Causes of Climate Change
A climate change indicates a change in the general circulation of the atmosphere on which climate ultimately depends, but the climate does not include the atmosphere only but also consists of the hydrosphere, biosphere, lithosphere, and cryosphere. The climatic system is also subjected to extra-terrestrial influences particularly, that of the Sun. Climate, therefore, depends on two major factors. These are-
- The nature of the above components makes up the climatic system and the interaction between various components.
- The nature of the geophysical conditions outside the climatic system and the influence they exert on the climatic system. The climatic state at any given period depends on three crucial factors-
- The amount of energy received by the climatic system from the Sun.
- How this energy is distributed and absorbed over the Earth’s surface.
- The nature of the interaction processes between the various components, which makes the climatic system.
Climatic changes are associated with variations in the above three factors over time. However, variations in climate occur on different time scales and therefore, they are to be explained differently. That is why not a single theory of climatic change has been found acceptable in explaining all the variations that have been known to occur in world climate. In the last century, various theories have been proposed to explain the reasons for climate change and can be discussed under various categories. The principles that have been discussed here are as follows: –
- Astronomical theories
- Continental displacement theories
- Theories involving a change in the composition of the atmosphere.
- Extra-terrestrial theories
Astronomical theories
Astronomical causes of climatic change involve changes in the Earth’s geometry and are also called Earth geometry theories or the Croll-Milankovitch hypothesis. As we know that the most important source of energy received by the earth is the sun. Therefore, even a slight change in the relative position of the Earth and the Sun brings about a change in the amount of solar energy. Three periodic variations affect the seasonal distribution of solar radiation on Earth, which are found in the relative positions of the Earth and the Sun.
- Changes in the eccentricity of the Earth’s orbit (a 96,000-year cycle);
- The precession of equinoxes (with a periodicity of 21000 years); and
- Changes in the obliquity of the ecliptic orbit (the angle between the plane of the Earth’s orbit and the plane of its rotational Equator) have a periodicity of about 41,000 years.
1. Eccentricity of the Earth’s orbit
The Earth’s orbit around the Sun is not a perfect circle but an ellipse. The distance from the Earth to the Sun varies between 147 and 152 million km. This can be as large as 142.5 to 156.5 million km. This factor is known as the eccentricity of the Earth’s orbit. In the cycle of 90,000 to 100,000 years, there is some change in the shape of the Earth’s orbit. It can stretch by departing much farther from a circle and then revert to almost true circularity.
Fluctuations in the eccentricity of the Earth’s orbit cause variations in the reception of solar energy by the Earth. The distance of the sun from the centre of the elliptical orbit controls the distance of the Earth from the Sun at different times of the year as well as the duration of the four seasons. The smaller the eccentricity of the elliptical orbit the smaller will be the differences in the length of the seasons and vice versa. At the perihelion (on January 3), when the Earth is nearest (147 million kilometres) to the Sun, solar energy received is 6 per cent more than at the aphelion (on July 4), when the Earth is farthest (152 million kilometres) away from the Sun.
However, at the time of high eccentricity of the Earth’s orbit, the amount of solar radiation received by the Earth is 20 to 30 per cent more during the perihelion than at the aphelion. This type of change in the amount of solar radiation means that a completely different type of climate will develop from the present climate.
Since the periodicity of the eccentricity is 96,000 years, in about 50,000 years the earth, in its orbit, will be nearest to the Sun in July and not in January as at present.
2. Precession of equinoxes
The precession of the equinoxes simply means that the time of year when the Earth is closest to the Sun (perihelion) changes. This is because the Earth wobbles like a top and swivels while rotating on its axis. Precession controls the change in length between the astronomical half-years and the Earth-Sun distances at both solstices. Currently, the equinoxes occur on March 21 and September 23, while the solstices occur on June 21 (summer solstice) and December 22 (winter solstice).
