But question is what causes global warming? Mere industrialization cannot warm the entire weather system. Similarly industrialization might raise some smog and make the sunlight dull, but it cannot cause such a haze which would disrupt the solar heat to cool the earth so much that it is wrapped in an ice sheet. Then who is behind all such phenomenon? To get answer, just read on:

A bolt from the blue and the dinosaurs perished some 65 million years ago (Ma). Likewise there have been four major extinctions in the past around 251 Ma, 256 Ma, and 204 Ma and of course the latest one around 65 Ma that changed the course of the evolutionary history of the present day life. Similarly we know that our earth was often shrouded under a cover of ice or it had become so hot that Svante Arrhenius (1896), a Swedish Nobel Laureate called it a "Hot House earth". Who was behind all these upheavals? Was it due to merely earth bound processes as commonly thought or was it due to some 'external hand'?

Researches since 1980 have put forth facts which point towards 'some galactic forces' of Milky Way, our home galaxy were behind the major turmoil like recurring mass extinctions of both terrestrial and marine species, global ice ages, major tectonic impacts, periodic boosts in fossil diversity, formation of large igneous provinces (LIP), climatic changes causing perturbations to global climate cycle leading to carbon isotope excursions (CIE), says, octogenarian scientist and free lance science writer A.V. Sankaran, in the September 2008 issue of Current Science.

What has the galaxy to do with the turmoil on the Planet? Well the answer to 'external hand' appears to lie in the galaxy! Our home galaxy, the Milky Way has four major arms arranged in a spiral, viz. Cygnus-Norma, Crux-Scutum, Carina-Sagittarius and Perseus. Before understanding the 'might' of the 'external hand' one must know the dimensions of our galaxy. It is 100,000 light years (l y) (One l y=9460730472580.0 km) in diameter with an average thickness of 1000 l y. Compared to the galaxy our Solar system is a minuscule located in a smaller arm Orion between Perseus and Sagittarius arms.

The galactic arms rotate around the galactic centre and so do the Sun, albeit at a slightly higher speed and completes one rotation in about 250 million years (m. y.). Besides, the solar system is also known to oscillate up and down the galactic plane; each of these motions and locations relative to the galactic centre correlate with some of the geological events in the earth's history says Sankaran.

The scientists synthesized the so far available galactic, geological, biological and atmospheric data and the conclusion was amazing! It was found that the geologic events occur at an approximate interval of 176 m. y., which is incidentally the time taken by the solar system to travel from one arm of the galaxy to the same position on the other. 

The Solar system of which our planet, the Earth is a part makes an orbit of all the four galactic arms in 250 m. y. While the solar system traverses across a galactic arm, there is a marked turbulence on our planet in the form of catastrophic geologic events; whereas during the passage of the solar system through the inter-arm space there is a 'quieter phase', free from cosmic ray flux (CRF) and geomagnetic reversals. The quieter phase called as Superchron, lasts for about 20 m. y. or more. Sankaran says that three such superchrons in the earth's geologic past are now well established. The latest was during the Cretaceous period of the earth's history lasting for 36 m. y. between 120 and 84 Ma. There were two other older superchrons, one lasting for 48 m. y. between 312 and 264 Ma and for 22 m. y. between 485 and 463 Ma. 

The 'quieter phase' is not that quiet as understood from the term. It is a period of volcanic eruptions on a large scale, leading to large igneous provinces (LIPs). The Panjal igneous province (250-160 Ma) and Deccan igneous province 60 Ma are the two examples of such LIPs from India. The former is present as the Pir Panjal range that separates the Kashmir valley from Jammu and the later forms the Deccan Plateau. In addition there are many other LIPs, world over through different geological periods, they are: Emeishan (China) (250-260 Ma); Central Atlantic, Karoo-Ferrar (South Africa) (180-200 Ma); Parana-Etendeka (Brazil and Namibia), Ontong-Java (Papua New Guinea) (120-130 Ma) and Caribbean (90 Ma).

The superchrons are also catalysts to great biological diversity, because they offer ideal conditions for the proliferation of life. For example, the Cambrian explosion of life that took place as early as 542 Ma had occurred during the inter-arm passage after the Solar system crossed Perseus arm. Amongst the four mass extinctions of the geologic past the latest, extinction of the dinosaurs took place during the transit of the Sun across Carina-Sagittarius arm of the galaxy.

There are other impacts, like geomagnetic reversals and quasi-periodic gravitational perturbations on the Oort cloud too faced by the earth during the inter-arm crossing of the solar system. The Oort cloud is a hypothetical spherical cosmic cloud lying about a light year away from the Sun, responsible for earth impacting materials. There are evidences to suggest that an asteroid hit the earth some 65 million years ago which was responsible for wiping off the dinosaurs.

Similarly the galactic triggers like CRF and geomagnetic reversals are thought to be powerful enough to modulate the earth's climate. It is also believed that an increased CRF in the vicinity of the earth promotes extensive cloud formation and consequent reduction in global temperatures, leading to Ice ages. The presence of certain isotopes of beryllium, carbon and oxygen found only in the space are found in the sediments of the past ice ages gives credence to the thought that there is an 'external hand in triggering the ice ages!

It is apparent that the galactic forces are much more powerful than thought and need to be carefully studied. As far as the galactic influence on mankind is concerned, Dr Sankaran says, "If the galactic forces had swayed various earth processes in past geological periods, mankind as well as other life as part of the plant can be affected, if not in one's life time, but certainly in geological time scales."

Only time will tell who is going to be the next victim of the galactic forces!