Because of the global warming controversy, it might sometimes seem as if carbon dioxide were one of the great evils of our time. One might be forgiven for thinking of it as a foul pollutant, a 20th-century aberration, an environmental vandal of which the world would be well rid.
But nothing could be further from the truth. CO 2 is an essential ingredient of our atmosphere which, among other things, plays an important role in regulating the temperature of our planet. Without it life on Earth could not exist.
First, carbon dioxide keeps us warm. CO 2 is an important contributor to the protective blanket which prevents the loss of too much of the Earth's heat to outer space.
Both water vapour and CO 2 absorb radiation very efficiently at the relatively long infra-red wavelengths of the spectrum, and it so happens that it is at precisely at these wavelengths that the Earth radiates most of its energy.
Much of the heat which might otherwise be lost to the cosmic cold of space is trapped by this protective shield, and reradiated back to Earth. If this were not so, the temperature of our planet would drop to unimaginable depths; it would be like Mars, and life as we know it would be quite impossible.
Carbon dioxide is also an essential fuel for plant life. Under the influence of sunlight, through the process of photosynthesis, plants take up carbon dioxide from the air around them and combine it with water to form the living tissue that results in the phenomenon of growth; in doing so, they exude oxygen as a byproduct, the life support substance for the metabolism of most animals.
But CO 2, again through photosynthesis, has another important benefit. The oxygen produced by photosynthesis reacts with sunlight in the stratosphere, high above the surface of the Earth, to produce ozone. Ozone acts as a filter to the sun's rays and protects our planet from ultraviolet radiation, potentially harmful solar radiation at the short-wavelength end of the spectrum.
In fact there is only a tiny amount of carbon dioxide in the atmosphere, comprising about 0.035 per cent of its total volume. But directly or indirectly, this tenuous ingredient controls the radiation balance of the Earth; it leaves us with a small clear window into space between the regions of ultra-violet and infra-red opacity. It is through this small window that the sunlight streams towards us, and through which we, looking outward, see the planets and the stars beyond.
The system is uniquely and precariously balanced, but it works, and has done so for three billion years.