The Availability of Limestone and Other Raw Materials for Ocean Alkalinity Enhancement

To achieve the Paris climate target, deep emissions reductions have to be complemented with carbon dioxide removal (CDR). However, a portfolio of CDR options is necessary to reduce risks and potential negative side effects. Despite a large theoretical potential, ocean-based CDR such as ocean alkalinity enhancement (OAE) has been omitted in climate change mitigation scenarios so far. In this study, we provide a techno-economic assessment of large-scale OAE using hydrated lime (‘ocean liming’). We address key uncertainties that determine the overall cost of ocean liming such as the CO2 uptake efficiency per unit of material, distribution strategies avoiding carbonate precipitation which would compromise efficiency, and technology availability (e.g., solar calciners). We find that at economic costs of 130 to 295 $/tCO2 net-removed, ocean liming could be a competitive CDR option which could make a significant contribution towards the Paris climate target. As the techno-economic assessment identified no showstoppers, we argue for more research on ecosystem impacts, governance, monitoring, reporting, and verification, and technology development and assessment to determine whether ocean liming and other OAE should be considered as part of a broader CDR portfolio.

Section: Methodsmentioning

confidence: 99%

Marine carbon dioxide removal by alkalinization should no longer be overlooked

Kowalczyk,

Amann,

Strefler

et al. 2024

“…This corresponds respectively to 10.3, 31.0, and 51.8% of current (2020) raw mineral (non-metallic) and ore use [43], and to 5.6, 16.9, and 28.3% of estimated raw mineral and ore use in 2060 [44]. For comparison, current global cement production is ∼4 Gt y −1 [45] and has been projected to roughly double by 2100 [46,47], while global available carbonate mineral deposits shallower than 25 m and within 50 km of coastlines amount to ∼570 Gt y −1 of raw feedstock potential if deployed between 2030 and 2100 [48].…”

Section: Earth System Modelling Of Ocean Alkalinity Enhancementmentioning

confidence: 99%

A biogeochemical model of mineral-based ocean alkalinity enhancement: impacts on the biological pump and ocean carbon uptake

Fakhraee

Planavsky

et al. 2023

Minimizing anthropogenic climate disruption in the coming century will likely require carbon dioxide removal (CDR) from Earth’s atmosphere in addition to deep and rapid cuts to greenhouse gas emissions. Ocean alkalinity enhancement — the modification of surface ocean chemistry to drive marine uptake of atmospheric CO2 — is seen as a potentially significant component of ocean-based CDR portfolios. However, there has been limited mechanistic exploration of the large-scale CDR potential of mineral-based ocean alkalinity enhancement, potential bottlenecks in alkalinity release, and the biophysical impacts of alkaline mineral feedstocks on marine ecology and the marine biological carbon pump. Here we a series of biogeochemical models to evaluate the gross CDR potential and environmental impacts of ocean alkalinity enhancement using solid mineral feedstocks. We find that natural alkalinity sources — basalt and olivine — lead to very low CDR efficiency while strongly perturbing marine food quality and fecal pellet production by marine zooplankton. Artificial alkalinity sources — the synthetic metal oxides MgO and CaO — are potentially capable of significant CDR with reduced environmental impact, but their deployment at scale faces major challenges associated with substrate limitation and process CO2 emissions during feedstock production. Taken together, our results highlight distinct challenges for ocean alkalinity enhancement as a CDR strategy and indicate that mineral-based ocean alkalinity enhancement should be pursued with caution.

“…A further distinction can be made between methods that seek to accelerate the uptake of atmospheric CO 2 by enhancing natural sinks and methods that seek to engineer the removal and subsequent storage of CO 2 . An ocean-based approach that seeks to enhance a natural process called chemical weathering, which naturally removes up to 0.29 GtC yr −1 (Hartmann et al 2009), is ocean alkalinity enhancement (OAE, Ilyina et al 2013, González and Ilyina 2016, Caserini et al 2022, which is the addition of alkalinity to the surface ocean. This might be achieved by adding minerals (e.g.…”

Section: Introductionmentioning

confidence: 99%

Earth system responses to carbon dioxide removal as exemplified by ocean alkalinity enhancement: tradeoffs and lags

Jeltsch-Thömmes,

Tran,

Lienert

et al. 2024

Carbon Dioxide Removal (CDR) is discussed for offsetting residual greenhouse gas emissions or even reversing climate change. All emissions scenarios of the Intergovernmental Panel on Climate Change that meet the ”well below 2 ◦C” warming target of the Paris Agreement include CDR. Ocean alkalinity enhancement (OAE) may be one possible CDR where the carbon uptake of the ocean is increased by artificial alkalinity addition. Here, we investigate the effect of OAE on modelled carbon reservoirs and fluxes in two observationally-constrained large perturbed parameter ensembles. OAE is assumed to be technically successful and deployed as an additional CDR in the SSP5-3.4 temperature overshoot scenario. Tradeoffs involving feedbacks with atmospheric CO2 result in a low efficiency of an alkalinity-driven atmospheric CO2 reduction of -0.35 [-0.37 – -0.33] mol C per mol alkalinity addition (skill-weighted mean and 68% c.i.). The realized atmospheric CO2 reduction, and correspondingly the efficiency, is more than two times smaller than the direct alkalinity-driven enhancement of ocean uptake. The alkalinity-driven ocean carbon uptake is partly offset by the release of carbon from the land biosphere and a reduced ocean carbon sink in response to lowered atmospheric CO2 under OAE.In a second step we use the Bern3D-LPX model in CO2 peak-decline simulations to address hysteresis and temporal lags of surface air temperature change (ΔSAT) in an idealized scenario where ΔSAT increases to ∼2 ◦C and then declines to ∼1.5 ◦C as result of CDR. ΔSAT lags the decline in CO2-forcing by 18 [14 – 22] years, depending close to linearly on the equilibrium climate sensitivity of the respective ensemble member. These tradeoffs and lags are an inherent feature of the Earth system response to changes in atmospheric CO2 and will therefore be equally important for other CDR methods.