Efficiency metrics for ocean alkalinity enhancements under responsive and prescribed atmospheric <i>p</i>CO<sub>2</sub> conditions
<p>Ocean alkalinity enhancement (OAE) and direct ocean removal (DOR) are emerging as promising technologies for enacting negative emissions. The long equilibration timescales, potential for premature subduction of surface water parcels, and extensive horizontal transport and dilution of added...
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Copernicus Publications
2025-01-01
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| Series: | Biogeosciences |
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| author | M. D. Tyka |
| author_facet | M. D. Tyka |
| author_sort | M. D. Tyka |
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| description | <p>Ocean alkalinity enhancement (OAE) and direct ocean removal (DOR) are emerging as promising technologies for enacting negative emissions. The long equilibration timescales, potential for premature subduction of surface water parcels, and extensive horizontal transport and dilution of added alkalinity make direct experimental measurement of induced <span class="inline-formula">CO<sub>2</sub></span> uptake challenging. Therefore, the challenge of measurement, reporting, and verification (MRV) will rely to a great extent on general circulation models, parameterized and constrained by experimental measurements. A number of recent studies have assessed the efficiency of OAE using different model setups and different metrics. Some models use prescribed atmospheric <span class="inline-formula">CO<sub>2</sub></span> levels, while others use fully coupled Earth system models. The former ignores atmospheric feedback effects, while the latter explicitly models them. In this paper it is shown that, even for very small OAE deployments, which do not substantially change atmospheric <span class="inline-formula"><i>p</i>CO<sub>2</sub></span>, the change in oceanic <span class="inline-formula">CO<sub>2</sub></span> inventories differs significantly between these methods due to atmospheric feedback causing some ocean <span class="inline-formula">CO<sub>2</sub></span> off-gassing. An analogous off-gassing occurs during direct air capture (DAC). Due to these feedback effects, care must be taken to compute the correct metrics when assessing OAE efficiency with respect to determining negative emissions credits, as opposed to determining the effect on global temperatures. This paper examines the commonly used metrics of OAE efficiency, their exact physical meanings, the assumptions inherent in their use, and the relationship between them. It is shown that the efficiency metric <span class="inline-formula"><i>η</i>(<i>t</i>)</span>, used in prescribed <span class="inline-formula"><i>p</i></span>CO<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M10" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi/><mn mathvariant="normal">2</mn><mi mathvariant="normal">atm</mi></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="16pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="2b4bdf2e6c26723c23f0ea148a8eb081"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-22-341-2025-ie00001.svg" width="16pt" height="16pt" src="bg-22-341-2025-ie00001.png"/></svg:svg></span></span> simulations, equals the equivalent schedule of a gradual DAC removal and storage in a fully coupled system.</p> |
| format | Article |
| id | doaj-art-913d92014f394f5f89395a537b3fbb44 |
| institution | Kabale University |
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| language | English |
| publishDate | 2025-01-01 |
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| spelling | doaj-art-913d92014f394f5f89395a537b3fbb442025-01-16T09:35:14ZengCopernicus PublicationsBiogeosciences1726-41701726-41892025-01-012234135310.5194/bg-22-341-2025Efficiency metrics for ocean alkalinity enhancements under responsive and prescribed atmospheric <i>p</i>CO<sub>2</sub> conditionsM. D. Tyka0Applied Research, Google Inc., 601 N 34th St, Seattle, WA 98103, USA<p>Ocean alkalinity enhancement (OAE) and direct ocean removal (DOR) are emerging as promising technologies for enacting negative emissions. The long equilibration timescales, potential for premature subduction of surface water parcels, and extensive horizontal transport and dilution of added alkalinity make direct experimental measurement of induced <span class="inline-formula">CO<sub>2</sub></span> uptake challenging. Therefore, the challenge of measurement, reporting, and verification (MRV) will rely to a great extent on general circulation models, parameterized and constrained by experimental measurements. A number of recent studies have assessed the efficiency of OAE using different model setups and different metrics. Some models use prescribed atmospheric <span class="inline-formula">CO<sub>2</sub></span> levels, while others use fully coupled Earth system models. The former ignores atmospheric feedback effects, while the latter explicitly models them. In this paper it is shown that, even for very small OAE deployments, which do not substantially change atmospheric <span class="inline-formula"><i>p</i>CO<sub>2</sub></span>, the change in oceanic <span class="inline-formula">CO<sub>2</sub></span> inventories differs significantly between these methods due to atmospheric feedback causing some ocean <span class="inline-formula">CO<sub>2</sub></span> off-gassing. An analogous off-gassing occurs during direct air capture (DAC). Due to these feedback effects, care must be taken to compute the correct metrics when assessing OAE efficiency with respect to determining negative emissions credits, as opposed to determining the effect on global temperatures. This paper examines the commonly used metrics of OAE efficiency, their exact physical meanings, the assumptions inherent in their use, and the relationship between them. It is shown that the efficiency metric <span class="inline-formula"><i>η</i>(<i>t</i>)</span>, used in prescribed <span class="inline-formula"><i>p</i></span>CO<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M10" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi/><mn mathvariant="normal">2</mn><mi mathvariant="normal">atm</mi></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="16pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="2b4bdf2e6c26723c23f0ea148a8eb081"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-22-341-2025-ie00001.svg" width="16pt" height="16pt" src="bg-22-341-2025-ie00001.png"/></svg:svg></span></span> simulations, equals the equivalent schedule of a gradual DAC removal and storage in a fully coupled system.</p>https://bg.copernicus.org/articles/22/341/2025/bg-22-341-2025.pdf |
| spellingShingle | M. D. Tyka Efficiency metrics for ocean alkalinity enhancements under responsive and prescribed atmospheric <i>p</i>CO<sub>2</sub> conditions Biogeosciences |
| title | Efficiency metrics for ocean alkalinity enhancements under responsive and prescribed atmospheric <i>p</i>CO<sub>2</sub> conditions |
| title_full | Efficiency metrics for ocean alkalinity enhancements under responsive and prescribed atmospheric <i>p</i>CO<sub>2</sub> conditions |
| title_fullStr | Efficiency metrics for ocean alkalinity enhancements under responsive and prescribed atmospheric <i>p</i>CO<sub>2</sub> conditions |
| title_full_unstemmed | Efficiency metrics for ocean alkalinity enhancements under responsive and prescribed atmospheric <i>p</i>CO<sub>2</sub> conditions |
| title_short | Efficiency metrics for ocean alkalinity enhancements under responsive and prescribed atmospheric <i>p</i>CO<sub>2</sub> conditions |
| title_sort | efficiency metrics for ocean alkalinity enhancements under responsive and prescribed atmospheric i p i co sub 2 sub conditions |
| url | https://bg.copernicus.org/articles/22/341/2025/bg-22-341-2025.pdf |
| work_keys_str_mv | AT mdtyka efficiencymetricsforoceanalkalinityenhancementsunderresponsiveandprescribedatmosphericipicosub2subconditions |