In recent years, the temperature of the earth's atmosphere has been warmer than at any time since measurements were first taken in 1860. Overall, it is estimated that the average overall temperature increase is between 0.3 and 0.6°C. In April 1997, it was announced that satellite measurements of the northern hemisphere of the globe showed that spring was arriving seven days earlier than ten years before and that the leaf fall of autumn was taking place four days later. These were just two examples of global warming caused by the so-called greenhouse effect.

The way the greenhouse effect is reported in the media, it would seem that it's a bad thing but in fact it's what keeps our planet habitable. If it wasn't for the presence of carbon dioxide and other gases in the atmosphere above the earth, we would be much colder – probably about –18°C! The greenhouse effect is essential to life on earth.

The greenhouse effect is the mechanism whereby heat from the sun reaches the atmosphere, and the layer of gases allows the short-wave radiation to pass through and warm the air and the earth's surface – just like the glass in a greenhouse. Some of the long-wave radiation (the infrared) given off by the warming process returns to space but most is retained because it cannot pass through the gas layer. The process is summarized in Figure 33.

The main gases responsible for causing the greenhouse effect are carbon dioxide, water vapour, methane, nitrous oxide and chlorofluorocarbons (CFCs). The problems arise when the levels of these gases increase in the atmosphere and make the greenhouse effect stronger. It's like putting thicker glass into a greenhouse and making it more efficient at trapping heat.

Of particular concern is the increase in the amount of carbon dioxide. As a result of human activities, the growing population and our increasing use of fossil fuels, the amount of carbon dioxide entering the atmosphere is rising.