Declining ocean chlorophyll under unabated anthropogenic CO2 emissions
Declining ocean chlorophyll under unabated anthropogenic CO2 emissions
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DOI：
10.1088/1748-9326/6/3/034035
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发表时间：
2011-07-01
影响因子：
6.7
通讯作者：
Schellnhuber, H. J.
中科院分区：
文献类型：
--
作者：
Hofmann, M.;Worm, B.;Schellnhuber, H. J.
浮游植物对二氧化碳和无机营养物的光合同化作用是维持海洋生物的必要先决条件。这一过程与海洋真光层中叶绿素的浓度密切相关。根据最近的一项实地研究，海洋叶绿素（a）浓度在上个世纪有所下降，估计全球每年下降率为全球中位数的1.0%。在这里，我们试图找出可能的机制，可以解释这种趋势。我们探索这些问题，使用海洋环流模式，迫使记录的历史和预测的未来人为二氧化碳排放量根据IPCC SRES A1 FI排放情景，直到2100年。我们进一步延长了A1 FI情景所涵盖的时间段，假设从2100年到2200年排放量线性下降，并将其保持在零水平，直到2400年。我们的数值模拟显示，在20世纪世纪，叶绿素（a）浓度只有微弱的减少，但预计在2000年和2200年之间下降50%。我们确定了一个本地和远程的作用机制，这种减少在北大西洋：（I）增加海面温度降低当地的深层混合，因此，减少营养供应从沃茨在中间深度;（II）一个稳定的浅滩的大西洋翻转细胞倾向于运输越来越多的营养耗尽的沃茨从南半球向北，导致进一步减少营养供应。这些结果为温度驱动的海洋叶绿素a和生产力下降提供了支持，但表明需要调用其他机制来解释近几十年来观察到的下降。
Photosynthetic assimilation of carbon dioxide and inorganic nutrients by phytoplankton constitutes a necessary prerequisite for sustaining marine life. This process is tightly linked to the concentration of chlorophyll in the ocean's euphotic zone. According to a recent field study marine chlorophyll(a) concentrations have declined over the last century with an estimated global rate of 1.0% of the global median per year. Here we attempt to identify possible mechanisms which could explain such trends. We explore these questions using an ocean general circulation model forced with documented historic and projected future anthropogenic emissions of carbon dioxide according to the IPCC SRES A1FI emission scenario until the year 2100. We further extend the time period covered by the A1FI scenario by assuming a linear decline in emissions from 2100 to 2200 and keeping them at zero levels until 2400. Our numerical simulations reveal only weak reductions in chlorophyll(a) concentrations during the twentieth century, but project a 50% decline between 2000 and 2200. We identify a local and a remotely acting mechanism for this reduction in the North Atlantic: (I) increased sea surface temperatures reduce local deep mixing and, hence, reduce the nutrient supply from waters at intermediate depths; (II) a steady shoaling of the Atlantic overturning cell tends to transport increasingly nutrient depleted waters from the Southern Hemisphere toward the north, leading to further diminishment of nutrient supply. These results provide support for a temperature-driven decline in ocean chlorophyll(a) and productivity, but suggest that additional mechanisms need to be invoked to explain observed declines in recent decades.