Tuesday, February 26, 2008

QUATERNARY CLIMATE CHANGE

QUATERNARY CLIMATE CHANGE

By

Dr. Nitish Priyadarshi

In recent years, the global climate has been exhibiting signs of change that can neither be overlooked nor fully explained in the terms of natural climate variations. In this context, scientists, environmentalists, policy makers and others interested in the subject of climate change have been posing several fundamental questions like:

· Is the earth’s climate really changing?

· How has the climate changed in the past?

· Why does climate change?

· How much is the human influence on climate?

· What will the future climate be like?

· What are the likely impacts of climate change?

These questions are not new, but we are in a much better position to answer them today than ever before. We have come a long way towards a clearer understanding about connections among the different components of the climate system, the nature of the feedback mechanisms and the natural dynamics of the natural system.

The climate system is subjected to external forcings like the incoming solar radiation, and is also influenced by internal interactions among its five components viz., the atmosphere, hydrosphere, cryosphere, lithosphere and the biosphere. In the past, scientists have treated them individually and it is only now that the linkages among these components are being brought into proper perspective.

The term Quaternary is derived from the Latin word “quatern” and refer to the most recent period of the Earth’s history covering a span of the past ca 1.77 m.y. (million years) and extends upto the present day. The Quaternary period is subdivided into Pleistocene and Holocene epochs. Originally the term Pleistocene was proposed for the epoch which comprises 70% of the species that are still living. The term Holocene was introduced for the part of the Quaternary that contains only species that are still living.

During the short interval of the Quaternary, some very important events took place, which are of great consequence for us today. It is characterized by dramatic climatic changes, including repeated glacial and interglacial events.

Climate change is a dynamic phenomenon. Climate is continuously changing, sometimes at a slow pace, sometimes faster. In the geological past, the earth was probably warmer for most of the time than at present, with little or no polar ice. However at frequent intervals, extensive glaciations occurred in the past. The last glaciation, prior to the Quaternary ice ages, took place during late Carboniferous to basal Permian ca 300 m.y. ago.

The world began to cool rapidly in late Tertiary about 40 m.y. ago. By about 2.6 m.y. ago, the world had cooled so much that ice sheets began to form in northern Canada and Scandinavia. In the Indian sub-continent, glaciers spread far and wide over Potwar, Kashmir, Kangra, and in Ladakh-Tibet regions. From this time onwards, climate changes were more dramatic than during earlier geological times. During certain time intervals of Quaternary, temperatures dropped too low forming large ice sheets. However at other times the climate was warm, comparable to that of today, or even warmer.

Early only four glacial periods, of varying duration and severity, with the intervening interglacial periods, during which the climate became warmer, were recognized during the quaternary. Now it is certain that the number of such episodes exceeded four.

Another important feature of Quaternary was intense dune-building activity from time to time. In the deserts of Sahara (Africa), India, the Middle East and Australia, the time of peak aridity and maximum dune activity was during the last glacial period about 20 k.y. to 12 k.y. ago. One estimate is that the tropical arid zones were five times larger during the times of peak glaciation. Sea level was lower at these times (consequently exposing large continental shelf area to erosion), water was colder, and tropical cyclones were less extensive, resulting in decreased rainfall.

During glaciations, lower sea-level brought about another important geological effect on the land. The rivers started down-cutting their valleys and sedimentation pattern changed. There were many other similar changes consequent to the climate changes during Quaternary, like lake-level change and lowering of river discharge because of changes in precipitation.

Indian Monsoon during Holocene:

From palaeoclimate studies of the Indian monsoon, it is postulated that four monsoon maxima occurred during the last 150,000 years in association with solar radiation changes.

Pollen density sequences from lake beds of northwestern India and lake levels suggest that the monsoon were weak before 18,000 years B.P., that a warm and humid climate existed between 10,000 and 5,000 years B.P. or early Holocene, and that trend towards aridity set in around 3,500 years B.P. Tibetan ice core analysis indicate prolonged drought during the 18th century A.D. Extensive sampling of tree rings over western Himalayas has shown signatures both of the little Ice Age and the Medieval Warm Period. The northern limit of the monsoon reaching the desert margins has undergone wide fluctuations in the past, coinciding with the rise and decline of civilization such as the Indus Valley Civilization (2300-1700 B.C.), ‘Painted Grayware’ Culture (700-300 B.C.) and the Rangamahal Culture (100-200 A.D.).

Until recently, human activities have been considered to be an insignificant force in natural dynamics. Today, their impact has begun to match, and even exceed, that of natural processes. Fossil fuel reserves that were generated over a period of several hundred million years are now on the verge of depletion. As much as 50% of the land surface has been transformed by direct human action, with significant consequences for biodiversity, nutrient cycling, soil structure, biology and climate.

It appears that we are in a process of transition from the Holocene into a new epoch, for which the Nobel Prize winner, Paul Crutzen, and other scientists have coined the term “Anthropocene”.

Reference:

  1. Quaternary Geology Indian Perspective (2005) by U.B. Mathur. Geological Society of India.
  2. The Indian Monsoon as a component of the climate system during the Holocene,(2006) by R.R. Kelkar, In journal of Geological Society of India, Vol.68, no.3.

Nitish Priyadarshi

Geologist

Rch_nitishp@sancharnet.in

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