Human beings manipulate the weather both intentionally and unintentionally. So far, people have done little of the former but, potentially, a great deal of the latter. A growing number of scientists fear that our industrial activity will change weather and climate with disastrous effects. Most attempts to make a short-term change in the weather have been confined to ‘seeding’ clouds by scattering crystals of silver iodide into them. This can cause rain to fall. The crystals act as ‘condensation nuclei’ on which tiny water droplets are formed. These then collide, forming larger drops that fall as rain. This technique has also been used to try to modify violent tropical storms. When forecasters predicted that a depression might grow into a hurricane, planes were sent to drop the crystals over the clouds near the centre of the wind system. The rain that was triggered fell through up welling warm air currents, cooled then and possible prevented the storm clouds from building up to a dangerous extent.
Trees affect the weather in several ways. At the ‘micro’ level they can act as windbreakers, preventing valuable topsoil from being blown away. Trees also give out large amount of water vapour from their leaves and this can moisten the air sufficiently to increase the rainfall locally. On the arid eastern side of the Mediterranean island of Crete, the Greek government is planting trees to increase rainfall and thus promote agriculture. There is a long-term reason for planting trees. Like all plants they absorb carbon dioxide, this is one of the major gases involved in the ‘greenhouse effect’, which may be causing the world to warm up.
The greenhouse effect is the trapping of the Earth’s heat by the atmosphere, which keeps the Earth about 33 degrees Celsius hotter than it would otherwise be. The glass of the greenhouse acts similarly – it lets visible light through to warm the interior of the greenhouse, but traps the heat radiation that tries to escape. The various gases making the atmosphere contribute to the greenhouse effect by different amounts. The most important is carbon dioxide, which is released when any fossil fuel (oil, wood, gas) is burned to provide heat or light, and when trees are burned to clear forests. To counteract all the carbon dioxide produced by burning fossil fuels, it has been calculated that an additional seven million square kilometres of forests would need to be planted.
There are several other important greenhouse gases. Methane is released in natural gas fields and by bacterial action in the ever-growing number of rice paddy fields. Nitrous oxide is given out in car exhausts. Chlorofluorocarbons (CFC’s) are used in spray can propellants and refrigerator coolants, and for cleaning electrical components. The extra amounts of these gases put into the atmosphere by human activity man have the effect of raising the temperature of the Earth – possibly by 1 degree by the year 2025, although scientific opinion is very divided over this. Such a temperature rise could have devastating effects. The weather might change radically. Deserts could spread further north and south from the equator. The US grain belt might become a dustbowl and monsoons of the Indian subcontinent and Far East could become more intense.
CFC’s harm the atmosphere’s ozone layer as well. Ozone gas is distributed throughout the atmosphere, but is found in the greatest concentrations 20-30 km up. The ozone layer screens the Earth from much of the ultraviolet present in sunlight. Whenever the weather is sunny, many people take the opportunity to sunbathe and get a tan. It is the ultraviolet light the causes tanning, but it ages the skin, and in excess it causes skin cancer. In the mid-1980’s a hole (actually a reduction in concentration) was discovered in the ozone layer over the Antarctic. Research has shown that ozone levels in the atmosphere are down, worldwide. This is very se rious, since a one per cent fall in ozone level could cause a five per cent rise in skin cancer. International efforts are being made to avert this weather threat, in 1987 the Montreal Protocol on substances that deplete the ozone layer – an agreement to reduce the amount of harmful CFC’s released – has been signed.