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極域増幅における大気海洋陸域相互作用 大石龍太

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極域増幅における大気海洋陸域相互作用

大石龍太1,2、阿部彩子2,3

1国立極地研究所

2東大大気海洋研

3

JAMSTEC

Atmosphere-Ocean-Land interction in the Arctic Amplification

Ryouta O'ishi

1,2

, Ayako Abe-Ouchi

2,3

1

National Institute of Polar Research

2

AORI, the University of Tokyo

3

JAMSTEC

When we quaitify the warming in the Arctic, centenial scale feedback from vegetation change such as shift of forest line must be considered as well as snow albedo feedback and sea ice feedback. In the present study, we quantified vegetation feedback o n climate in the mid-Holocene (ca. 6,000 yeasrs ago) and the last interglacial (ca. 125,000 years ago) by using a vegetation cou pled atmosphere and ocean general circulation model MIROC-LPJ. The result indicates that the stronger summer insolation ca used by the Earth's orbit induces the northward expansion of boreal forest and thus decrease of surface albedo amplifies arctic warming. This warming is not only due to the increase of forest itself due to summer insolation change, but also the ocean, sea ice and snow albedo feedback in autmn, winter and spring, respectively, induced by the forest expansion. This result also show s that the response to the increase of summer insolation affects throughout seasons by individual feedback mechanisms. We als o quantified the vegetation feedback due to the increase of atmosphere CO2. Vegetation change induced by the increase of atm ospheric CO2 amplifies global warming and the Arctic warming. However, compared with the orbit induced experiments, the past warm period is not considered to be an analogy of high atmospheric CO2 environment.

極域における温暖化を定量化する際には、積雪アルベドフィードバック、海氷フィードバックとともに、数百年の タイムスケールでは森林帯の移動に代表される植生分布の変化によるフィードバックを考慮する必要がある。本研 究では、大気海洋植生結合

GCM MIROC-LPJ

を用いて約

6,000

年前の完新世中期と、約

125,000

年前の最終間氷期 における植生フィードバックを定量化した。その結果、軌道要素の違いによる夏の日射が原因で、高緯度の森林が 北上し、アルベドが低下することによって温暖化が強化されることが示された。また植生の増加によって、秋冬の 海洋海氷フィードバックと春先の積雪フィードバックがが強化されることが明かになった。すなわち、夏の日射が 増加することによるレスポンスシグナルが植生フィードバックに仲介されて季節をまたいで伝播することが示され た。同様に、大気二酸化炭素増加時の植生フィードバックを定量化すると、やはり植生フィードバックによって温 暖化は強化された。軌道要素由来の温暖化の場合との比較を行った結果、両者ともに植生フィードバックは極域増 幅を強化するが、軌道要素由来の温暖な時代は大気二酸化炭素増加時の単純なアナロジーとはならないと考えられ る。

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