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Tropical Rainforest will Survive the Future global warming: finds IIT Kharagpur study

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.

Position of Indian landmass 56 million years back with location of Vastan, Gujarat (Yellow asterisk)

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 Coal beds of Vastan that was once a dense tropical rain forest

“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.

Fossil remain of plant that grew during the super greenhouse earth

“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.

Pollens of 56 million year old rain forest trees

“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.

An extinct fly preserved in tree amber of Vastan (photo courtesy Dr. H.S. Rana, Birbal Sahni Institute of Palaeosciences, Lucknow)

“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.

Reference: The Temperature-Precipitation Duel and Tropical Greening during the Early Eocene Greenhouse Episode by Samanta A. et al., Global and Planetary Change, Available online 16 October 2024, 104603:

https://www.sciencedirect.com/science/article/pii/S0921818124002509

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By : Prof. Anindya Sarkar, Department of Geology and Geophysics, IIT Kharagpur
Email: sarkaranindya@hotmail.com

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In the quest for the hidden past

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Indian researchers have for the first time connected the decline of Harappan city Dholavira to the disappearance of a Himalayan snow-fed river which once flowed in the Rann of Kutch. They have connected the dots between the growth and decline of the Dholavira, the most spectacular and largest excavated Harappan city in India located in the Rann with this river which resembles the mythical Himalayan River Saraswati. The study has just been published online in prestigious Wiley Journal of Quaternary Science.

Studies so far made were indirect attempts to find out the river courses, its connection to climate and civilization in areas far away from these ancient cities. But this research team from IIT Kharagpur, Archaeological Survey of India, Deccan College PGRI Pune, Physical research laboratory, and Department of Culture, Gujarat, dated archaeological remains from all the stages and also inferred climate shifts through time which led to the rise and fall of the Harappan city.

“Our data suggest that prolific mangroves grew around the Rann and distributaries of Indus or other palaeochannels dumped water in the Rann near southern margin of Thar Desert. This is the first direct evidence of glacial fed rivers quite like the supposedly mythological Saraswati, in the vicinity of Rann” said IIT Kharagpur’s Prof. Anindya Sarkar who led the research.

Dr. Ravi Bhushan and Navin Juyal from PRL, Ahmedabad dated the carbonates from human bangles, fish otolith and molluscan shells by accelerator mass spectrometer and found that the site was occupied from ~5500 years back i.e. Pre-Harappan period to ~3800 years before present i.e. Late Harappan period. The Dholavirans were probably the original inhabitants in the region, had a fairly advanced level of culture even at its earliest stage. They built spectacular city and survived for nearly 1700 years by adopting water conservation suggested the researchers. The study indicates that the city expanded till 4400 years followed by an abrupt decline nearly ~4000 years back, onset of the newly proposed Meghalayan geological stage.

Researchers Dr. R.S. Bisht and Y.S. Rawat from the Archaeological Survey of India who originally excavated the site concluded this based on degeneration of architecture, craftsmanship, and material culture.

Dr. Arati Deshpande Mukherjee of Deccan College pointed out at the climate evidence coming from high resolution oxygen isotopes in snail shells Terebralia palustris which typically grow in mangroves and was a source of food for the Dholavirans.

The lead author of the paper and a PhD student at IIT Kharagpur Torsa Sengupta said, “the early to Mature Harappan snail isotopes suggested that the mangrove was fed by Glacier River debouching in the Rann of Kutch. However, during late Harappan period the meltwater contribution and seasonality reduced coinciding with the fall of Dholavira.”

“Though the Dholavirans adopted excellent water conservation strategy by building dams, reservoirs and pipelines, but were pushed to the limit by a catastrophic Meghalayan mega-drought collapsing the city. Indeed Dholavira presents a classic case for understanding how climate change can increase future drought risk as predicted by the IPCC working group” added Prof. Anindya Sarkar.

The research indicates that the collapse of Harappan Dholavira was near-synchronous to the decline at all the Harappan sites in India as well as societal collapse of Mesopotamia, Greece, China and the Old Kingdom of Egypt.

A New Page in India’s History

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A breakthrough discovery by a study led by IIT Kharagpur has made the colourful Rann Festival of Gujarat a little more vibrant. The researchers have uncovered the earliest traces of the Old Iron Age which is older than 3000 years in the deep stretches of the Rann of Kutch and the Thar Desert. Most importantly the study reaffirmed the theories of human migration from the west to the east induced by climate change.

The region of Gujarat has been a part of many tales from the times of our mytho-history through various historical periods until modern times. Even the Harappan period can be traced to a few rocky islands in the Kutch region of Gujarat. However, till now the Rann which is a prominent geological feature of Gujarat was devoid of any sign of continued human settlement throughout the Early Iron Age to Early Historic Age (~3100 – 2300 years). The lack of evidence even led archaeologists term this period as ‘Dark Age of Gujarat’.

