Background

About 56 million years ago, Earth experienced an extreme warming period with temperatures rising by approximately 5 to 8 degrees Celsius due to rapid carbon emissions. Academia refers to this period as the Paleocene-Eocene Thermal Maximum (PETM) and studies it as the most analogous past case to the climate change currently faced by modern humans. A hypothesis gained traction suggesting that the high concentration of atmospheric carbon dioxide at that time acted as a fertilizer, promoting plant growth and making forests lush. However, the incomplete nature of fossil records has made it challenging to quantitatively prove the density of ancient forests. Most studies have only compared the distribution of discovered plant species. To understand the real impact of rising temperatures and water scarcity on ancient plants, a new analytical method to restore canopy density was urgently needed.

Key Findings

Led by Dr. Regan Dunn, a paleobotanist at the La Brea Tar Pits Museum, the research team derived answers from the fossils of ancient plant leaves. The team focused on the changes in the epidermal cell morphology of leaf fossils collected from the Hanna Basin in Wyoming. Leaves receiving more sunlight tend to have round and symmetrical epidermal cells, whereas those growing in the shade develop elongated and narrow cells to capture more light. The team devised a method to calculate the Leaf Area Index (LAI), representing the canopy density of ancient forests, by precisely analyzing the ratio of cell width to length. The results derived from this analytical method clearly demonstrated the harshness of ancient climate change. As the Paleocene-Eocene Thermal Maximum began, the canopy density of forests in the Hanna Basin area dropped sharply by about 60% compared to before. With temperatures rising by approximately 5 to 9 degrees Celsius, the once-lush forests rapidly dried up and transformed into sparse grasslands. This forest collapse did not remain a temporary phenomenon but persisted for more than 100,000 years. The extreme drought and heat stress accompanying the warming completely offset the plant growth promotion effects of increased carbon dioxide concentration.

Significance and Outlook

This study holds significant academic value in elucidating how temperature rise-induced water supply imbalances reduce the carbon absorption efficiency of plants. It directly refutes the optimistic expectation that increased carbon dioxide concentration will lead to more vigorous plant growth and a greener Earth. The current rate of carbon emissions by humans is about ten times faster than that of the ancient thermal maximum. Even if atmospheric carbon levels surge, if extreme heat and drought accompany it, forests may lose their ability to store carbon and are likely to become degraded. The collapse of forests could create a feedback loop by reducing carbon absorption capacity and accelerating warming. However, this analytical method is limited to fossil sites with well-preserved cuticles. The research team plans to conduct follow-up studies to verify whether ancient tropical forests and high-latitude forests also experienced similar levels of canopy collapse.