Earth is closest to the Sun on January 3rd and farthest from the Sun on July 4th. In about 12,000 years, Earth will be closest to the Sun on July 4th, and in 22,000 years, things will reappear to their existing conditions. This is called the precession of the equinoxes and has a cycle of 22,000 years. This factor is important because it now makes winters milder and summers colder than normal in the Northern Hemisphere, and the opposite is true in the Southern Hemisphere.
This means that any climate change caused by this factor would occur out of phase in each hemisphere. The displacement of the four seasonal points will lead to the migration of the seasons along the orbit. Fluctuations in the precession of the equinoxes cause the seasons to shift.
The equinoxes complete a full cycle approximately every 22 ka. (a) Present-day position of the Northern Hemisphere equinoxes and solstices. The summer solstices coincide with an aphelion (the longest Earth-Sun distance). (b) Positions of the Northern Hemisphere equinoxes and solstices at the opposite phase of the processional cycle (e.g., 11 ka BP and 11 ka AP). The summer solstices coincide with perihelion (the shortest Earth-Sun distance).
3. Obliquity of the ecliptic orbit
This refers to the angle of the axis with the plane in which the Earth revolves around the Sun. Currently, the angle is 66.5° which refers to an obliquity angle or tilt of 23.5°. Towards or away from the Sun, this angle is not constant. It is estimated that the tilt of the axis changes from 22.1° to 24.5° in a cycle of 41,000 years. Such changes affect the amount of annual radiation and intensity of the seasonal variations in both hemispheres. The greater the tilt, the more pronounced the difference between winter and summer and vice versa.
Continental displacement theories
It is worth noting that the glaciated topographies found in Africa, Australia, South America, and India do not match the present climatic conditions of these regions. There are clear indications from these topographies that there must have been an ice age in these landmasses during the geological extreme.
Today all scientists are unanimous in accepting the fact that there must have been an ice age in these regions about 23 million years ago. How difficult it is to believe at present that the climate of these regions located in the tropics would ever have been similar to the climate of Greenland and the Antarctic.
According to the ‘Plate Tectonic Theory’, all the above-mentioned areas with glaciated topographies or all the seven continents were linked together as a single landmass and the name of that huge landmass was ‘Pangea’. This huge plot of land was situated far to the south of the present position of the continents in the high latitudes of the Southern Hemisphere.
Over time, due to the internal forces of the earth, cracks occurred in Pangea and gradually it got divided into many sections, which came to their present positions by sliding on different plates. This revolutionary theory derived from geology was fully capable of solving the puzzle of how these glaciated terrains formed in their scattered subtropical conditions.
Today, geologists and other scientists believe that different types of climate changes would have occurred as a result of continental drift in ancient times. As a result of these changes, there would have been radical changes in the ocean circulation system and due to this, there would have been a corresponding change in the transfer pattern of heat and moisture.
The plates that make up the Earth’s crust move at a relatively slow rate (i.e., a few centimeters per year). Because of this, it takes crores of years for the conditions of the continents to change. This is the reason why long-term climate changes take millions of years. In other words, the ‘plate tectonic theory’ proved to help explain long-term climate changes. But this theory is not capable of explaining the short-term climate changes that take place throughout hundreds-thousands of years. The need for other theories was felt to explain these short-term climate changes.
Atmospheric Theories
Changes in atmospheric composition can moderate the effects of incoming radiation from the Sun, which modifies the flow of energy through the atmosphere and retains energy through the greenhouse effect. Variations in the transmissivity and absorptivity of the atmosphere are the primary causes of climatic change.
Gases such as carbon dioxide, ammonia, and water vapor, which are transparent to incoming shortwave radiation but relatively opaque to outgoing longwave radiation, are radiatively active and variations in their concentration can alter the thermal structure of the Earth’s atmosphere and disturb its energy balance. This mechanism has played a significant role in past climatic changes and could influence future climate.
