Why can't we just measure the carbon we remove?
The fundamental challenge is that the ocean is vast, turbulent, and already full of natural carbon. When you add alkalinity to seawater to enhance carbon dioxide removal, the chemical signal—a shift in pH or CO2 pressure—gets diluted to levels that are indistinguishable from normal background noise within weeks to months [4]. One expert analysis notes that because CO2 equilibration between ocean and atmosphere can take several months or longer, the added alkalinity becomes undetectable above natural variability on timescales relevant for monitoring [4]. This means that direct measurement alone cannot prove how much carbon was removed, a problem that is often glossed over in public discussions of ocean CDR.
The same paper makes a stark conclusion: comprehensive quantification of carbon removal via ocean alkalinity enhancement will be impossible through observational methods alone [4]. Numerical simulations—computer models—will be required. But those models must be carefully validated against real-world data, and such fit-for-purpose models are still being developed [4]. So the monitoring gap is not just a technical inconvenience; it is a fundamental scientific bottleneck that currently prevents us from verifying whether these methods actually work at scale.
What environmental side effects are we missing?
Beyond the challenge of measuring carbon removal, there is a parallel blind spot: we are not adequately monitoring the environmental side effects of these techniques. A systematic review of 20 life cycle assessments (LCAs) on ocean-based carbon dioxide removal found that current studies have a limited scope and routinely overlook environmental impacts beyond global warming [1]. Specifically, the review notes that LCA as a method is currently limited in capturing aquatic impacts, meaning we have very little understanding of how large-scale deployment might affect marine life, food webs, or ecosystem health [1].
The authors emphasize that the potential environmental side-effects of deploying ocean CDR on a large scale are largely unknown, and they caution that current LCA models cannot assess the full impacts on marine environments [1]. This is not a minor gap—it means that decisions about whether to deploy these technologies are being made without a complete picture of the ecological costs. The review recommends that future work must include more marine environmental impacts and develop new LCA methodology specifically for the ocean [1].
Are there any solutions being developed?
Researchers are actively working on frameworks to address these monitoring gaps, but the work is still in early stages. The SEAO2-CDR project, for example, is developing multidisciplinary assessment processes that will characterize the temporal and spatial monitoring, reporting, and verification (MRV) requirements for different ocean CDR techniques [2]. This includes determining what needs to be measured, where, and how often—a critical step that has been largely absent from earlier discussions [2].
However, the same project acknowledges that additional research is needed to facilitate environmentally safe, socially acceptable, and economically viable implementation [2]. Another expert group recommends that early-stage MRV for ocean alkalinity enhancement research should aim for comprehensiveness, reproducibility, and transparency, and that observational approaches must monitor for secondary precipitation, biotic calcification, and other ecosystem changes that could feedback on greenhouse gas sources or sinks [4]. The bottom line is that while the monitoring challenges are real and underestimated, there is a growing recognition that solving them requires a combination of better sensors, better models, and a willingness to accept that we may never have perfect verification—only probabilistic estimates.
About These Sources
This answer is built on 4 peer-reviewed studies — published from 2023 to 2025, 2 from 2024 or later, 1 in Q1 journals, collectively cited 76 times — selected as the most relevant from 4 studies that passed quality screening, drawn from 40 papers retrieved from a database of over 500 million.
Sources used in this answer
Life cycle assessment of ocean-based carbon dioxide removal approaches: A systematic literature review
A systematic review of 20 life cycle assessments found that current models cannot assess the full impacts of ocean CDR on marine environments and routinely overlook environmental side effects beyond global warming [1].
Strategies for the evaluation and assessment of ocean-based carbon dioxide removal
The SEAO2-CDR project is developing multidisciplinary frameworks to characterize monitoring, reporting, and verification requirements for ocean CDR, but notes that additional research is needed for safe and viable implementation [2].
Operational Monitoring of Open-Ocean Carbon Dioxide Removal Deployments: Detection, Attribution, and Determination of Side Effects
This perspective piece frames the scale of the CDR challenge—100–1000 Gt CO2 over the 21st century—and the need for operational monitoring to detect, attribute, and determine side effects of open-ocean deployments [3].
Monitoring, reporting, and verification for ocean alkalinity enhancement
For ocean alkalinity enhancement, comprehensive quantification of carbon removal will be impossible through observations alone due to dilution below natural variability; fit-for-purpose numerical models validated against data are essential [4].