Another problem caused by humans is as a result of the increase in demand for extensive farming throughout the world. In order for crops to be grown trees have to be cut down and I have explained the effects of this already. As well as deforestation, agriculture has also called for the use of fertilisers and pesticides. Fertilisers (and raw sewage) cause eutrophication. Eutrophication occurs when farmers are forced to increase the productivity of their crops and so use fertilisers. These fertilisers run off the land into the waterways and increase the levels of nitrates and phosphates in the water. In turn this increases the speed of the algae growth. Then, if the algae dies it is decomposed by aerobic decomposers and these use up oxygen present in the river or stream. This causes the death of aerobes in the water and brings about anaerobic conditions. The living algae can also damage the rivers by blocking out the light to the bottom dwelling plants and so restricting their photosynthesis. Pesticides also have an effect on the environment and one of the most harmful ones used is DDT. This is a persistent insecticide. This means that it does not break down and if it gets into a creature’s body it will not go away easily. DDT has especially affected the peregrine falcon. The numbers of the birds dramatically fell and it was only in around 1970 that it was realised that the pesticide was so harmful. Fortunately, its use has been stopped in the UK and Europe, but it is still being used in less developed countries as without it too many people would die of malaria or starve.
Acid rain is also having an increasingly large effect on the environment. Acid rain is formed when oxides of sulphur and nitrogen combine with atmospheric moisture to yield sulphuric and nitric acids, which are then carried variable distances before being dropped as acid rain. The pollution may also be deposited in the form of snow or fog or even as dry precipitation. The problem originated from the advent of the industrial revolution and has been growing ever since. The severity of its effects has long been recognised in local settings, as exemplified by the periods of acid smog in heavily industrialised areas. But, unfortunately, the widespread effects that acid rain has has only been recognised in recent years. One large area that has been studied extensively is northern Europe, where acid rain has eroded structures, injured crops and forests, and threatened or depleted life in freshwater lakes. In 1984, for example, environmental reports indicated that almost half of the trees in Germany's Black Forest had been damaged by acid rain. This form of pollution has also particularly affected the northeastern United States and eastern Canada; damage has also been detected in other areas of these countries and other regions of the world. The main causes of air pollution are thought to be industrial emissions, but industries have tended to challenge such assessments, and because of the cost of pollution reduction, governments have tended to support this attitude. To attempt to tackle the problem the United Nations Economic Commission for Europe produced the first Sulphur Protocol. This called for a70% reduction in sulphur emissions and the UK achieved this as well as implementing compulsory catalytic converters for all new cars.
But the most discussed problem caused by humans is probably global warming. Global warming is an increase in the earth's temperature due to the use of fossil fuels and other industrial processes leading to a build-up of "greenhouse gases" (carbon dioxide, methane, nitrous oxide, and chlorofluorocarbons) in the atmosphere. It has been known since 1896 that carbon dioxide helps stop the sun's infrared radiation from escaping into space and thus functions to maintain the earth's relatively warm temperature (this is called the greenhouse effect). The question is whether the measurably increasing levels of carbon dioxide in the atmosphere over the last century will lead to elevated global temperatures, which could result in coastal flooding (through a rise in sea level) and major climatic changes, and have serious implications for agricultural productivity. Since 1850 there has been a rise in temperature of around 1 degree Celsius but this could just could be part of a natural fluctuation. As a result of this doubt there is no solid evidence that global warming is a major threat, although many scientists have urged immediate action.
Connected to the problem of Global warming is the greenhouse effect. It is the term for the role the atmosphere plays in helping warm the earth's surface. . The atmosphere is largely transparent to incoming short-wave solar radiation, which is absorbed by the earth's surface. Much of this radiation is then re-emitted at infrared wavelengths, but it is reflected back by gases such as carbon dioxide, methane, nitrous oxide, halocarbons, and ozone in the atmosphere. This heating effect is at the root of the theories concerning global warming.
Another problem is the ozone layer. This is, in nature, a region of the atmosphere from 19 to 48 km above the earth's surface. Ozone concentrations of as much as 10 parts per million occur in the ozone layer. The ozone forms there by the action of sunlight on oxygen. This action has been taking place for many millions of years, but naturally occurring nitrogen compounds in the atmosphere apparently have kept the ozone concentration at a fairly stable level. Concentrations this great at ground level are dangerous to human health; but because the ozone layer protects life on earth from the full force of the sun's cancer-causing ultraviolet radiation, it is critically important. Therefore, scientists were concerned when they discovered, in the 1970s, that certain chemicals called chlorofluorocarbons, or CFCs (compounds of fluorine) - long used as refrigerants and in aerosol spray cans-posed a possible threat to the ozone layer. Released into the atmosphere, the chlorine-containing chemicals rise and are broken down by sunlight, resulting in the chlorine reacting with and destroying ozone molecules. For this reason, the use of CFCs in aerosols has been banned in many countries. Other chemicals, such as bromine halocarbons, and nitrous oxides from fertilisers, may also attack the ozone layer. Scientists discovered that the largest problem lay in Antarctica, where a periodic loss of ozone was located. A similar problem was found in the Arctic and as a result the “Montreal Protocol” was signed by 49 countries. Within it the countries declared that they would phase out the use of CFCs by the end of the century. In addition to this NASA launched the 7-ton Upper Atmosphere Research Satellite. This measures ozone variations at different altitudes, and is providing the first complete picture of upper atmosphere chemistry.
