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The greenhouse effect

Cloudy sky at sunset, Strathspey, Scotland

Image: Andy Hay

The term 'greenhouse effect' describes the actual process of temperature rise. 'Global warming' and 'climate change' describe the result of that process.

What is the greenhouse effect?

About 30 gases produced by human activity have been identified as contributing to the greenhouse effect. These are sometimes called greenhouse gases. The main ones are carbon dioxide, methane, CFCs and nitrous oxide. Sunlight reaches the Earth's surface, is reflected off it, and is prevented from escaping from the atmosphere by this layer of gases, which acts like glass in a greenhouse.

Up to a point, the greenhouse effect is a natural process, and naturally occuring carbon dioxide is not dangerous. Without greenhouse gases, the average temperature of the Earth would be -19C , in other words, too cold for us to live. However, the concentration of the greenhouse gases in the atmosphere has changed dramatically: since the 18th century, carbon dioxide has increased by 30%.

What part have humans had in the greenhouse effect?

We have increased the amount of carbon dioxide in the atmosphere by burning fossil fuels, such as coal, oil and petrol. Cows, rice fields and rubbish rotting in landfill sites produce methane, and aerosols and fridges use CFCs.

The removal of trees - deforestation - is also thought to increase the greenhouse effect. Trees absorb carbon dioxide and produce oxygen. When they are removed, less carbon dioxide is absorbed, more is present in the atmosphere, and the greenhouse effect worsens.

The longterm outlook

There is disagreement among scientists about the seriousness of global warming. Most scientists agree that the global temperature is rising. They do not agree on more specific elements of the issue: How much will it warm up? What will happen if it does warm up? How far are humans responsible? What should we do to stop it?

Scientists believe that if we go on producing gases at the rate we do now, it is likely that the Earth's average temperature will rise by about 0.3 degrees C every ten years. This does not sound much, and it might be pleasant to have hotter summers in Britain, but it could have a huge effect on all living things on Earth. Changes in global temperature of the kind predicted could have considerable effects on rainfall and wind, as well as warmer weather. They can also affect ocean currents, which can greatly affect the weather, and the unusual weather experienced during El Nino is an example of the effect the ocean has on the weather.

Sea level rise is another effect of climate change. This will happen as temperatures rise, and warm water will occupy more space than cold water, flooding low lying areas (sea level rise is not primarily due to melting polar ice caps). Again, there is disagreement as to how much the sea level will rise. Many of the world's cities and much agricultural land are in the threatened zones.

As the global temperature rises, the distribution of plants and animals will be changed. Again, the consequences are unpredictable, but countries that now have rich agricultrual land could find these less productive: others may find poor areas improved.

Wildlife already faces a number of threats and many species and habitats are vulnerable to human activities. Although we. re not sure how climate change will affect wildlife, it is clear that its impact will make things worse.

Many types of wildlife depend on natural signals, such as temperature or day length, to time their life cycles, and if some of these signals alter due to climate change, the timing of life cycles will change. A study of birds across the UK from 1971 to 1995 shows that 63% of the species are showing a tendency to nest earlier. Twenty species of UK breeding bird are laying their first egg an average of nine days earlier than they did 20 years ago.

Most scientists accept that global warming will cause wildlife to shift northwards, or to move higher in altititude: this is what happened, very slowly, after previous Ice Ages, as the Earth has warmed up again. However, some species may not be able to move if climate change is so rapid they they, or the other species on which they depend for food, cannot move into new areas where the climate is suitable for them. Plants, for example, may not be able to disperse themselves fast enough to keep pace with the shift in the location of their optimal climate conditions.

Animals that migrate may be affected by global warming. The success of migration depends upon the right places being available at the right times of year, with the right food needed to complete the migration. If the links in the chain of areas vital to migration are damaged, migrants' survival is threatened. In the US, Delaware Bay is a vital feeding ground for knots, which feed on the eggs of the horseshoe crab. Eggs are a vital fuel source to sustain the knots in the Arctic. Any changes in the timing and availability of eggs or the arrival of the knots will have an adverse impact on the success of the birds in reaching their destination. Food supplies at staging areas during the spring migration may affect not only a bird. s ability to reach its destination, but also its breeding success, as is the case for some arctic nesting geese.

What does global warming mean for the UK? Scientists believe we will have more dry summers, like the drought of 1995 which cost the UK water, agriculture and insurance industries millions of pounds. Mountain top habitats like those in the Cairngorms may be lost, along with their special birds, such as ptarmigans and dotterels. We could lose some 40 plant species. The threat from sea level rise means that birds that live on the coasts could be threatened. Wintering wildfowl and wading birds on estuaries will be threatened by the loss of intertidal feeding areas, caused by increased erosion of the shoreline, particularly where there are sea defences.

Other common misconceptions

The greenhouse effect is caused because heat cannot escape from the earth because it cannot find the holes in the ozone layer.