Radiation Balance of Earth
The earth’s surface obtains nearly all its heat energy by radiation from the sun. Subsequently, the heat energy received by the earth is lost by reflection and by radiation. The net result is that the earth loses as much heat (radiation) as it received.
In this way, a near steady state of the average global temperature is maintained. Over millions of years, the earth has neither progressively cooled nor warmed and the temperature of the earth has remained more or less constant.The atmosphere plays a decisive role in maintaining the radiation balance of the earth. It is only through the atmosphere that the solar radiations reach the surface of the earth and it is also the atmosphere through which the surface of the earth radiates or reflects back the solar radiations. It is estimated that approximately 22% of the solar radiation received from the sun are reflected back by the combined effect of the clouds, snow and ice sheets on the earth surface. The reflecting capacity of the earth is called albedo. Of the remaining 68% of the solar energy, the atmosphere absorbs about 16-17% and 48-51% is absorbed by the surface of the earth. Finally, both the atmosphere and the earth’s surface radiate back the absorbed energy as heat. It is the energy transport which plays a significant role in the earth’s radiation balance. This transport of energy takes place by radiation of energy from the earth, conduction of energy through the interaction of atoms and molecules with atmosphere and convection of energy through massive air circulation. The last two factors are responsible for the loss of heat from the earth’s surface via transport to clouds and consequent radiation from the clouds. The amount of heat energy radiated into the outer space is approximately 67%. Thus, the heat gained and heat lost is balanced over the global scale. On an average, the temperature of the earth is maintained around 15oC due to reabsorption of most of the outgoing radiation by water vapours and carbon dioxide.
This combined effect of water vapours and carbon dioxide leads to the so called greenhouse effect (For more details see Chapter 4).The heat balance varies from one region of the earth to another and this aspect is very significant. The imbalance between the heat absorbed and radiated causes atmospheric and ocean circulations to occur. This, in turn, controls the climate and weather patterns of the globe.
The earth’s radiation balance is greatly affected by many human activities. Many Industrial processes are responsible for injecting a large amount of green house gases like carbon dioxide, ozone, nitrous oxide, methane, water vapours and particulate matter into the atmosphere. Deforestation, soil erosion and eruption of volcanoes also adds to the misery. The gases released into the atmosphere tend to increase the average temperature of the earth. The pollutants discharged by human activities create an imbalance in the global temperature. In fact, it has been found that over the last decades, the average global temperature has steadily risen. The discharge of pollutants in the atmosphere, if not controlled, will be disastrous for the environment.
3.5