) olivine revealed that CEW's life-cycle carbon emissions and total environmental footprint, i.e., carbon and environmental penalty, amount to around 51 kg CO
eq and 3.2 Ecopoint (Pt) units per tonne of captured atmospheric CO
, respectively, and these will be recaptured within a few months. Smaller particle sizes dissolve and uptake atmospheric CO
even faster; however, their high carbon and environmental footprints (e.g., 223 kg CO
eq and 10.6 Pt tCO
, respectively, for 1 μm olivine), engineering challenges in comminution and transportation, and possible environmental stresses (e.g., airborne and/or silt pollution) might restrict their applicability. Alternatively, larger particle sizes exhibit lower footprints (e.g., 14.2 kg CO
eq tCO
and 1.6 Pt tCO
, respectively, for 1000 μm olivine) and could be incorporated in coastal zone management schemes, thus possibly crediting CEW with avoided emissions. However, they dissolve much slower, requiring 5 and 37 years before the 1000 μm olivine becomes carbon and environmental net negative, respectively. The differences between the carbon and environmental penalties highlight the need for using multi-issue life cycle impact assessment methods rather than focusing on carbon balances alone. When CEW's full environmental profile was considered, it was identified that fossil fuel-dependent electricity for olivine comminution is the main environmental hotspot, followed by nickel releases, which may have a large impact on marine ecotoxicity. Results were also sensitive to transportation means and distance. Renewable energy and low-nickel olivine can minimize CEW's carbon and environmental profile.
The world's oceans are an important part of the global carbon cycle, having already absorbed one-quarter of the anthropogenic carbon emissions, however, at the expense of surface ocean acidity, which ...has increased around 30% since the Industrial Revolution, affecting marine ecosystems. Ocean liming, whereby particulate calcium oxide or, more likely, hydroxide is spread to surface ocean waters can address, at least partly, both the need for carbon dioxide removal (CDR) and ocean acidification. While the idea was proposed almost three decades ago, previous studies have focused on techno-economic feasibility but not on environmental sustainability. Life cycle assessment revealed that limestone calcination is the main environmental hotspot followed by the capture and storage of the calcination CO2 emissions. Mining, comminution, and hydration had a small impact, while results were sensitive to the kiln technology, fuel type, electricity mix, and transportation. Differences between the carbon and environmental footprint highlight that multi-issue life cycle impact assessment methods may be more appropriate when assessing CDR rather than only using carbon balances. Clean and energy efficient kilns (e.g., solar calciners) and the use of renewable energy optimize the system's environmental performance (total carbon and environmental footprint −1031 kgCO2eq and −15.1 Pt per ton of lime spread in the ocean, respectively). The valorisation of the CO2 emissions from limestone calcination, e.g., for fuels, chemicals, or plastics production, could potentially further improve ocean liming's environmental profile, through avoided emissions, however net removal would depend on the longevity of the use. Results imply that CO2 removal at the Gt yr.−1 scale can be achieved, however more research is required on the biological and ecological implications of this CDR approach.
•Ocean liming's environmental performance was examined using LCA.•Environmental hotspots include calcination heat and electricity consumption.•CCS or CCU during calcination is required.•Sensitivity analysis revealed future environmentally sustainable systems.•At optimum conditions 1034 kg CO2 tCaO−1 can be removed/avoided.
The negative emissions technology, artificial ocean alkalinization (AOA), aims to store atmospheric carbon dioxide (CO
2
) in the ocean by increasing total alkalinity (TA). Calcium carbonate ...saturation state (ΩCaCO
3
) and pH would also increase meaning that AOA could alleviate sensitive regions and ecosystems from ocean acidification. However, AOA could raise pH and ΩCaCO
3
well above modern-day levels, and very little is known about the environmental and biological impact of this. After treating a red calcifying algae (
Corallina
spp.) to elevated TA seawater, carbonate production increased by 60% over a control. This has implication for carbon cycling in the past, but also constrains the environmental impact and efficiency of AOA. Carbonate production could reduce the efficiency of CO
2
removal. Increasing TA, however, did not significantly influence
Corallina
spp. primary productivity, respiration, or photophysiology. These results show that AOA may not be intrinsically detrimental for
Corallina
spp. and that AOA has the potential to lessen the impacts of ocean acidification. However, the experiment tested a single species within a controlled environment to constrain a specific unknown, the rate change of calcification, and additional work is required to understand the impact of AOA on other organisms, whole ecosystems, and the global carbon cycle.
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•CO2 capture via enhanced weathering (EW) of limestone with seawater was studied.•A sophisticated model was developed for EW in a packed bubble column reactor.•Key mechanisms adopted ...in the model were experimentally validated.•CO2 capture rate and energy consumption in continuous operation were investigated.•Trade-off of ground area requirement and energy requirement was revealed.
Enhanced weathering of minerals is one option being considered for removing CO2 from the atmosphere to help combat climate change. In this work, we consider the weathering of calcite with seawater in a reactor using air enriched with CO2. A mathematical model of the packed bubble column reactor was constructed with the key mass transfer and chemical reaction components validated with experimental data. The modelling results for a continuous process reveal the performance in terms of the specific energy consumption and the CO2 capture rate, which are affected by parameters including particle size, superficial velocities of gas and liquid, reactor bed height and feed CO2 concentration. The major energy requirements are for pumping liquid and compressing gas, and for CO2 enrichment; energy needed for supplying solid particles (mining operations, transport and comminution) was found to be comparatively minor. A trade-off was possible between ground area requirement (determined by CO2 capture rate) and energy requirement. To capture 1 tonne of CO2 at the reactor, optimal designs were predicted to consume 2.1–2.3 GJ of electricity and occupy 1.8–5.2 m2 year of space, depending on the feed CO2 concentration. These would increase to 5.7–8.2 GJ and 7.1–13.1 m2 year per tonne of CO2 captured, after allowing for degassing of the weathering product in the ocean. This increased energy intensity is still within the range of the CO2 removal options previously reported, while the space requirement quantification provides essential information for future feasibility assessment of this scheme.
Land-management options for greenhouse gas removal (GGR) include afforestation or reforestation (AR), wetland restoration, soil carbon sequestration (SCS), biochar, terrestrial enhanced weathering ...(TEW), and bioenergy with carbon capture and storage (BECCS). We assess the opportunities and risks associated with these options through the lens of their potential impacts on ecosystem services (Nature's Contributions to People; NCPs) and the United Nations Sustainable Development Goals (SDGs). We find that all land-based GGR options contribute positively to at least some NCPs and SDGs. Wetland restoration and SCS almost exclusively deliver positive impacts. A few GGR options, such as afforestation, BECCS, and biochar potentially impact negatively some NCPs and SDGs, particularly when implemented at scale, largely through competition for land. For those that present risks or are least understood, more research is required, and demonstration projects need to proceed with caution. For options that present low risks and provide cobenefits, implementation can proceed more rapidly following no-regrets principles.
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