The greenhouse effect is a naturally occurring process that aids in heating the Earth’s surface and atmosphere. It results from the fact that certain atmospheric gases, such as carbon dioxide, water vapor, and methane, are able to change the energy balance of the planet by absorbing longwave radiation emitted from the Earth’s surface. Without the greenhouse effect life on this planet would probably not exist as the average temperature of the Earth would be a chilly -18Â° Celsius, rather than the present 15Â° Celsius.
As energy from the Sun passes through the atmosphere a number of things take place . A portion of the energy (26% globally) is reflected or scattered back to space by clouds and other atmospheric particles. About 19% of the energy available is absorbed by clouds, gases (like ozone), and particles in the atmosphere. Of the remaining 55% of the solar energy passing through the Earth’s atmosphere, 4% is reflected from the surface back to space. On average, about 51% of the Sun’s radiation reaches the surface. This energy is then used in a number of processes, including the heating of the ground surface; the melting of ice and snow and the evaporation of water; and plant photosynthesis.
The heating of the ground by sunlight causes the Earth’s surface to become a radiator of energy in the longwave band (sometimes called infrared radiation). This emission of energy is generally directed to space . However, only a small portion of this energy actually makes it back to space. The majority of the outgoing infrared radiation is absorbed by the greenhouse gases .
Absorption of longwave radiation by the atmosphere causes additional heat energy to be added to the Earth’s atmospheric system. The now warmer atmospheric greenhouse gas molecules begin radiating longwave energy in all directions. Over 90% of this emission of longwave energy is directed back to the Earth’s surface where it once again is absorbed by the surface. The heating of the ground by the longwave radiation causes the ground surface to once again radiate, repeating the cycle described above, again and again, until no more longwave is available for absorption.
The amount of heat energy added to the atmosphere by the greenhouse effect is controlled by the concentration of greenhouse gases in the Earth’s atmosphere. All of the major greenhouse gases have increased in concentration since the beginning of the Industrial Revolution (about 1700 AD). As a result of these higher concentrations, scientists predict that the greenhouse effect will be enhanced and the Earth’s climate will become warmer.
In summary, the greenhouse effect causes the atmosphere to trap more heat energy at the Earth’s surface and within the atmosphere by absorbing and re-emitting longwave energy. Of the longwave energy emitted back to space, 90% is intercepted and absorbed by greenhouse gases. Without the greenhouse effect the Earth’s average global temperature would be -18Â° Celsius, rather than the present 15Â° Celsius. In the last few centuries, the activities of humans have directly or indirectly caused the concentration of the major greenhouse gases to increase. Scientists predict that this increase may enhance the greenhouse effect making the planet warmer. Some experts estimate that the Earth’s average global temperature has already increased by 0.3 to 0.6Â° Celsius, since the beginning of this century, because of this enhancement. Predictions of future climates indicate that by the middle of the next century the Earth’s global temperature may be 1 to 3Â° Celsius higher than today.