Research press release


Nature Communications

Earth sciences: Plants may have reduced CO2 levels before forests evolved

前期デボン紀と中期デボン紀(4億1000万年前から3億8000万年前まで)の大気中CO2濃度は、これまで考えられていたよりもはるかに低かったという可能性が浮上した。この新たな知見は、最も初期の維管束植物が、森林の出現よりずっと前の時代に大気中CO2濃度を大幅に低下させたことを示唆している。こうした初期のCO2濃度の低下は、その時期の著しい地球寒冷化と氷河作用につながった可能性がある。これらの知見について報告する論文が、Nature Communicationsに掲載される。


今回、Tais Dahlたちは、ヒカゲノカズラ類の現生の子孫種、植物化石と地球化学的データの解析を行って、4億1000万年前から3億8000万年前までの大気中CO2濃度が、現在のレベルと比べて、およそ1.5倍とそれほど高くはなかったことを明らかにした。この点について、以前の研究では10倍と推定されていた。Dahlたちは、古気候と地球システムのモデル化を行って、最も初期の陸上植物による大気中CO2濃度の上昇とそれと同時に起こった大気の酸素化が、気候の著しい寒冷化と部分氷河作用をもたらすために十分であり、地質学的証拠と一致することを明らかにした。


Atmospheric CO2 levels in the Early and Middle Devonian period, between 410–380 million years ago, may have been much lower than previously thought, according to a study published in Nature Communications. The findings suggest that the earliest vascular plants substantially reduced CO2 levels long before the evolution of forests. This early CO2 decline may have led to significant global cooling and glaciation during this period.

The emergence of forests about 385 million years ago has previously been linked to a decrease in atmospheric CO2 levels and related global climatic cooling. The evolution of trees with deep roots was thought to have increased continental weathering and led to the sequestration of carbon in the oceans. However, geochemical evidence suggests that CO2 levels may have been much lower millions of years before the emergence of forests. The significance of biology in the carbon cycle is debated, and large uncertainties involved in the modelling of these processes remain.

Tais Dahl and colleagues analysed modern descendants of club mosses, plant fossils, and geochemical data and found that 410–380 million years ago, CO2 levels were only modestly elevated compared to the present day. They found that CO2 levels were only about 1.5 times greater than current levels, compared to 10 times greater as previously assumed. Using palaeoclimate and Earth systems modelling, they found that CO2 decline and simultaneous O2 increase, even by the earliest land plants, was enough to have led to significant climatic cooling and partial glaciation, consistent with geological evidence.

The authors conclude that the evolution of trees with deep roots did not dramatically enhance CO2 removal, with the earlier shallow-rooted ecosystems simultaneously causing abrupt atmospheric oxygenation and climatic cooling long before the rise of forests.

doi: 10.1038/s41467-022-35085-9


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