Carbon dioxide theory
Carbon dioxide theory was propounded by T.C. Chamberlin in the last decade of the nineteenth century. According to the theory a change in the amount of CO2, there is a change in climate on a global scale. The main characteristic of this gas is that it is transparent to the solar radiation coming towards the earth’s surface in the form of short waves, while it absorbs the outgoing long-wave terrestrial radiation and reradiated absorbed territorial radiation towards the earth.
Therefore, CO2 generates the greenhouse effect. It is thus clear that any change in the amount of carbon dioxide in the atmosphere brings about a change in the temperature in the lower part of the atmosphere.
In the modern era, industries have grown at an unprecedented rate, due to which a huge amount of carbon dioxide gas has reached the atmosphere. This gas is partially used by plants in photosynthesis and some part of it dissolves in ocean water. Then, 50 per cent of it remains present in the atmosphere. It is estimated that between 1860 and 1970, the amount of this gas in the atmosphere increased by 10%.
As a result, it is obvious to increase the temperature of the atmosphere during this period. But point is that the increase in the atmospheric temperature on a large scale was recorded only till the middle part of the fourth decade of the twentieth century. After that, a gradual decrease in the atmospheric temperature was marked. On the contrary, some scientists believe that in the absence of the greenhouse effect caused by the increase of carbon dioxide in the atmosphere, the temperature would have decreased more.
(Image Credit: Global Monitoring Laboratory – Carbon Cycle Greenhouse Gases (noaa.gov))
The amount of carbon dioxide in the atmosphere at this time is 330 ppm (parts per million. If the current rate of increase of carbon dioxide continues, then by the year 2040 the quantity of this gas may double, as a result of which the world temperature will increase by 2°C by year 2040.
Studies of ice-core oxygen isotope records in Greenland suggest that the current cooling trend that began in the 1940s is a temporary phenomenon. After a decade or two, the atmospheric temperature will start rising again. Scientists predict that in the following decade of this century, the temperature of the atmosphere will increase to such an extent that it has never happened in the last 1000 years. Therefore, due to the continuous increase in the amount of this gas, there is a danger of a huge increase in temperature.
There is also a possibility that due to the increase of carbon dioxide gas in the atmosphere, there will be a fall in the temperature. This is possible in the following way – excess of this gas first increases the temperature due to which the rate of evaporation increases, and more cloudiness is generated. As a result of greater cloud cover, a relatively smaller amount of solar radiation is received at the surface. Thus, the temperature of the atmosphere decreases. The atmosphere is a very complex interactive system, so there are many possibilities regarding the consequences of increasing the amount of carbon dioxide.
Criticism
The most important thing is that it is not possible to determine the quantity of CO2 in the atmosphere in ancient times. Secondly, this theory does not throw light on the causes of short-term climate changes. For example, this theory does not explain the cause of the fluctuations in climate that occur in post-glacial or historical periods.
According to Huntington and Vischer, this theory should be partially accepted. This theory does not provide a complete solution to the problem of glaciation. Nevertheless, there is no doubt that fluctuations in the amount of carbon dioxide are one of the main causes of long-term climate change.
Volcanic dust theory
The dust particles emitted during volcanic activities deflect the incoming short-wave solar radiation. But long-wave terrestrial radiation passes through volcanic dust without any decay. So, it is obvious that due to volcanic dust, there is a decrease in the temperature on the earth’s surface. Volcanic dust is probably one of the factors responsible for the ‘Little Ice Age’. According to this theory, the onset of ice ages is believed to occur during the period of maximum eruption of volcanoes.
In 1883, there was such a terrible explosion of the Krakatoa volcano in the Eastern Islands that most of the island was destroyed and its explosion was heard 4,800 kilometers away. As a result of this so much dust entered the atmosphere that for two years the color of the sky appeared redder at sunset. Some scientists revealed that due to the high amount of dust in the atmosphere, the temperature of the whole world fell.
