The temperature recorded in the month of July 2016 was the highest since recording of global temperature began in the year 1880. Summer season has become increasingly harsh, especially in the tropical countries. This is primarily due to increase in Green House Gases (GHGs) in the atmosphere around the globe. The average global temperature may increase by 4 C by the end of the 21st century. The international community has agreed to reduce emission of GHGs and signed the Paris Agreement to reduce global warming and limit it to 0.8 C.
What are the possibilities of earth cooling down spontaneously at some point of time in the futureIJ Did such global warming happen earlier in the geological ages followed by spontaneous cooling and return of the Ice Age in the Pleistocene epochIJ
We are now living in the Holocene epoch, an interglacial warm period, which started 11,700 years ago after the retreat of Pleistocene glaciers (Ice Age). According to Paleoclimatic data, the glacial periods alternate with interglacial warm periods, which are cyclic in nature.
Milutin Milankovitch (1930), a Serbian astronomer and mathematician, proposed a hypothesis based on planetary movement, which suggested that the cycle of glacial and interglacial periods are determined by the cyclic nature of some of theorbital features of the earthwhile it circumnavigates around the sun. These are its eccentricity, axial tilt and precession.
It is generally known that earth moves around the sun in an elliptical orbit. According to Milankovitch the orbital shape fluctuates between less or more elliptic (0-5 percent ellipticity) in a cycle of 100,000 years. Such fluctuations alter the distance of the earth from the sun and as a result increase or reduce solar radiation received on the surface of the earth in different seasons. This causes significant changes in earth’s climate and glacial regime. At present orbital-eccentricity is almost at its minimum.
It is also known that the earth’s axial tilt is at 23.5 degrees, which largely accounts for our seasons. According to Milankovitch, this oscillates between 21.5 to 24.5 degrees in a periodicity of 41,000 years. A smaller degree of axial tilt promotes glaciation. At present axial tilt is in the middle of its range.
The third hypothesis states that the earth wobbles as it spins on its axis, known as its precession. As a result, sometimes its axis points to the star Polaris (North star) and later points to the star Vegas (which then becomes the North star).This causes significant climatic alterations. This has a periodicity of 23,000 years.
Milankovitch cycle has been questioned by several scientists. Based on recent experimental evidences, the Journal Science in 1978 declared that Milankovitch hypothesis which fairly predicts the cyclical occurrences of glacial and interglacial periods should be accepted by the Scientific community.
By the end of 19th century, geologists agreed from their field studies that in the Earth’s history of Pleistocene epoch, there were several ice ages, when ice sheets retraced back and forth intervened by warmer interglacial periods, each cycle lasting tens of thousands of years.
Significant studies included examination of the ice sheets of Greenland and Antarctic. Ice cores were dug up to depths of kilometers until bedrocks were reached. Some of the ice cores were dated as much as 100,000 years old.
These studies indicated that in the past there were short interglacial periods with spurts of rapid warming followed by more gradual and, irregular descent into a cold ice age, which lasted for tens of thousands of years. Warm periods did not last long. One of the interglacial periods has been dated to last for 13,000 years.
Ice core studies also revealed that concentrations of carbon dioxide and methane were higher in interglacial than in glacial periods. In the Antarctic record, the carbon dioxide concentration in the atmosphere in the past 750,000 years was between 180 and 280ppm.
Some of the scientific methods used to arrive at probable dates of geological periods include, radio carbon dating of glacial moraine, fragments of trees or any other organic matter buried deep thousands of years ago, dating ancient coral reefs with radioactive isotopes of uranium and other radioactive atoms with long periods of decay.
The ratio of oxygen-18 and oxygen-16 isotopes has been used to predict fairly the temperature prevailing at that time.
As predicted by Milankovitch’s calculations, there were several cycles of glacial and interglacial periods of duration of 20,000 and 40,000 years with very strong cycles of 100,000 years.
Based on past climate model studies, scientists in 1970-1980s expected a swift onset of the next ice-age. With the improvement of climate model studies and new calculations, it is now predicted that Ice age is not going to start soon. Green House Gas warming and other human influence appear to overwhelm the cooling effects caused by orbital changes of the earth and cancel the onset of the ice age.
According to Professor Franklin Hadley Cocks (2009) of Duke University, if the GHGs increase up to 500ppm, the increase in global temperature would be 5 C. Even such warming will not prevent onset of ice age, which lasts for millennia and interglacial, a few thousands of years. The major source of GHGs, the fossil fuels will be exhausted in next 300 years. In about 2000 years, planetary forces will initiate global cooling. The excess carbon dioxide will dissolve in the oceans as they cool down and locked there as carbonates.GHG levels will decrease to the pre-industrial revolution era.
However, the current challenge, especially in tropical countries is to survive global warming, which threatens to incinerate us. All efforts need to be made to slow down global warming, reduce emissions of GHGs by replacing fossil fuels with solar energy, nuclear energy and all other forms of non-conventional energy. let us allow the planetary forces to initiate global cooling. Equipped with our evolving intellectual excellence we will be able to fend of cold and save our civilization, when ice age sets in. let not an angry earth punish us with the ‘Sixth Mass Extinction’!
(Dr Mitra is a former Dean, Agriculture, OUAT)