The recent explorations in the coastal settlement of Karim Shahi region of the Rann near Indo-Pak border, led by Prof. Anindya Sarkar from the Dept. of Geology and Geophysics at IIT Kharagpur, however, have unraveled the secrets behind this curtain of silence. The team has found the earliest evidence of human habitation dating back to 3000 years. They also found evidence of Historic to Medieval (~1500–900 years old) human settlement at Vigakot in the Thar Desert.

“We were conducting geological investigations for finding out the past climate change during and after the collapse of Harappan civilization funded by the INFOSYS foundation and IIT Kharagpur. During our explorations we came across numerous archaeological artefacts strewn over the surface of Karim Shahi and Vigakot,” said Prof. Sarkar.

The most intriguing finding is how the human habitation thrived in such water-deprived inhospitable terrain and survived from Iron Age to Medieval period although major Harappan cities were abandoned by that time, he opined. The researchers also did a total station survey to determine the landscape.

“What seems to be an arid landscape today have indications of an active river system and some amount of rainfall during that period as evidenced from the analysis of sediments, botanical remains like pollens and isotopes of oxygen in fossil molluscan shells,” said collaborator Dr. Navin Juyal from Physical Research Laboratory, Ahmedabad.

The researchers also referred to the historical travelogue of Al Beruni of 1030 AD which mentioned the presence of rivers in Kutch.

“Our study suggests that the Rann of Kutch and part of Thar desert were still a hospitable terrain for the sustenance of human settlements from the Early Iron Age till at least medieval times which led to the survival of the civilization under such climate threat situation following the Harappan decline,” he said.

It is long known that from Mature to Late Harappan period (5200 to 3300 years) the number of human settlements continuously increased from the Indus River valley in the west to the Ghaggar-Hakra in the east. This migration following the collapse of the old Indus Valley (Harappan) civilization has been attributed to the decline of monsoon or major droughts by many scientists. But what happened to the people after such collapse? Little is known about the people after the post-urban Harappan period. Some archaeologists believe that there was no demographic collapse as such. Rather populations persisted in smaller less complex settlements dispersed from the original river valleys of the Indus and Ghaggar-Hakra to more distant areas of the Ganga-Yamuna interfluves or Gujarat and Rajasthan until 3000 years before the present time. In northern India new kind of civilization rose afterward, namely, the Iron Age (or Painted Gray ware) between ~3000 and 2500 years before the present time that was followed by the Kushan period (~1900–1500 years).

“This was indeed a very critical transition, wherein human migration, as suggested by our findings, was far more expansive than thought before. We suspect that the gradual shift of Intertropical Convergence Zone, the main driver of monsoon from west to east over the last seven thousand years, forced people to migrate for greener pastures,” remarked Prof. Sarkar.

An earlier study by Prof. Sarkar on Haryana’s Bhirrana region had shown human migration from west to east due to the weakening of the monsoon. In a way, this created large climate refugees who took refugia were still some little rainfall was available.

“The United Nations framework convention on climate change and high commissioner for refugees in its report warned about such climate refugees due to impending climate change. If it could have happened in the past it will happen in the future too” reminded Prof. Sarkar.

Apart from the climatic conditions and sustenance of the settlements, the researchers have been successful in stitching a critical section of India’s Iron Age history which lay buried under the deep stretches of the Rann. The recovery of artefacts like pitcher, jars and bull figurines and also numerous animal remains like bones, teeth etc. have helped in reconstructing the social subsistence pattern of the region during the study period confirmed co-researcher Dr. Arati Despande Mukherjee from Deccan College PGRI Pune. The earliest evidence of Iron Age found in Gujarat till now was 2500 years old which has now been pushed back by several centuries in antiquity. At Motichher, a place close to Karim Shahi, iron objects, nuggets, and slags have been found and which would need further investigation. The researchers acknowledged the Indian Army for facilitating the explorations. The areas are so remote and close to an international border that no scientific investigation could have been carried out without the permission of the Indian army.

Talking about a probable occupation of those people, Prof. Sarkar remarked, “both Karim Shahi and Vigakot probably acted as trade centers during this time. In fact, at Vigakot we found 1100 years old Chinese Qingbai porcelain probably manufactured in Guangdong province of south China and Sgraffiato potteries of 10th century Persia suggesting it to be a part of long-distance trade between West Asia and China”.

Prof. M.G. Thakkar from Kutch University and a collaborator emphasized the fact that the multidisciplinary study has proven the near-cultural continuity after the Harappans which the experts till now only hypothesized. He also harped on the point that this finding is going to bring Kutch under international limelight.

The findings have been published online in the prestigious Elsevier journal ‘Archaeological Research in Asia’. Download Paper

Graphic Credit: Suman Sutradhar