As global temperatures continue to rise and CO2 levels reach unprecedented heights, scientists are increasingly focused on understanding past climate events that could inform future scenarios.
A recent study published in the journal Global and Planetary Change by Arpita Samanta, assistant professor at Asutosh College, Calcutta and the lead author of the paper, investigates how tropical rainforests responded to significant climatic changes during the Early Eocene period around 51-56 million years ago.
This study by professor Anindya Sarkar, the lead researcher of IIT Kharagpur, and her team, published on October 16, 2024 is essential for understanding how today’s tropical ecosystems might cope with ongoing climate change and highlights the critical link between temperature, precipitation, and biodiversity.
As we approach levels of CO2 in the atmosphere that could exceed 1,000 parts per million by 2100, it is essential to learn how tropical rainforests, the ‘lungs of our planet’ responded to past warm climates.
The early Eocene was marked by extremely high levels of atmospheric carbon dioxide, estimated to be between 1,200 and 2,500 parts per million. This period experienced global temperatures around 14 degree Celsius higher than what we see today, creating conditions notably warmer than our current climate.
Understanding these ancient warm periods can provide insight into how current ecosystems may respond as we face increasing greenhouse gas emissions.
To conduct this research, scientists gathered data from two locations in India; the Vastan and Valia lignite mines in Gujarat. These areas are part of Cambay Shale formation, which dates back to the Early Eocene.
The researchers used geological analysis, where they studied the layers of rocks and sediments, especially gray shales and lignites to identify the time period of the sediments and the climate events that occurred.
They also conducted isotopic analysis, where they examined the hydrogen and oxygen isotopes from clay minerals found in the sediments. This helped them study the past temperatures and rainfall conditions.
The researchers also did pollen analysis, where the team looked at fossil pollen to identify the types of plants that thrived during this time and help them understand the diversity and abundance of tropical rainforest species.
The study was carried out for a long time and utilised over 800 samples collected at various depth intervals from the mine sites to get a comprehensive look at climatic conditions during the early Eocene.
The study found that average land surface temperature was similar to today’s, despite significant changes in temperature and rainfall during the early Eocene.
Increased rainfall during the Paleocene-Eocene Thermal Maximum (PETM) helped stabilise temperatures, supporting the growth of tropical rainforests. While, during the Eocene Thermal Maximum 2 (ETM2), the increase in rainfall was not as significant, leading to less stable climate conditions and the presence of more dry loving plant species.
The Paleocene-Eocene Thermal Maximum (PETM) and the Eocene Thermal Maximum 2 (ETM2) are two significant periods in Earth’s climatic history, characterised by rapid warming and major shifts in climate and ecosystems.
PETM was warmer, humid conditions favourable to tropical rainforest expansion, driven by significant increases in rainfall and temperature stabilisation while ETM2, still a warming event, was marked by a milder increase in rainfall, resulting in less stability and a shift toward drier ecosystems.
The analysis revealed a high diversity of tropical rainforest plants during this period, with significant expansions during warmer, wetter phases like the PETM.
The researchers identified 256 taxa from various families such as Araceae, Arecaceae (palms), and Bombacaceae, indicating a rich variety of plant life.
The study indicated that plant diversity diminished slightly during the ETM2 due to less favourable climatic conditions but remained robust overall, pointing to the resilience of tropical rainforests during periods of climate change.
While the study confirmed that regions experienced swings in temperature and rainfall amounts, the data indicated a long-term cooling trend in temperatures post-PETM, while rainfall patterns during the Early Eocene were inconsistent, reflecting on complex climate dynamics.
There have been similar studies that investigate the climate dynamics, vegetation response, and ecological changes during significant warming periods in Earth's geological history.
In a paper titled Floral response to rapid warming in the earliest Eocene published in 2001 by SL Wing and GJ Harrington where the researchers investigated plant community changes in response to warming at the onset of the Eocene, with an emphasis on how temperature impacted flowering plant diversity. The study documented the increase in warm loving plant species and recognition of floral assemblages during global warming Temperature sensitivity of the terrestrial biosphere during the early Eocene.
Another paper titled Global warming and neotropical rainforests: a historical perspective by Carlos Jaramillo and Andres Cardenas published in 2013 assessed how tropical rainforest biodiversity changed over time, particularly during the early Eocene, in response to extreme greenhouse conditions. They concluded that high CO2 levels and high temperatures typically promote biodiversity, demonstrating resilience in tropical ecosystems.
By examining how tropical rainforests thrived in a high CO2 world, researchers hope to uncover strategies that could help preserve biodiversity in the face of modern climate challenges.
The need for more comprehensive studies remains critical to drawing firmer conclusions about the interplay between climate and plant diversity during pivotal periods in earth’s history.