Arctic Ocean acidification over the 21st century co-driven by anthropogenic carbon increases and freshening in the CMIP6 model ensemble
<p>The uptake of anthropogenic carbon (<span class="inline-formula">C<sub>ant</sub></span>) by the ocean leads to ocean acidification, causing the reduction of pH and the saturation states of aragonite (<span class="inline-formula">Ω<sub>...
| Main Authors: | , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2021-04-01
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| Series: | Biogeosciences |
| Online Access: | https://bg.copernicus.org/articles/18/2221/2021/bg-18-2221-2021.pdf |
| Summary: | <p>The uptake of anthropogenic carbon (<span class="inline-formula">C<sub>ant</sub></span>) by the ocean leads to ocean acidification, causing the reduction of pH and the saturation states of aragonite (<span class="inline-formula">Ω<sub>arag</sub></span>) and calcite (<span class="inline-formula">Ω<sub>calc</sub></span>). The Arctic Ocean is particularly vulnerable to ocean acidification due to its naturally low pH and saturation states and due to ongoing freshening and the concurrent reduction in total alkalinity in this region. Here, we analyse ocean acidification in the Arctic Ocean over the 21st century across 14 Earth system models (ESMs) from the latest Coupled Model Intercomparison Project Phase 6 (CMIP6). Compared to the previous model generation (CMIP5), models generally better simulate maximum sea surface densities in the Arctic Ocean and consequently the transport of <span class="inline-formula">C<sub>ant</sub></span> into the Arctic Ocean interior, with simulated historical increases in <span class="inline-formula">C<sub>ant</sub></span> in improved agreement with observational products. Moreover, in CMIP6 the inter-model uncertainty of projected changes over the 21st century in Arctic Ocean <span class="inline-formula">Ω<sub>arag</sub></span> and <span class="inline-formula">Ω<sub>calc</sub></span> averaged over the upper 1000 m is reduced by 44–64 %. The strong reduction in projection uncertainties of <span class="inline-formula">Ω<sub>arag</sub></span> and <span class="inline-formula">Ω<sub>calc</sub></span> can be attributed to compensation between <span class="inline-formula">C<sub>ant</sub></span> uptake and total alkalinity reduction in the latest models. Specifically, ESMs with a large increase in Arctic Ocean <span class="inline-formula">C<sub>ant</sub></span> over the 21st century tend to simulate a relatively weak concurrent freshening and alkalinity reduction, while ESMs with a small increase in <span class="inline-formula">C<sub>ant</sub></span> simulate a relatively strong freshening and concurrent total alkalinity reduction. Although both mechanisms contribute to Arctic Ocean acidification over the 21st century, the increase in <span class="inline-formula">C<sub>ant</sub></span> remains the dominant driver. Even under the low-emissions Shared Socioeconomic Pathway 1-2.6 (SSP1-2.6), basin-wide averaged <span class="inline-formula">Ω<sub>arag</sub></span> undersaturation in the upper 1000 m occurs before the end of the century. While under the high-emissions pathway SSP5-8.5, the Arctic Ocean mesopelagic is projected to even become undersaturated with respect to calcite. An emergent constraint identified in CMIP5 which relates present-day maximum sea surface densities in the Arctic Ocean to the projected end-of-century Arctic Ocean <span class="inline-formula">C<sub>ant</sub></span> inventory is found to generally hold in CMIP6. However, a coincident constraint on Arctic declines in <span class="inline-formula">Ω<sub>arag</sub></span> and <span class="inline-formula">Ω<sub>calc</sub></span> is not apparent in the new generation of models. This is due to both the reduction in <span class="inline-formula">Ω<sub>arag</sub></span> and <span class="inline-formula">Ω<sub>calc</sub></span> projection uncertainty and the weaker direct relationship between projected changes in Arctic Ocean <span class="inline-formula">C<sub>ant</sub></span> and changes in <span class="inline-formula">Ω<sub>arag</sub></span> and <span class="inline-formula">Ω<sub>calc</sub></span>.</p> |
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| ISSN: | 1726-4170 1726-4189 |