11:45 〜 12:00
[MIS14-19] 過去の間氷期において溶け残り氷床により抑制された北極温暖化
キーワード：間氷期、溶け残り氷床、北極海氷、大気海洋植生結合モデル
The Arctic climate of past interglacials (MIS1, MIS5e, MIS11) are simulated by the MIROC4m-LPJ AOVGCM with prescribed ice sheet distributions in time slice experiments. Different climate responses are observed during periods before and after glacial ice sheets have disappeared, especially in autumn over the Arctic Ocean. When glacial ice sheets are absent (5ka, 125ka, and 410ka), higher summer insolation peaks result in higher annual mean SAT and greater Arctic sea ice melting, consistent with Yin & Berger (2012). This sea ice retreat further amplifies the annual warming at higher NH latitudes due to heat exchange between the atmosphere and oceans, consistent with the 127ka PMIP experiments (Guarino et al., 2020; Otto-Bliesner et al., 2020; Kageyama et al., 2021; O’ishi et al., 2021; Sicard et al., 2022). Vegetation feedback, in addition to snow/ice feedback and cloud feedback, plays an important role in Arctic warming amplification during glacial ice sheet-free interglacial periods.
On the other hand, when glacial ice sheets remain (10ka, 130ka, and 415ka), Arctic warming and sea ice melting are suppressed throughout the year. The periods with remnant glacial ice sheets do not show Arctic warming amplification when compared to global mean annual surface temperature changes. Even if the surface is heated by solar radiation in summer, the anomalous energy is transported toward glacial ice sheets and is not used effectively to amplify the warmth. In MIS5e, annual mean temperatures are much lower at 130ka than at 125ka despite the larger solar radiation contribution to warming at 130ka, and this temperature difference is too large to be explained by GHG forcing alone. Similarly, despite the stronger insolation at 10ka, which is near the peak in MIS1, the annual mean SAT at that time is lower than that at 5ka. A robust approach was taken by performing systematic sensitivity experiments with an AOVGCM to quantify the influence of glacial ice sheets on the amplitude of Arctic warming in comparison to that of orbital forcings. Our sensitivity experiments confirm that ice sheets could cancel the high summer insolation impact on Arctic climate and even cool down the climate not only in the Arctic but globally, suggesting a stronger influence of ice sheets on the climate response than summer insolation anomaly.
Our study demonstrates that there are times when remnant NH ice sheets exert a greater influence on climate than orbital forcing. This finding emphasizes the need for careful consideration of remnant ice sheets in addition to the orbital and GHG forcings in order to fully understand how interglacial climates and sea levels are shaped.
The climatic response in MIS11, which is as strong as in MIS5e, is often considered paradoxical despite its weak boreal summer insolation. The most significant difference between MIS5e and MIS11 is that during the summer insolation peaks (130ka and 410ka, respectively), there were still remnant glacial ice sheets in MIS5e at 130ka whereas glacial ice sheets melted completely in MIS11 at 410ka due to its anomalously long duration (Tzedakis et al., 2022). The effect of remnant ice sheets suggested in this study implies that warming did not develop despite strong summer insolation intensity in MIS5e. In MIS11, ice sheets retreated at around the first peak of climatic precession when the obliquity was rising, and had completely disappeared by 410ka at the second peak of climatic precession after obliquity maximum (Past Interglacials Working Group of PAGES, 2016; Tzedakis et al., 2022). As a result, the duration of the ice-free interglacial was very long due to the phase relation between precession and obliquity (Yin and Berger, 2015; Watanabe et al., 2023), which allowed feedbacks contributing to warming to work strongly at NH high latitudes. The long ice-free period over North America with strong summer insolation was favorable for the retreat of the Greenland ice sheet in MIS11 (Robinson et al, 2011; Crow et al., 2024). On the other hand, the duration of ice-free MIS5e was shorter than MIS11 because of the large eccentricity and the phase relation between climatic precession and obliquity (Yin and Berger, 2015). In fact, ice sheet simulations show that the NH ice sheets shrank quickly at the end of MIS6, followed by a short interglacial (MIS5e) and inception of the next glacial (Ganopolski and Calov, 2011; Abe-Ouchi et al., 2013; Obase et al., 2021; Ganopolski, 2024). This further implies that the persistence of the Greenland ice sheet today may be partly due to remnant glacial ice sheets present at the onset of the current interglacial, around 10ka.
On the other hand, when glacial ice sheets remain (10ka, 130ka, and 415ka), Arctic warming and sea ice melting are suppressed throughout the year. The periods with remnant glacial ice sheets do not show Arctic warming amplification when compared to global mean annual surface temperature changes. Even if the surface is heated by solar radiation in summer, the anomalous energy is transported toward glacial ice sheets and is not used effectively to amplify the warmth. In MIS5e, annual mean temperatures are much lower at 130ka than at 125ka despite the larger solar radiation contribution to warming at 130ka, and this temperature difference is too large to be explained by GHG forcing alone. Similarly, despite the stronger insolation at 10ka, which is near the peak in MIS1, the annual mean SAT at that time is lower than that at 5ka. A robust approach was taken by performing systematic sensitivity experiments with an AOVGCM to quantify the influence of glacial ice sheets on the amplitude of Arctic warming in comparison to that of orbital forcings. Our sensitivity experiments confirm that ice sheets could cancel the high summer insolation impact on Arctic climate and even cool down the climate not only in the Arctic but globally, suggesting a stronger influence of ice sheets on the climate response than summer insolation anomaly.
Our study demonstrates that there are times when remnant NH ice sheets exert a greater influence on climate than orbital forcing. This finding emphasizes the need for careful consideration of remnant ice sheets in addition to the orbital and GHG forcings in order to fully understand how interglacial climates and sea levels are shaped.
The climatic response in MIS11, which is as strong as in MIS5e, is often considered paradoxical despite its weak boreal summer insolation. The most significant difference between MIS5e and MIS11 is that during the summer insolation peaks (130ka and 410ka, respectively), there were still remnant glacial ice sheets in MIS5e at 130ka whereas glacial ice sheets melted completely in MIS11 at 410ka due to its anomalously long duration (Tzedakis et al., 2022). The effect of remnant ice sheets suggested in this study implies that warming did not develop despite strong summer insolation intensity in MIS5e. In MIS11, ice sheets retreated at around the first peak of climatic precession when the obliquity was rising, and had completely disappeared by 410ka at the second peak of climatic precession after obliquity maximum (Past Interglacials Working Group of PAGES, 2016; Tzedakis et al., 2022). As a result, the duration of the ice-free interglacial was very long due to the phase relation between precession and obliquity (Yin and Berger, 2015; Watanabe et al., 2023), which allowed feedbacks contributing to warming to work strongly at NH high latitudes. The long ice-free period over North America with strong summer insolation was favorable for the retreat of the Greenland ice sheet in MIS11 (Robinson et al, 2011; Crow et al., 2024). On the other hand, the duration of ice-free MIS5e was shorter than MIS11 because of the large eccentricity and the phase relation between climatic precession and obliquity (Yin and Berger, 2015). In fact, ice sheet simulations show that the NH ice sheets shrank quickly at the end of MIS6, followed by a short interglacial (MIS5e) and inception of the next glacial (Ganopolski and Calov, 2011; Abe-Ouchi et al., 2013; Obase et al., 2021; Ganopolski, 2024). This further implies that the persistence of the Greenland ice sheet today may be partly due to remnant glacial ice sheets present at the onset of the current interglacial, around 10ka.