Research press release





今回、Samuel Munozたちの研究グループは、ミシシッピ川の近くにある湖で採取された樹木と堆積物コアを観察して、これまでより長期間に及ぶ16世紀初頭以後の洪水歴を組み立てた。洪水によって浸水したカシの木は、異常な成長を示すため、年輪に洪水の年表が残されている。一方、ミシシッピ川が氾濫すると上流から運ばれてきた堆積物が氾濫原の湖に流れ込み、それぞれの氾濫に関する詳細な記録が得られる。Munozたちは、こうした研究結果を過去の気候記録と比較した。


Management of the Mississippi River could be making floods worse and more frequent, with the current overflow risk at a level unprecedented in the last half-century, reports a paper published in this week’s Nature.

The 2011 flooding of the Mississippi led to US$3.2 billion in agricultural losses and infrastructure damage - this notwithstanding 150 years of river engineering in the form of channel straightening, man-made embankments and overflows. Whether these measures have the desired effects is challenging to assess, however, because stream flow records dating back to 1897 cannot capture the river’s natural variability before human interference began to take place.

Samuel Munoz and colleagues assembled a longer flooding history - reaching back to the early sixteenth century - by looking at both trees and sediment cores taken from nearby lakes. When inundated by floodwaters, oak trees grow abnormally, preserving a timeline of floods in their growth rings. When the Mississippi overflows, meanwhile, it dumps carried sediments into lakes on the floodplain, producing a detailed register of each deluge. The authors compared these with historical climate records.

They found that the Mississippi’s flood magnitude has increased by about 20 per cent over the last 500 years, with the highest levels in the last century. Although the river does slowly rise and fall with North Atlantic climate fluctuations, the authors found that this can only account for a quarter of the recent increase in flood intensity. The remaining 75 per cent of the heightened flood hazard, they suggest, comes from human changes to the Mississippi and its basin.

doi: 10.1038/nature26145

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