The role of CO2 decline for the onset of Northern Hemisphere glaciation
The role of CO2 decline for the onset of Northern Hemisphere glaciation
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DOI：
10.1016/j.quascirev.2015.04.015
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发表时间：
2015-07
影响因子：
4
通讯作者：
M. Willeit;A. Ganopolski;R. Calov;A. Robinson;M. Maslin
中科院分区：
文献类型：
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作者：
M. Willeit;A. Ganopolski;R. Calov;A. Robinson;M. Maslin
上新世-更新世过渡期（PPT）发生在大约320万年至250万年前（Ma），代表了气候系统的重大转变，其特征是逐渐冷却的趋势和欧亚大陆北方和北美出现大型大陆冰盖。古证据表明，PPT是伴随着，并可能导致大气中CO2的减少，但CO2重建的时间分辨率低，这段时间的不确定性仍然很大。因此，而不是应用现有的CO2重建，我们解决了一个“逆”问题，找到一个示意性的CO2浓度的情况下，使我们能够模拟的时间演变的关键气候特征与古气候记录。为此，我们进行了一个合奏的瞬态模拟与地球系统模型的中间复杂性，从中我们得出了一个最好的猜测瞬态CO2情景的时间间隔为3.2至2.4 Ma，使模拟和重建的底栖δ 18 O和全球海表温度演变之间的最佳拟合。我们的数据约束的CO2情景与最近的CO2重建是一致的，并建议从375-425到275-300 ppm，3.2和2.4 Ma之间的CO2逐渐下降。除了逐渐下降，最适合古气候数据需要存在明显的CO2变化与41-kyr（1 kyr = 1000年）的降水周期一致。在我们的模拟中，长期CO2下降伴随着北方半球冰川作用在2.7 Ma左右相对突然的加剧。这是冰盖对逐渐减少的CO2和轨道强迫的响应的阈值行为的结果。模拟的北方半球冰盖在早更新世冰期旋回达到最大体积相当于海平面下降约40 m。冰量和底栖生物δ 18 O均受41-kyr旋回的控制。我们的模拟表明，在2.7马格陵兰岛是无冰的夏季日照最大值，只有部分冰覆盖在夏季日照最小的时期。一个完全冰川格陵兰相当于其现今的冰量是模拟只有在冰川极大期后2.7马和更连续的2.5马。
The Pliocene–Pleistocene Transition (PPT), from around 3.2 to 2.5 million years ago (Ma), represented a major shift in the climate system and was characterized by a gradual cooling trend and the appearance of large continental ice sheets over northern Eurasia and North America. Paleo evidence indicates that the PPT was accompanied and possibly caused by a decrease in atmospheric CO2, but the temporal resolution of CO2reconstructions is low for this period of time and uncertainties remain large. Therefore, instead of applying existent CO2reconstructions we solved an ‘inverse’ problem by finding a schematic CO2concentration scenario that allows us to simulate the temporal evolution of key climate characteristics in agreement with paleoclimate records. To this end, we performed an ensemble of transient simulations with an Earth system model of intermediate complexity from which we derived a best guess transient CO2scenario for the interval from 3.2 to 2.4 Ma that gives the best fit between the simulated and reconstructed benthic δ18O and global sea surface temperature evolution. Our data-constrained CO2scenarios are consistent with recent CO2reconstructions and suggest a gradual CO2decline from 375–425 to 275–300 ppm, between 3.2 and 2.4 Ma. In addition to a gradual decline, the best fit to paleoclimate data requires the existence of pronounced CO2variability coherent with the 41-kyr (1 kyr = 1000 years) obliquity cycle. In our simulations the long-term CO2decline is accompanied by a relatively abrupt intensification of Northern Hemisphere glaciation at around 2.7 Ma. This is the result of a threshold behaviour of the ice sheets response to gradual CO2decrease and orbital forcing. The simulated Northern Hemisphere ice sheets during the early Pleistocene glacial cycles reach a maximum volume equivalent to a sea level drop of about 40 m. Both ice volume and benthic δ18O are dominated by 41-kyr cyclicity. Our simulations suggest that before 2.7 Ma Greenland was ice free during summer insolation maxima and only partly ice covered during periods of minimum summer insolation. A fully glaciated Greenland comparable to its present-day ice volume is modelled only during glacial maxima after 2.7 Ma and more continuously after 2.5 Ma.