Climatic Change

Introduction

All the scientists agree that climatic changes have been great and diverse during the Earth’s approximate history of about 4.6 billion years. Climate, in fact, has changed in space and time. Some of the important causes of climatic change have been described briefly in the following.

1. Astronomical or Orbital Theory

Sun is the ultimate source of heat and light to the Earth. It has been established by the scientists that the eccentricity of the Earth’s orbit around the Sun changes. The Earth’s orbit undergoes a significant change during a cycle of 90, 000 to 100,000 years. Sometimes, the orbit forms a longer ellipse and returns to more circular shape. At the time of greater eccentricity of the Earth’s orbit, the amount of heat received at the Earth’s surface at perihelion may be 20 to 30 per cent greater than that of aphelion. Variation of this magnitude in the solar radiation would lead to change in solar constant, thereby affecting the temperature and climate of the Earth.

As a result of variation in the angle between Earth’s axis and the elliptic, the position of Equator, Tropic of Cancer, and Tropic of Capricorn will change. At present the Earth’s axis is inclined at an angle of 23.5 degrees. But according to experts, this angle undergoes a change. The inclination of the Earth varies from 22 degrees to 24.5 degrees during the cycle of 41,000 years. These changes in the axis of the Earth may lead to gradual and more warm and cool phases on the Earth’s surface.

2. Sunspot Theory

In the opinion of geo-physicists, sunspots are responsible for changes in climate. Sunspots are the dark, circular areas of the outer-face (photosphere) of the sun. These are the areas in the surface where temperatures drop some 1400oC lower than the surrounding areas. Sunspots were seen and recorded as early as 28 B.C. in Eastern Asia. They have been studied intensively since the invention of the telescope shortly after 1600 A.D. The number of sunspots occurring at any one time varies from a few as 5 or 6 to as many as 100. The fluctuation in sunspot follows a 11 year cycle, and multiples of that cycle appear 22 and 33 years. Two periods of major deviation from normal existed in the 1000 years.

Theories about the Changes in Atmospheric Composition the contents of carbon dioxide, nitrous oxide, methane, water vapour, etc. are not constant in the atmosphere. The proportion of carbon dioxide changed in the past because of the volcanic eruption. It is changing more fast at present because of the heavy consumption of fossil-fuel by man.

The carbon dioxide theory about the climatic change was put forward by T.C. Chamberlin. According to this theory variations in the carbon dioxide content of the atmosphere play an important role in causing the world-wide climatic change. This gas is transparent to the incoming solar radiation but absorbs outgoing long-wave terrestrial radiation. This leads to greenhouse effect. Thus any change in carbon content in the atmosphere would bring about changes in the temperature of the atmosphere. The high rate of industrialisation and urbanisation during the last about 250 years has been directly adding more carbon dioxide to the atmosphere, which has been attributed as the main cause of climatic change According to one estimate, the present carbon dioxide content of the atmosphere is 410 ppm (parts per million). It is likely to reach 500 ppm by the end of 21st century. If the present rate of increase in CO2 continues, this atmospheric gas may double by the year 2050. Consequently, the global temperature may rise by about 1.5o to 2o C by 2050.

The CO 2, CH4, NO2, and CFCs are continuously increasing in the atmosphere. It has been estimated that between 1980 and 2001 the surface temperature of the ocean has raised at the rate of 0.11oC per annum (Fig. 7.3). This increase in temperature will result into sea-level change. Methane (CH4) is also a radically active gas contributing to overall greenhouse effect. According to experts, methane is increasing in concentration at about 1 percent per year. Air bubbles in ice show that concentrations of methane in past 500-27,000 years ago were approximately 0.77 ppm, where as current atmospheric concentrations are 1.7 ppm. The CFCs are thought to contribute about 20 percent of the global warming. CFCs absorb infrared in wavelengths missed by carbon dioxide and water vapour in the lower troposphere. As radio actively active gases, CFCs enhance greenhouse and also play a negative role in stratospheric ozone depletion.

4. Volcanic Dust Theory

Volcanoes are another important factor of climatic change. The eruption of volcanoes sends enormous smoke and dust in the atmosphere. The ejected sulphur dioxide gas reacts with water vapour found in the atmosphere to form a dense optically bright haze layer that reduces the atmospheric transmission of some of the Sun’s incoming radiation. Thus dust and sulphur dioxide effect light of short-wave lengths coming from the Sun. Contrary to this, the long wave terrestrial radiation can easily pass through volcanic dust without loss. Thus volcanic dust may lower down the earth’s temperature to certain extent. The volcanic dust is considered as responsible for ‘Little Ice Age’. In the geographic past the frequent volcanic eruptions initiated the process of Ice Age. According to one estimate a reduction in radiation absorbed by earth by as little as 1 per cent can produce a change in surface temperatures by as much as 1.2oC to 1.5oC.

The volcanic eruptions of El-Chichon (Mexico) sent heavy quantities of sulphuric gases and sulphuric aerosols into the stratosphere, where they remained for longer periods. The presence of excessive dust particles and smoke reduces the process of albedo (reflect back sunlight to space). The low albedo rate produces a cooling effect on surface temperature of the Earth.

5. Theories of Continental Drift

The Continental Drift theory of Alfred Wegener, the Se a-Floor Spreading of H.H. Hess and theory of Plate Tectonics by Morgan reveal that the continents and ocean basins have dried apart. According to geo-physicists, the drift was started about 300 million years back. As a consequence of drift the positions of equator and poles have undergone significant change that affected the horizontal distribution of continents and ocean-basins. The drift of continents has thus brought significant climatic changes. The distribution of coal in the different latitudes, including the Greenland and Antarctica are a testimony of climatic change. Human Induced Changes in Earth’s Surface the appearance of man on the earth surface is very recent in terms of Geological Time. Man has however, brought about massive changes in the environment. These changes have had a significant impact on the Earth’s climate.

Humans have been altering the environment since they first controlled the domesticated plants and animals and started agriculture in about 8000 B.C. The result of these activities was deforestation of large areas of the world. In the tropical realm, it may be that the savanna grasslands are a response to deforestation by e.

The beginning and development of agriculture was made possible by deforestation. With the growth of technology and rapid increase in population, deforestation became even more extensive. In many areas deforestation achieved an alarming dimension. Moreover, the destruction of the forest and damage to environment in the quest for raw materials for industries, reservoirs of dams, creation of artificial lakes, generation of energy, expansion of agricultural lands, irrigation, industrialisation, urbanisation, and other processes have significantly changed the face of the earth. With the passage of time there is increasing use of fossil fuel. All these human activities resulted into substantial increase in greenhouse gases, the main cause of climatic change.

7. Other Theories

Some of the scientists have introduced ideas ranging from the possible influence of periodic passage of the earth through an interstellar dust cloud to variations in atmospheric water vapour caused by both natural and human activities. This process results in an increase in temperature. Despite all these ideas, there is no single theory that can account for all the observed events; it is evident that earth’s climates result from a spectrum of causal elements. At present, the human activities are largely responsible for greenhouse gases, global warming and climatic change.