By Ashok Sahni
For those worried about climate and how it is going to change the world in the next 50 or 100 years, it may come as a surprise to note the number of times the Earth has changed face—from a snowball to a sizzling hothouse since its origins some 4.6 billion years (By) ago.
The Earth, unlike other solar planets, has always been in a delicate equilibrium with the sun–its main energy provider—which is just the right distance away for water on Earth to exist in all three interchangeable states: vapour, liquid and ice. As the energy emanated from the sun fluctuates through time, the Earth has till now suffered about six ice ages. The Ice Age of around 290 million years (Ma) was extensive and intercontinental in extent with remnants in India—in Rajasthan and Odisha. However, for the last 400 Ma, by and large, the Earth has sizzled as a greenhouse with elevated sea levels and no polar ice representing its normal state.
Evolution of life in the face of catastrophe
Around 3.5 By ago, life originated and lowly forms of life such as fungi, bacteria and algae evolved and survived; but complex multicellular life required more drastic measures. About 2.5 to 2.2 By ago, an event took place that changed the way the earth and life on it had been evolving. This was the origin of photosynthesis by lowly plants including blue-green algae. As our atmosphere got enriched in oxygen, complex metabolisms became possible and CO2 levels drastically reduced. Evolution created complex organisms that could walk, swim, fly and burrow, some 600 Ma ago. Life diversified, creatures conquered the land and grew tall withstanding the force of gravity but then a catastrophe struck.
About 250 Ma ago, almost 96% of all life on Earth living in the sea, land and in the air suddenly became extinct. This extinction was attributed to an event popularly known as The Great Dying that had to do with a sudden drop in atmospheric oxygen levels. How? We are not quite sure at present.
The 4% of life that survived this great extinction event limped on, slowly growing in numbers and diversity. The dinosaurs evolved just after this great extinction and some grew to sizes that tested the limits of their bodies on land and against the force of gravity. However, a major catastrophe struck yet again some 65 million years back. A giant asteroid hit Mexico. Together with the lava flows erupting in Deccan India, this caused the extinction of 65 % of all forms of life. Again, the survivors representing the remaining 35 %, hung on tenaciously and we humans are one of these.
The advent of modern day diversification
After 65 Ma, life appeared and it was a bit more modern in aspect. Recent researches have shown that around 56 to 54 Ma ago, India appeared to be a virtual Garden of Eden—witnessing the evolution of primates, primitive horse-like creatures, and a multitude of two-toed mammals. They were somewhat like the modern-day sheep and deer, rabbits, and even parrot-like birds. This extraordinary radiation and diversification are believed to have been triggered by a runaway greenhouse caused by a thermal event with greatly elevated temperatures, high sea levels and absence of polar ice. Flora and fauna blossomed during this time and this serves as a red flag to all those who are scared of global warming.
Living in the current Ice Age
Our current Ice Age consists of cycles of Glacial and intervening Interglacial episodes. The last glacial event (Last Glacial Maxima, LGM) occurred around 24000 BCE. At its peak, polar ice sheets were widespread, and as the water turned to ice, sea levels dropped to a remarkable 125 meters. The LGM gradually petered out to the present Interglacial which, for the last 10,000 years, has supported human civilisation.
If events of the immediate geologic past are any indication, the next Glacial should have been just around the corner. However, anthropogenic activities may have disrupted earth cycles established during the last 4,00,000 years and may have prolonged the onset of the next Glacial by several thousand years. This is something positive for us, humans, as Glacials are cold, nasty, life-threatening, wind-driven dust bowls.
Dr Ashok Sahni is a Professor Emeritus at the Department of Geology, Panjab University.
Featured image credits: Flickr
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