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

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.