Toward reconciliation of Late Ordovician (∼440 Ma) glaciation with very high CO2 levels
Toward reconciliation of Late Ordovician (∼440 Ma) glaciation with very high CO2 levels
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努力协调晚奥陶世（~440 Ma）冰川作用与极高的二氧化碳水平
DOI：
10.1029/91jd02449
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发表时间：
1991
影响因子：
--
通讯作者：
S. Baum
中科院分区：
文献类型：
--
作者：
T. Crowley;S. Baum
虽然中生代冰川通常与估计的低大气CO2的时间相吻合，晚奥陶纪（440马）冰川是一个显着的例外。在此期间，二氧化碳水平可能比现在高出10倍。在早期的一篇论文中，我们提出冈瓦纳大陆独特的地理配置可以解释这种反应，因为超大陆的边缘基本上与南极相切，附近海洋的调节作用可能抑制了陆地夏季变暖的幅度，从而允许冰川开始。早期研究的一个局限性是它使用了线性能量平衡模型（EBM）。在本文中，我们进一步测试上述假设在一套实验中的非线性EBM，允许雪反馈。我们还利用更新的估计CO2水平，太阳光度的下降，和轨道强迫的变化。在光度或二氧化碳没有变化的基线实验中，冰覆盖面积为6.3 ×106平方公里，占奥陶纪冰盖估计面积的53%。在轨道强迫、7 X/13 X CO2和-3.5%/-5.0%光度的不同组合下进行的额外实验得出了晚奥陶世估计冰面积的0-35%。对可能的地形影响的粗略估计将这些数字增加到总估计冰面积的7 -47%。与冰盖增长有关的其他因素应该会使这些值有所增加。这些结果为高CO2/冰川作用解释提供了额外的支持，但需要注意的是，即使是部分成功，也只有当参数处于其允许范围的极端时才会发生。奥陶纪冰川作用的估计持续时间也与冈瓦纳大陆跨越南极的迁移相一致，位于中心的南极在夏季产生无冰条件。因此，相同水平的外部强迫产生冰川或无冰的条件下，解决方案取决于陆块的位置。虽然在这个问题上还需要更多的工作，但我们的实验表明，对于这个令人困惑的古气候悖论，可能有一个相对简单的解释。
Although Phanerozoic glaciations usually coincided with times of estimated low atmospheric CO2, the Late Ordovician (440 Ma) glaciation is a significant exception. CO2 levels during that time may have been as much as 10 times greater than present. In an earlier paper we suggested that the unique geographic configuration of Gondwanaland may explain such a response, as the edge of the supercontinent was essentially tangent to the south pole, and the moderating effect of the nearby ocean may have suppressed the magnitude of summer warming on the landmass, thereby allowing glacial inception. One limitation to the earlier study was that it used a linear energy balance model (EBM). In this paper we further test the above hypothesis in a suite of experiments with a nonlinear EBM that allows for snow-albedo feedback. We also utilize updated estimates for CO2 levels, decreases in solar luminosity, and variations in orbital forcing. Baseline experiments with no changes in luminosity or CO2 resulted in an ice-covered area of 6.3 ×106 km2, 53% of the estimated area covered by the Ordovician ice sheet. Additional experiments for different combinations of orbital forcing, 7X/13X CO2, and −3.5%/−5.0% luminosity yielded 0–35% of the estimated ice area in the Late Ordovician. A crude estimate of possible topographic influences increased these numbers to7–47% of total estimated ice area. Additional factors related to ice sheet growth should increase these values somewhat. These results provide additional support for the high CO2/glaciation explanation, with the caveat that even the partial success occurs only when parameters are at the extreme end of their permissible range. The estimated duration of Ordovician glaciation is also consistent with the migration of Gondwanaland across the south pole, with a centrally located pole yielding ice-free conditions in the summer. Thus identical levels of external forcing yield either glaciated or ice-free conditions, with the solution dependent on location of the landmass. Although more work is required on this topic, our experiments suggest that there may be a relatively parsimonious explanation for this perplexing paleoclimate paradox.The results lend further support to the proposition that paleogeography significantly modifies the role of CO2 in the long-term evolution of climate.