Less than 15,000 years ago, an ice sheet covered much of Europe and North America. Since then, the global climate has naturally got warmer. Some scientists, however, have recently become concerned that human activity is accelerating the pace of global warming. The carbon dioxide (CO2) content of the atmosphere has been increasing from an average value of 290 parts per million in the late 19th century to some 350 parts per million in 1990.

This is almost certainly a result of the great increase in the use of fossil fuels such as coal, oil and natural gas, which when burned, release CO2 into the atmosphere at a faster rate than it can be absorbed by vegetation or the oceans. The destruction of the forests, particularly in the tropics, adds to this effect. Forest clearance - smoke from a burning rainforest can be seen from Space. Most scientific evidence suggests that the persistent use of such methods to clear large areas of land is changing the Earth's climate. But it is still unclear exactly what will be the result of these changes.

Some estimates suggest that by the year 2050 the CO2 content of the atmosphere may be double its present level. CO2 is called a 'greenhouse' gas because in the atmosphere it acts, together with cloud and water vapour to trap outgoing longwave radiation just as the glass in a greenhouse does. Thus an increase in this green house effect would raise the Earth's surface temperature. The precise magnitude of this effect is uncertain; estimates range from 1-5 degrees C (2-9 degrees F). The effect would probably be greatest in high latitudes and least in the tropics. The present pattern of climatic types could be drastically changed: some areas would benefit from the greater warmth but other areas could experience reduce rainfall, seasonal drought, or more frequent severe storms. In the longer term the polar icecaps would melt and sea levels rise. At present evidence for the beginning of such changes is by no means clear, but a slight increase in the surface temperatures in the sea may be the first sign of this effect.