No increase is detected and modeled for the seasonal cycle amplitude of <i>δ</i><sup>13</sup>C of atmospheric carbon dioxide
<p>Measurements of the seasonal cycle of <span class="inline-formula"><i>δ</i><sup>13</sup></span>C of atmospheric CO<span class="inline-formula"><sub>2</sub></span> (<span class="inline-formula">&...
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| Main Authors: | , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2025-01-01
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| Series: | Biogeosciences |
| Online Access: | https://bg.copernicus.org/articles/22/19/2025/bg-22-19-2025.pdf |
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| Summary: | <p>Measurements of the seasonal cycle of <span class="inline-formula"><i>δ</i><sup>13</sup></span>C of atmospheric CO<span class="inline-formula"><sub>2</sub></span> (<span class="inline-formula"><i>δ</i><sup>13</sup></span>C<span class="inline-formula"><sub>a</sub></span>) provide information on the global carbon cycle and the regulation of carbon and water fluxes by leaf stomatal openings on ecosystem and decadal scales. Land biosphere carbon exchange is the primary driver of <span class="inline-formula"><i>δ</i><sup>13</sup></span>C<span class="inline-formula"><sub>a</sub></span> seasonality in the Northern Hemisphere (NH). We use isotope-enabled simulations of the Bern3D-LPX (Land surface Processes and eXchanges) Earth system model of intermediate complexity and fossil fuel emission estimates with a model of atmospheric transport to simulate atmospheric <span class="inline-formula"><i>δ</i><sup>13</sup></span>C<span class="inline-formula"><sub>a</sub></span> at globally distributed monitoring sites. Unlike the observed growth of the seasonal amplitude of CO<span class="inline-formula"><sub>2</sub></span> at northern sites, no significant temporal trend in the seasonal amplitude of <span class="inline-formula"><i>δ</i><sup>13</sup></span>C<span class="inline-formula"><sub>a</sub></span> was detected at most sites, consistent with the insignificant model trends. Comparing the preindustrial (1700) and modern (1982–2012) periods, the modeled small-amplitude changes at northern sites are linked to the near-equal increase in background atmospheric CO<span class="inline-formula"><sub>2</sub></span> and the seasonal signal of the net atmosphere–land <span class="inline-formula"><i>δ</i><sup>13</sup></span>C flux in the northern extratropical region, with no long-term temporal changes in the isotopic fractionation in these ecosystems dominated by C<span class="inline-formula"><sub>3</sub></span> plants. The good data–model agreement in the seasonal amplitude of <span class="inline-formula"><i>δ</i><sup>13</sup></span>C<span class="inline-formula"><sub>a</sub></span> and in its decadal trend provides implicit support for the regulation of stomatal conductance by C<span class="inline-formula"><sub>3</sub></span> plants towards intrinsic water use efficiency growing proportionally to atmospheric CO<span class="inline-formula"><sub>2</sub></span> over recent decades. Disequilibrium fluxes contribute little to the seasonal amplitude of the net land isotope flux north of 40° N but contribute near equally to the isotopic flux associated with growing season net carbon uptake in tropical and Southern Hemisphere (SH) ecosystems, pointing to the importance of monitoring <span class="inline-formula"><i>δ</i><sup>13</sup></span>C<span class="inline-formula"><sub>a</sub></span> over these ecosystems. We propose applying seasonally resolved <span class="inline-formula"><i>δ</i><sup>13</sup></span>C<span class="inline-formula"><sub>a</sub></span> observations as an additional constraint for land biosphere models and underlying processes for improved projections of the anthropogenic carbon sink.</p> |
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| ISSN: | 1726-4170 1726-4189 |