Green House Effect

In the Green House Effect, the thermal radiation from a planetary surface is absorbed by atmospheric greenhouse gases, and is re-radiated. Due to this, the temperature of the body is higher than what it would have been if there was no atmosphere.

Process of Green House Gas on Earth

Earth receives the energy from Sun in the form of Ultraviolet, Visible and Near Infra Red radiation. Except most of the UV radiation, almost all of them pass through the atmosphere without being absorbed. Out of this 50% is absorbed by the surface of the Earth. When surface becomes warm, it radiates the far Infrared thermal radiation, which has longer wavelengths than that of the radiation absorbed. This thermal radiation is absorbed by the atmosphere, and the atmosphere reradiates it both upwards and downwards. The radiation that is sent downward again raises the temperature of the Earth.

Thus, the long wave radiation is trapped and the equilibrium temperature of earth is higher than if there was no atmosphere.

The Incoming sun light is mostly in the form of visible light and nearby wavelengths,  in the range 0.2–4 μm. The loss of the Radiation is almost nothing at the surface level but maximum at higher in the atmosphere because of the decreasing concentration of water vapor, an important greenhouse gas. While the major atmospheric components (Nitrogen and Oxygen) absorb little or no radiation, some of the minor components are effective absorbers. Particularly effective is water vapor and CO2 which absorb effectively in the IR wavelength range.

These absorbing gases and their surrounding air warm up, emitting radiation downward, towards the Earth’s surface, as well as upward, towards space. This effectively traps part of the IR radiation between ground and the lower 10 km of the atmosphere. This reduction in the efficiency of the Earth to lose heat causes the surface temperature to rise above the effective temperature until finally, enough heat is able to escape to space to balance the incoming solar radiation. The effect is analogous to that of a blanket that traps the body heat preventing it from escaping into the room and thus keeps us warm on cold nights.

Green House Effect versus Planetary Albedo

If Earth was an ideal black body which absorbs all the radiation from the Sun and emit the radiation due to this heating, its temperature would have been approximately 5.3 °C. The Earth and other planets are not perfect black bodies, as they do not absorb all the incoming solar radiation but reflected part of it back to space. The ratio between the reflected and the incoming energies is termed the planetary albedo. Earth reflects 36-37% of this incoming light and it corresponds to the Earth Albedo 0.367. So, Earth’s mean temperature is 14 °C. If there were no atmosphere and no radiation was lost due to reflection, its mean temperature would have been −18 or −19 °C. This difference is due to the Green House Effect.

Green House on higher altitudes

At high elevations, air temperatures are generally cooler and have a greater day- to-night range. Since both the thickness and density of the air column decrease with elevation; the greenhouse effect is weaker at high elevations.

Green House Gases

The Major Green House Gases are Methane, Water Vapour, Carbon dioxide, Nitrous Oxide (N2O), Ozone, Sulphur Hexafluoride (SF6), Hydrofluorocarbons (HFCs) and Perfluorocarbons (PFCs). Out of them, most potent greenhouse gas is water vapour, which causes about 36-70% of the greenhouse effect. Carbon dioxide (9-26%), methane (4-9%) and Ozone (3-7%) are other major greenhouse gases.

GHG Emission by sectors

Maximum anthropogenic GHG emission is by Power Stations (over 21%) as shown in the following chart. It is followed by the Industrial Processes (around 17%).

Anti-greenhouse effect

In Solar system, Mars and Venus show the Green House Effect, but Titan (the largest planet of Saturn) and Pluto, show Anti-Green House Effect.

Greenhouse effect occurs because the atmosphere transparent to solar radiation, but largely opaque to infrared and far infrared emitted by the planet / body. But in anti-greenhouse effect, the atmosphere is opaque to solar radiation but lets out infrared. The effect is that the body is cooler than the actual temperature would have been.

In case of Titan, both Green House Effect and Anti Green House Effect have been proved. Due to Green House Effect, the temperature goes up by 21K while, due to Anti-Green House effect, the temperature goes down by 9K. The result is that surface temperature is 12 K warmer than without atmosphere.

At Pluto, there is different mechanism. Here, the sunlight causes the Nitrogen ice to sublimate which cools the body.