Another suggested cause of the rise is deforestation. Photosynthesis removes carbon dioxide from the atmosphere and locks up the carbon as organic molecules in plants. However a mature tree which is not growing takes in only about as much carbon dioxide as it gives out (as a waste product of respiration), so just cutting down trees does not affect the carbon dioxide levels. It is not until these trees are burnt, or decay, that the carbon they contain is released into the atmosphere. But even then the land on which the trees are growing is usually rapidly recolonised by plants or planted with crops, which grow and photosynthesise, probably taking in as much or more carbon dioxide then the trees were doing. Moreover, recent measurements of the rate of deforestation in many areas of the world suggest its extent was previously overestimated. It is important to also realise that the land is not the only place where photosynthesis occurs. Seas cover much more of the Earth’s surface than land does and there is much photosynthetic activity by marine life.
The Vostok ice core was also used to estimate the temperatures on Earth at the time air bubbles were trapped. These bubbles were analysed to measure the ratio of two isotopes of oxygen, O and O, which varies with global temperature. The estimate of global temperatures over the last 160 000 years show that the temperature fluctuations closely match the carbon dioxide fluctuations. There is no direct proof that carbon dioxide concentrations directly affect temperatures, it could equally mean that temperatures affect carbon dioxide levels or there is some other factor affecting the both of them.
Other factors which may be relevant include sunspot activity and the variation in the distance of the Earth from the Sun. But because we do know that carbon dioxide acts as a greenhouse gas, most scientists currently accept the hypothesis that carbon dioxide levels do affect global temperatures.
The exact effect of the current rise might have on global temperatures is impossible to predict. It is not known how much of the Earth’s average temperature is likely to rise nor is it known that how such changes would affect particular parts of the Earth. For example, Britain is kept warmer then it otherwise would be by a current of warm water, the Gulf Stream that flows across the Atlantic Ocean. Ocean currents are produced by uneven heating of the Earth’s atmosphere and surface which produce wind and water movements. Just a small change in global temperatures could cause a current suddenly to change direction. If the Gulf Stream no longer flowed past Britain then our climate would become noticeably colder while other parts of the world would become warmer.
We also need to be aware of the ‘natural’ cycles of temperature change which have been taking place on Earth. In the last 850 000 years there have been about nine periods when the ice has advanced from the poles and covered much of North America and Western Europe.. These periods are called Ice Ages and they have alternated with warmer interglacials. At the moment we are in an interglacial which started about 14 000 years ago.
No one knows how global warming would affect the Earth. Many convincing arguments have been put forward suggesting various affects we could expect, but there are so many variables to take into account it is impossible predict just what might happen.
One suggestion is that the rise in temperature could cause melting of the ice caps, which could cause a rise in sea levels. This could have devastating consequences for low-lying countries such as Bangladesh. It could also be disastrous for many other countries as most capital cities lie close to sea level. A rise in sea level sounds a straightforward and reasonable suggestion but even this is controversial; some scientists argue for exactly the opposite effect. With warmer temperatures more water vapour evaporates from the sea; this produces more water vapour in the atmosphere which may increase the snowfall over the poles. More ice would build up in the Antarctic and Artic, while the amount of water in oceans might be reduced.
Temperature changes and an increase in carbon dioxide levels could also affect the rate of plant growth. Several long-term experiments are being conducted to investigate the effect of raised carbon dioxide levels on plants and these are producing mixed results. In increase carbon dioxide levels some species of plants grow faster, some initially grow faster and then slow down, while others actually grow more slowly. These responses vary also when temperatures are raised and they are also affected by the amounts of nutrients in the soil. These early results suggest as different species of plants are likely to respond differently, then the balance of species within plant communities is likely to change if carbon dioxide levels continue to increase and if global warming takes place.
Most people believe that we should attempt to reduce the rate at which carbon dioxide levels are rising. An international summit at Rio de Janeiro in 1992 brought agreement between in many countries to try and do this. Measures that can be taken include reducing the amount of fossil fuels which are burnt; this can be done by using other sources of energy to generate electricity (such as nuclear fuels, wind and water), by making our use of fossil fuels more efficient (such as by using ‘lean-burn’ engines in cars), and by reducing the demand for energy by industry and individuals (such as by installing insulation in homes, and building more energy efficient homes.) Other measures also include reducing the rate of deforestation and planting more trees where possible and also conserving soils, especially peat, which contain large amounts carbon. Some success has already bee achieved, but as always a balance has to be reached between what environmentalists think is needed and what industrialists think is needed.
In conclusion the greenhouse effect is currently very important to life on Earth but is seen to cause many pending problems by global warming in the future. But at present the effects of this are impossible to predict and what specifically is causing it is yet unknown.
