Colliding Continents and Climate
Bitten by the bug of climate change, we have of late started worrying about its consequences. But if you ask a geologist he will just feel amused, because he knows that climatic vicissitudes are not recent happenings. They are age old. The climate has been changing through the ages, even from the eras when the two-legged creatures called humans were not there.
The atmosphere of the yore was worse with a carbon dioxide content much higher than that of today and the sea-surface temperatures were much higher. Yet in those inhospitable climates fauna and flora survived after some adaptations. It was sheer luck of the human beings that with their evolution the climate also became congenial, till about 2 million years ago when the Ice Age set in. Death is cold and as such life on the planet was struggling to survive. It was around 18000 years ago that once again the planet began to warm up and glaciers began to retreat. The warmth ushered life again.
Humans had evolved considerably by then and over a period of time they began sowing seeds to grow crops. Being endowed with three seasons, summer, rains and winter and a wide network of rivers, India has been an agrarian economy since ages. The agriculture is largely dependent on monsoon and so is the economy. A good monsoon brings cheers for the farmer and gives strength to the government while a bad monsoon makes both shaky and weak.
The Indian monsoon is peculiar says Anil K Gupta, a Professor with IIT Kharagpur, India. Having researched on the past of the Indian monsoon he has published couple of outstanding papers in the recent past. Taking help of the marine records of the upwelling currents and sediment deposition and the change in the continental vegetation from dominant C3 type to dominant C4 type plants Gupta has postulated a major intensification of the Indian monsoon some 10 to 8 million years ago. Here it is pertinent to mention that there are three carbon isotopes in nature, viz. 12C, 13C and 14C. Though all of them are the same element Carbon with six protons, but their neutron counts vary. This gives them different atomic mass of 12, 13 and 14. The atmospheric carbon dioxide contains less heavy C12 in bulk. A small fraction 1.1% of the carbon dioxide is made of C13-a slightly heavier variety. In addition in the atmosphere a heavier type of carbon dioxide C14 also occurs. Being radioactive it is often used for building the chronologies of past events-for example the past climates.
We all know that the terrestrial and marine flora perform photosynthesis. While absorbing carbon dioxide, they are able to discriminate between the lighter C12 carbon and heavier C13 carbon. Broadly plants older than 10 million years are of C3 type as they had the mechanism to absorb carbon dioxide even when it was much higher in concentration in the atmosphere. With the reduction in the carbon dioxide concentration in the atmosphere in later periods the C4 type of plants evolved.
It was the Miocene chapter (23.5 to 5.30 million years ago) of the history of the earth, when the C4 type of plants had started to evolve. This was a peculiar period, full of turmoil. The Indian Plate was actively colliding with the Tibetan Plate. The uplift of the Himalayas was already on and this was causing a new problem - the newly formed slopes were being eroded fast by the rivulets and rivers and huge quantities of sediments were being transported to the oceans.
On the other hand we know that the oceanic currents have a big hand in controlling the climates of a large region. Since the plate movement of the continents was rather active in this period, some of the seaways of oceanic circulation were getting choked. There was no option because the collision of continents blocked the circulation currents of the intervening sea and it had to be switched. Cyrus Karas and Dirk Nurnberg of Leibniz Institute of Marine Sciences, Kiel, Germany, A.K. Gupta of IIT Kharagpur, Ralf Tiedemann Alfred Wegener Institute for Polar and Marine Research, Germany and Kuppusamy Mohan of Zentrum fur Marine Umweltwissesschaften, Bremen, Germany found evidences of one such marine reorganization in the Indonesian Sea between 4 and 3 million years ago.
The condition of the oceans of those days were simulated in the lab and it suggested that this kind of situation would have triggered a switch in the source of waters feeding the Indonesian through flow into the Indian Ocean, from the warm salty waters of the South Pacific Ocean leading to cooling of relatively fresh waters of the North Pacific Ocean. This caused shallowing of the thermocline. In general the sea water temperature decreases from the surface to the deepest levels, except in high latitudes where the configuration is complex. Most of the ocean areas except the polar and sub-polar regions there are depths at which the rate of decrease of temperature is much larger than compared to above and below. It is this depth which is called thermocline of the ocean. Though climatologists, oceanographers and marine biologists often use this term, but is also common amongst the fishermen and submarine crews.
In the Indonesian seas the marine, microscopic fauna (planktonic foraminifera) indicated increase in fresh water as evidenced from the content of isotopic oxygen 18O and from the magnesium-calcium ratios of the shells of the foraminifera. It may be noted that 18O content is higher in fresh water and polar ice, compared to that of atmospheric oxygen or seawater. The ratios of magnesium and calcium revealed that subsurface waters had cooled by about 4 degrees centigrade between 3.5 and 2.95 million years ago. Prior to 3.5 million years to about 5.5 million years ago the sea water temperatures had remained almost stable.
The cooling of ocean in this region possibly led to the development of equatorial Pacific cold tongue suggests Karas and his colleagues in the prestigious Science Journal, Nature.
Thus colliding continents caused the uplift of the Himalayas on one hand, which not only changed the geography of the region but also changed the climate pattern completely. Had the Himalayas not been there, the monsoon, so vital for our survival, would not be there. Similarly the collision of continents as explained brought about a change in the sea currents and oxygen content of marine waters. The lowering of sea surface temperatures and shoaling of thermocline brought down the temperatures on the land too.
We are contemporarily bugged by the phobia of global warming. While the climate change is inevitable, but as projected by many, holding the industry and the humanity solely responsible for expediting climate change is incorrect. The climate is affected by tectonic activities within the depths of the earth and also by the orbital changes of the earth’s trajectory etc. Various evidences have suggested that climate is certainly affected by the Solar activity as well.
Anthropogenic intervention in the earth’s environment, excessive and indiscriminate felling of trees, concretization of urban complexes, pollution of rivers etc are many agents which add up to hasten the climate change phenomenon. Thus no doubt a curb on our wrong deeds is imperative, though it will only help in postponing the climate change.
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V. K. Joshi (Bijji)
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