In course of time, when the dust settled down from the atmosphere, then the temperature became normal. There is evidence from historical records that due to volcanic eruptions in the last century, the temperature of the atmosphere decreased by several degrees.
The volcanic dust theory is proved by geological tests. If the beginning of the Ice Age was possible from a bag of volcanic dust, then the layers of sediments or the dust particles released from the destructive volcanic eruptions should have been preserved in the current glaciers or the lakes and oceans. This theory can be tested by comparing the record of ice age climate with the sedimentary record of volcanic activity. But practically it is impossible to test the validity of this theory by making accurate measurements on a large area.
Extra-terrestrial theories
Theories of extra-terrestrial causes of climatic change postulate changes in the amount of solar energy, reaching the Earth either because of changes in the solar output or changes in the amount of solar radiation absorbed outside the Earth’s atmosphere.
Solar output variation
As the Sun is the main source of energy on the Earth’s surface. Thus, variations in solar radiation must be considered a likely cause of climatic change. On different time scales i.e., days, years and centuries, empirical shreds of evidence have purported to show links between solar variations and various climatic events. An increase in solar radiation causes the atmosphere to heat up, and a decrease in its amount causes a drop in the atmospheric temperature.
One explanation accepts that the Sun contains substances other than hydrogen and helium. Diffusion of these substances from the core of the Sun towards the surface would produce a barrier to the flow of energy from the core to the surface.
The barrier should have the effect of ultimately reducing the amount of energy passing from the Sun’s surface and producing an ice age on the Earth. There is, however, no physical evidence that it could occur, and it cannot be considered, at least at present, a viable explanation. Tidal oscillations on the Sun raised by the planets in their orbits also cause fluctuations in solar output.
Sunspot Theory
Sunspots are dark circular patches on the Sun’s surface that represent a cool region on the photosphere having 4600 K temperature in comparison to 6000°C for the photosphere. Climatic changes may be attributed to the changing number of sunspots, that appear on the surface of the Sun. The reason behind the theory is that there may be a slight increase (less than 1 per cent) in the heat, coming from the Sun when there are more sunspots than usual.
Sunspots cause fluctuations of output with cycles of 11 years, 22 years, 44 years, etc., the best known being the 11-year cycle. Other cycles that have been recognized include the 80-year-old Gleissberg cycle and one with a frequency of 205 years. One of the major parts of evidence used in support of this theory is the fact that during the period 1645 to 1723, there were almost no sunspots.
This minimum called the Maunder minimum corresponds to the Little Ice Age on the Earth. In contrast, maximum sunspot activity took place between 1100 and 1250, a typically warm period in the Northern Hemisphere.
However, the influence of sunspot cycles on weather and climate is a matter of controversy. A few scientists claim to have found a correlation between sunspot activity and weather on Earth and others found no relationship. Even the physical mechanism, by which they influence the atmosphere, is not well defined. This is because for the wavelengths of emission have different effects on different parts of the atmosphere. A further complication is that sunspot activity coincides with an active solar wind, and it is difficult to separate the two effects.
Interstellar dust hypothesis
One possible reason for the variability in receipt of solar radiation on the Earth’s surface is the existence of clouds of fine interstellar matter (nebulae) through which the Earth might pass from time to time or which might interpose themselves between the Earth and the Sun. In encircling the Galaxy, the Earth passes through one of the rotating spiral arms of the Galaxy about once every 300 million years.
Passage through this and the adjacent dust lane might cause climatic effects as interstellar dust affects the radiative transmittance of interplanetary space or as infalling interstellar material affects the solar luminosity The Earth’s path around the Galaxy is elliptical and its passage nearest to the Galaxy’s center every 270 to 400 million years. might give rise to cyclic changes in climate over 100 million years. time scale.
On a more local scale, there is some suggestion of a correlation between the indirect climate record and the reversal of the Earth’s magnetic field but there is no obvious physical mechanism to account for the link.
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