The Greenhouse Effect

For millions of years, the “greenhouse effect” has kept the Earth at a comfortable temperature for living organisms.
The temperature has remained constant throughout our Earth’s history; this is due to the concentration of the greenhouse gases in the atmosphere.
There are now worries that human activities are increasing the concentration of greenhouse gases in our atmosphere.
This could have irreversible effects upon our planet.

Radiation Cycle

When objects get hot, they emit electromagnetic radiation. The hotter the object, the higher the energy of the radiation.
The surface temperature of the sun is around 6,000 K; it mainly emits energy in the visible region, although there is also radiation emitted between the ranges of infrared and ultraviolet radiation.
This radiation heats our planet.
The Earth’s average surface temperature is 285 K; the energy emitted from our planet is mainly in the infrared region. Our planet absorbs some of the energy from the sun, and then emits the energy back out.
This delicate balance keeps our planet at the right temperature, but as with all delicate balances, it can be disturbed by changes to the system; that is changes to the concentrations of atmospheric gases.
Methane is an example of one of these atmospheric gases.
Basically it traps some of the suns radiation that would otherwise be radiated into space.
It is the greenhouse effect that keeps our planet warm. As the name suggests, the process is similar to how a greenhouse traps heat.

The term greenhouse gas relates to a gas that absorbs infrared radiation, but does not absorb the visible or ultraviolet radiation from the sun.
Some greenhouse gases have a greater greenhouse effect than others.
The greenhouse factor can be used to compare the greenhouse effect of the different gases. The measure is relative to the same amount of carbon dioxide that is assigned the value of 1.
Without the greenhouse effect, we would not be able to survive on this planet.
Mars has an atmosphere that is mainly CO₂, however the atmosphere is not thick, so the greenhouse effect is not sufficient to keep the planet warm.

The Swedish chemist Arrhenius predicted, nearly 100 ago, that increasing the amounts of carbon dioxide in the atmosphere could lead to warming of the Earth.
Average temperatures did rise by around 0.25^oC between 1880-1940, but decreased again between 1940-70 by 0.2^oC.
During the 1970’s the amount of carbon dioxide in the atmosphere increased substantially.
Predictions about climate changes began to be made.
Just as the predictions were being made, measurements of the Earth’s temperature began to confirm that global warming was taking place.
Between the years of 1970-80, the Earth’s average temperature raised by 0.3^oC.
This global warming has been linked to human-made emissions of the greenhouse gases.

The two most significant greenhouse gases are water and carbon dioxide; this is due to their abundance in the atmosphere. CO₂ and H₂ O absorb radiation in two bands across the Earth’s radiation spectrum.
Between these two bands is a window where infrared radiation can escape.
70% of the Earth’s radiation escapes through this window.
Anthropogenic gases can increase the greenhouse effect of the atmosphere. There are two types of these gases:
1. Gases already present in the atmosphere which are increased by human activities. CO₂ is an important example; humans burn fossil fuels, which release CO₂ into the atmosphere.
2. Gases which are not naturally present in the atmosphere. These gases can absorb radiation that is within the vital “window”. CFC’s are an example.