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Global WarmingAn introduction to greenhouse gases in the atmosphere and the current information on global warmingUpdated 11 July, 2007
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The Effect
Greenhouse Gases
Carbon Storage
References Blewbury Energy Initiative Contacts Domestic Renewable Options Facts and Figures Legal Matters Reducing Energy Use Background Information: Energy Links Global Warming Green Energy Hydrogen as a Fuel Saving Energy Main Blewbury Site |
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There is no doubt that the amount of Carbon Dioxide in the Air is increasing, and there is almost universal agreement among scientists that this will have significant climatic effects. However there are differences of opinion on the magnitude and timing of the effects.
The climate of the earth results from a complex set of interactions largely driven by the heat from the sun. Over geological time, temperatures have varied, sometimes warmer and sometimes colder, while the composition of the atmosphere has also altered, and animals have adapted, moved or become extinct. The more rapid changes in climate, which have been ascribed to the impact of enormous meteors, or of vast volcanic eruptions, have been associated with widespread extinction. However the last 4000 years have been a period of relative stability. This is likely to change because of the impact of human ingenuity and our invention of electricity, motor cars, aeroplanes, central heating, refrigeration, air conditioning, televisions, computers and other ways of using energy - because we are using of fossil fuels to generate this energy.
The climate of the world is a result of so many different complex interactions that it is difficult to predict exactly what the effect of adding greenhouse gases to the atmosphere will be. However nearly all scientists studying the problem agree that the effect will be an increase in average temperatures, and a rise in sea level. There has been a significant rise in average temperatures over the last 100 years of over 0.5 of a degree Celsius.
The rise in temperature is the easiest effect to predict, and the predictions of mean global temperature made by the Intergovernmental Panel on Climate Change in 1991 have proved very accurate. No study which ignores the effect of greenhouse gases has predicted or explained the considerable rise in temperatures since 2000. Considerable changes in precipitation are also expected with the Middle East and China losing significant rainfall, while other locations will see more rainfall.
It is possible that this was partly caused by other effects than that of greenhouse gases - for example changes in the sun or the long term cyclic changes in the earth's orbit. However at the least, causing a significant change in the composition of the atmosphere is a risky thing to do - and it is believed that the current level of greenhouse gases has not been seen for 200 million years. Part of the increase is probably from human actions other than the burning of fossil fuels, for example by a reduction in the size of forests, which absorb carbon dioxide. However the fact that we are generating significantly more greenhouse gases from fossil fuels than the observed increase of these gases in the atmosphere means that this is likely to be the major cause.
There is a general consensus that the average temperature of the earth will rise, but estimates of the size of the rise differ. The effects will also be different in different parts of the world. Estimates are in the range of 1.4 to 5.8 degrees Celsius over the next 100 years. However it is possible to predict even worse outcomes. For example most models of the world used for climate prediction assume that the current ice sheets remain in place as they thin. This is because predicting ice movements is very difficult. In fact we are seeing major losses of sea ice in the Arctic and Antarctic. This leads to more warming as the ice has reflected the sun's rays into space but the ocean absorbs the light and converts it to heat.
Changes in temperature are likely to be associated with other changes. For example hurricanes are stronger over warmer seas. An estimate for Northern Europe is for wetter winters and hotter drier summers. The predicted effects are most serious for tropical countries. In some places, such as central Spain, a rise of average temperature of 7 degrees Celsius is predicted.
The average depth of the oceans is 4 kilometres. The expansion if the sea temperature rises by 1 degree Celsius is complicated. Water actually contracts as it warms from zero to 4 degrees, and the deep sea is at about this temperature. However the higher levels of the sea reach higher temperatures and will expand if they get warmer. Sea levels will also rise if glaciers melt, as they are doing over much of the world.
There is a general consensus that the sea level has risen about 15 to 20 cm over the last 100 years, and is still rising by about 2 mm per year, to put on another 20 cm in the next 100 years. Sea rise would be much more dramatic, nearly 60 metres, if the Antarctic Ice Cap was to melt. However any rapid thaw there is thought very unlikely as the surface of this ice is usually well below freezing point.
A rise of even 20 cm in sea level is serious for low lying countries and cities such as New Orleans.
Greenhouse gases are so named because they have an effect on the world similar to that of the glass of a greenhouse. They let the sun's rays in to warm the earth, but they then absorb some of the infrared energy being radiated out again, and radiate some of this back to earth again.
The existence of greenhouse gases in the atmosphere is very desirable. Without this effect the world would be very much colder. In addition without Carbon Dioxide in the atmosphere trees and plants could not grow.
The main greenhouse gases are Carbon Dioxide, Methane, Nitrous oxide and Fluorocarbons such as CFC. Carbon Dioxide is by far the most abundant but the other gases have much greater effect at a given concentration. Since 1750 the concentration of Carbon Dioxide in the atmosphere has increased by about 30%. The amount of Methane has increased by 150%, that of Nitrous oxide by 16%, and all the Fluorocarbons are new. The following diagram illustrates the relative greenhouse effect of the different gases at present.
The recent increase in Carbon Dioxide is estimated as follows.
The source of these increases is almost certainly human activity. The human input of Carbon into the atmosphere is illustrated below:
The burning of fossil fuels has also increased the density of very small particles in the atmosphere - so called aerosols. These have an overall cooling effect as they block and reflect incoming sunlight. In the 1970s it was thought that this effect might more than cancel out the effect of the greenhouse gases.
Carbon is a key element for all life. Only a very small proportion of the world's carbon is in the atmosphere.
Carbon is released into the atmosphere in several ways.
The Carbon balance may also be affected by other activities. About half the volume of the world's plants are in the tropical forests. As these are cleared, the amount of carbon being removed from the atmosphere each year is likely to decrease. Fishing is also reducing fish and other life in the sea, which is likely to reduce the amount of carbon being trapped in sediment.
The US Environmental Protection Agency provides a comprehensive summary of the current state of scientific thinking on global warming. This is used for the expected temperature and sea level rises. It also provides climate predictions for different parts of the world.
The Intergovernmental Panel on Climate Change provides a variety of comprehensive reports.
The wikipedia also provides a comprehensive coverage.
Energy Statistics from this US Government web site cover the world in some detail.
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