Kharagpur, India: The tropical rain forests (TRF) like Amazon or Western Ghats are considered as “the lungs of the planet”, contain about 200-300 petagram (10¹⁵) or approx. 1/3^rd of the total atmospheric carbon and plays a crucial role in modulating the global carbon cycle, biodiversity and hydrological cycle. The 2023 AR6 report by the Intergovernmental Panel on Climate Change (IPCC) warns if the CO₂ emission and global warming continues unabated the TRF community may altogether collapse much before the end of this century, and will drive a global catastrophe affecting nearly 800 million people worldwide. However, scientists are sharply divided on this issue. Predictions made through climate-vegetation models suggest that just a 2˚C increase in mean annual temperature could increase the respiration rates and push trees to their photosynthetic threshold causing their mortality. This will be compounded by increased extreme events like variability in rainfall, droughts and wildfires. Yet others think that over longer time scale plants will adapt to these changes by changing their diversities or invading into favourable climate zones. This has indeed been found in the Andes where low elevation warmer region trees are invading into the colder higher altitude region. In the Himalayas the Rhododendrons blooming time is slowly changing. The only way to test these contradicting predictions is to study the evolutionary record of the TRF plant community and the climate in the past when the earth went through natural warming phase due to high CO₂ emission.

A team of scientists from IIT Kharagpur, Calcutta University and University of Western Ontario have studied detail records of TRF in sediments from Vastan coal mines of Gujarat deposited in coastal lagoons around 56 million years back.  India was a tropical island then surrounded by oceans and Himalayas were yet to form. The period is known as Palaeocene-Eocene Thermal Maximum (PETM) when global carbon dioxide rose to >1000 ppmv, an abnormally high level that the future global warming might reach. The PETM is the most rapid global warming event known in Earth’s history. An amount of carbon approximately equal to the total modern fossil fuel reservoir was released in the ocean-atmosphere system due to release of carbon stored in sea-floor sediments. The coal layers in Vastan are nothing but a spectacularly fossilized tropical rain forest containing huge amount of plant and pollen remains as well as variety of mammals and insects those lived in these forests. In fact, world’s earliest mammals evolved here due to this climate shift at PETM.

“The study took several years of field and laboratory investigation. We had to date the sediments to confirm its PETM age and collected samples at centimeter intervals, analyzed the pollens to understand how the TRF community evolved in response to such extreme global warming. To understand how the climate changed during this super-greenhouse globe we analyzed isotopes of carbon in the plant organic matter and developed special techniques of measuring isotopes of oxygen and hydrogen in micron size clay mineral kaolinite that precipitated in these lagoonal water. The climate was also monitored by analyzing oxygen isotopes in fossil teeth of small horse-like ungulate mammals those once roamed in these forests,” said Prof. Anindya Sarkar, the lead researcher of IIT Kharagpur. The study has just been published online in prestigious El Sevier Journal Global and Planetary Change.

“Pollens are widely dispersed by air and water, resistant to decay and are invaluable indicators for reconstructing ancient biomes. Evidence of huge diversity (70 families and 256 taxa) of dense tropical rainforest trees like Sal, Mahogany, Palm, a variety of evergreen and mangrove plants are preserved in the sediment and coal beds of Vastan. No wonder that such rain forest harbored diverse animals including ancestors of early horses, snakes and insects,” said Prof. Subir K Bera of Calcutta University, an expert in ancient plants and co-author of the paper.

“We found a large anomaly in carbon isotopes exactly at 56 million year. This was such a characteristic signal for a super greenhouse globe with very high atmospheric CO₂. The hydrogen and oxygen isotope compositions in clays depend on land temperature and amount of rainfall and act as snapshots of past climate. Likewise, the isotopes in fossil teeth record the history of what water animals drank. As the CO₂ began to increase, the land became abnormally hot >40^oC. But to our surprise we found that the temperature came down to ~30^oC during the later period, almost similar to today. The rainforest not only survived but also diversified during and after this global warming phase,” said Arpita Samanta, a former PhD student at IIT Kharagpur currently Assistant Professor at Asutosh College, Calcutta and the lead author of the paper.

            “What helped the rainforest’s survival? We critically looked at the rainfall pattern and found that the warming intensified the rainfall and that possibly brought down the temperature. We call it rainfall buffered temperature. The increased rainfall and lowered temperature sustained these ancient rainforests of western India,” said Dr. Melinda K Bera, an isotope expert who painstakingly developed the novel clay based thermometer and a co-author.

            “The Vastan record is unique in many ways. This is the first record of how tropical rainforest responded to elevated levels of CO₂ and global warming in the past. The only other available record is from Neotropical South America. Vastan data shows that globally tropical plant community responded in tandem. Second, the increased rainfall during this super greenhouse earth exactly supports the IPCC prediction of intensified precipitation regime in case of a future extreme 4^oC warming of the planet. Vastan is an ancient analogue of what our future greenhouse earth can be. Fossil fuel emission has increased the CO₂ from pre-industrial level of 280 ppm to ~421 ppm in 2024. Climate models suggest that a doubling of CO₂ will intensify the atmospheric circulation and consequently the rainfall. Nature already did experiment in the past that has lessons for us to learn. Many experts believe that the climate change due to such fast rate of global warming is now irreversible and collapse of rainforest or ocean biosphere is just imminent. The Vastan record shows that there may be some hope. At least the rain forest may take the heat stress and survive,” added Prof. Sarkar.