Global Warming

The surface temperature of the Earth is largely governed by the way in which sun's rays which are allowed to penetrate the atmosphere but are then trapped and cannot escape back to space. For millions of years carbon dioxide in the upper atmosphere has been present at about 300 parts per million. Carbon dioxide, and other gasses, transmit the visible and Ultra Violet (UV) rays from the sun, but are not good at transmitting Infra Red (IR) rays. Since the higher energy rays from the sun are those in the visible and UV part of the spectrum, the filtering of IR is not a problem. Rays from the Earth are generally in the IR simply because the Earth is much colder than the sun. If all of the Earth's IR rays could escape then the Earth would cool down. The presence of the greenhouse gasses, like carbon dioxide inhibit the escape of IR rays and so keep our planet warm. This is the classical greenhouse effect, it is how a greenhouse works and is essential to the survival of life on Earth over millions of years. The increased atmospheric carbon dioxide levels from burning fossil fuels is further inhibiting IR ray transmission and is therefore leading to an enhanced greenhouse effect - that is to say that the Earth is getting warmer.

Ozone Depletion

Some of the CFCs (chemicals containing chlorine, fluorine and carbon - chloroflorocarbons), that are particularly good as refrigerants, degreasing agents and for driving foams from aerosols are now controlled by the Montreal Protocol. This is an international agreement to phase out their use because of the damage that they cause to the Ozone layer. The ozone layer, 25,000 m above sea level, is vital to life on Earth as it filters the ultra violet rays from the sun. Ozone is a gas of molecules of three atoms of oxygen, chemically written as O₃. In 1979 a hole was discovered in the Ozone layer which led to increased UV in the Southern Hemisphere and increased levels of skin cancer. The hole is seasonal but is larger each year. One of the problems is that the CFCs remain in the upper atmosphere for many years and so will continue to damage the ozone layer for years to come even if we manage to completely eliminate the use of CFCs.

Increased UV radiation into the Earth's atmosphere also contributes to global warming - see previous box.

Ground Level Ozone

Ozone in the upper atmosphere forms a protective layer which filters out dangerous UV rays from the Sun. Ground level ozone, on the other hand, is itself very damaging to human health. It is formed by complex chemical reactions from vehicle exhausts.