The representation of alkalinity and the carbonate pump from CMIP5 to CMIP6 Earth system models and implications for the carbon cycle
The representation of alkalinity and the carbonate pump from CMIP5 to CMIP6 Earth system models and implications for the carbon cycle
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DOI：
10.5194/bg-20-1195-2023
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发表时间：
2023-04
期刊：
影响因子：
4.9
通讯作者：
Alban Planchat;L. Kwiatkowski;L. Bopp;Olivier Torres;J. Christian;M. Butenschön;T. Lovato;R. Séférian;M. Chamberlain;O. Aumont;Michio Watanabe;A. Yamamoto;A. Yool;T. Ilyina;H. Tsujino;K. Krumhardt;J. Schwinger;J. Tjiputra;J. Dunne;C. Stock
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作者：
Alban Planchat;L. Kwiatkowski;L. Bopp;Olivier Torres;J. Christian;M. Butenschön;T. Lovato;R. Séférian;M. Chamberlain;O. Aumont;Michio Watanabe;A. Yamamoto;A. Yool;T. Ilyina;H. Tsujino;K. Krumhardt;J. Schwinger;J. Tjiputra;J. Dunne;C. Stock
抽象的。海洋碱度对于表层沃茨吸收大气碳至关重要，并为相关的酸化提供缓冲能力。然而，与溶解无机碳（DIC）不同，碱度不受人为碳排放的直接影响。在预测未来海洋碳吸收和潜在生态系统影响的背景下，特别是通过耦合模型相互比较项目，碱度的代表性及其在海洋内部分布的主要驱动因素，碳酸钙循环，往往被忽视。在这里，我们跟踪从CMIP 5到CMIP 6的变化，相对于地球系统模型（ESM）表示的碱度和碳酸盐泵，通过生物生产碳酸钙消耗表面海洋的碱度，并通过出口和溶解将其释放到深处。我们报告了CMIP 6 ESM相对于CMIP 5中的碱度表示的改进，CMIP 6 ESM模拟较低的表面碱度浓度，增加的垂直表面梯度和增强的全球垂直梯度。这一改善的部分原因是某些无害环境措施增加了碳酸钙（CaCO 3）的产量，从而重新分配了表面的碱度，并加强了其在水柱中的垂直梯度。我们能够限制颗粒无机碳（PIC）输出估计为44-55 Tmol yr−1在100 m的ESM匹配所观察到的垂直梯度的碱度。回顾整个CMIP 5/6的碳酸钙循环的表示，我们发现了大量的参数化。虽然目前所有生物地球化学模型都代表了中上层钙化，但它们是隐含的，并且它们不代表海底钙化。此外，大多数模型模拟的是海洋方解石，而不是文石。在CMIP 6中，某些模型组增加了模拟碳酸钙生产，下沉，溶解和沉积的复杂性。然而，这不足以解释碱度表示的总体改善，因此这可能是海洋生物地球化学模型调整或临时参数化的结果。虽然建模者的目标是平衡全球碱度预算的ESM，以限制漂移的海洋碳吸收在工业化前的条件下，不同的假设有关的预算和/或碱度初始化程序的关闭有可能影响未来的碳吸收的预测。例如，在许多模型中，碳酸盐的产生、溶解和埋藏与海水饱和状态无关，如果考虑到这一点，敏感性的范围很大。因此，海洋酸化对碳酸盐泵的未来影响，进而对海洋碳吸收的影响，在目前的紧急保障措施中可能被低估，而且没有得到充分的限制。
Abstract. Ocean alkalinity is critical to the uptake of atmospheric carbon in surface waters and provides buffering capacity towards the associated acidification. However, unlike dissolved inorganic carbon (DIC), alkalinity is not directly impacted by anthropogenic carbon emissions. Within the context of projections of future ocean carbon uptake and potential ecosystem impacts, especially through Coupled Model Intercomparison Projects (CMIPs), the representation of alkalinity and the main driver of its distribution in the ocean interior, the calcium carbonate cycle, have often been overlooked. Here we track the changes from CMIP5 to CMIP6 with respect to the Earth system model (ESM) representation of alkalinity and the carbonate pump which depletes the surface ocean in alkalinity through biological production of calcium carbonate and releases it at depth through export and dissolution. We report an improvement in the representation of alkalinity in CMIP6 ESMs relative to those in CMIP5, with CMIP6 ESMs simulating lower surface alkalinity concentrations, an increased meridional surface gradient and an enhanced global vertical gradient. This improvement can be explained in part by an increase in calcium carbonate (CaCO3) production for some ESMs, which redistributes alkalinity at the surface and strengthens its vertical gradient in the water column. We were able to constrain a particulate inorganic carbon (PIC) export estimate of 44–55 Tmol yr−1 at 100 m for the ESMs to match the observed vertical gradient of alkalinity. Reviewing the representation of the CaCO3 cycle across CMIP5/6, we find a substantial range of parameterizations. While all biogeochemical models currently represent pelagic calcification, they do so implicitly, and they do not represent benthic calcification. In addition, most models simulate marine calcite but not aragonite. In CMIP6, certain model groups have increased the complexity of simulated CaCO3 production, sinking, dissolution and sedimentation. However, this is insufficient to explain the overall improvement in the alkalinity representation, which is therefore likely a result of marine biogeochemistry model tuning or ad hoc parameterizations. Although modellers aim to balance the global alkalinity budget in ESMs in order to limit drift in ocean carbon uptake under pre-industrial conditions, varying assumptions related to the closure of the budget and/or the alkalinity initialization procedure have the potential to influence projections of future carbon uptake. For instance, in many models, carbonate production, dissolution and burial are independent of the seawater saturation state, and when considered, the range of sensitivities is substantial. As such, the future impact of ocean acidification on the carbonate pump, and in turn ocean carbon uptake, is potentially underestimated in current ESMs and is insufficiently constrained.