Global Warming has for many years now been an issue for scientists. With, the media reporting headlines such as "Rising concentrations of greenhouse gases may have more serious impacts than previously believed" are we right to believe what they are telling us? In this essay what evidence is available to support this statement, and the mechanisms that lead to global warming. Around 18000 years ago, the earth was in its last known "ice age". Glaciers extended a long way south from the poles well into Europe and North America. The ice was several kilometres thick and covered a vast area of the globe. In present times, glaciers only account for 10% of the earth's surface. Geological surveys have found evidence to support the changing earth's climate, showing the process is both slow and continuous. However, in recent time it has been observed that there has been a marked increase in temperature at the earth's surface. Figure 1 shows estimates of the average northern hemisphere temperature until 1850 after which detailed observations were available to produce the data. It is quite clear on the graph that although there have been both cooler and warmer periods leading up to about 1900, post 1900 there is a marked consistent temperature increase unlike before. Experts for some time have been trying to find some patterns in human and natural activities that might have led to this increase in temperature beyond what they would see as natural fluctuations. Greenhouse gases are those which scientists believe are the main causes of global warming. Naturally our earths atmosphere reflects some incoming radiation back out to space whereas other radiation can pass freely through into the atmosphere to heat the earth. As the earths surface is heated, it radiates the heat back out to the atmosphere. In a time when there were few greenhouses gases scientists believe more of the heat was being radiated back out into space, whereas now the greenhouse gases in the atmosphere are trapping some of the outgoing heat and causing it to be reflected back to earth. The effect was initially thought to be similar to that of a greenhouse; however studies proved it's not exactly like a greenhouse! The main greenhouse gases in our atmosphere today are water vapour, carbon dioxide, ozone, methane, nitrous oxide and CFC's. Most of the greenhouse gases in the atmosphere today have existed here naturally for many years and we rely on them to keep the earth at an average temperature of 15C. If the gases were not present then the earth's surface temperature would be somewhere is the region of -18C, which would be too cold to support a lot of living creatures on the planet. It was not until the western world industrialised that levels of greenhouses gases started to rise rapidly. Carbon dioxide, CO2 is probably the most well known gas that is associated with the global warming, produced as a by product of the burning of fossil fuels such as wood, coal and oil. CO2 is a selective absorber which is able to absorb strongly outgoing infrared radiation from the earth thereby trapping the energy associated with it in our atmosphere. Pre industrialising the amount of CO2 present in the atmosphere was about 280ppm and today levels are about 375ppm. In the last 25 years CO2 levels have risen around 10% and this looks set to continue with more of the world industrialising. Other greenhouse gases such as methane and nitrous oxide have also increase in concentration over the past few years. Both of these gases are also good absorbers of infrared radiation and add to the overall warming of the earth. Methane is given off by coal, natural gas and oil however more naturally it is produced by the decomposition of organic waste and by livestock. Methane is far more effective at trapping heat than CO2. Its level although have risen in recent years have tended to stabilise at around 1750ppb. Nitrous Oxide is given off by both industrial and natural processes. There has been a steady rise of NO 2 over recent years also and this too is adding to the greenhouse effect. Perhaps more worryingly is the effects caused by pollutants created by man in recent years which are not natural greenhouse gases. CFC's were first created during World War Two as propellants. These gases were so useful that after the war they became propellants for whole host of products including domestic products such as deodorants. The use of CFC's has all but been phased out worldwide now, and production at least in the European Union finished around 1996. CFC's had a destructive effect on the ozone layer. In 1991 scientists worldwide conducted a survey on the ozone layer and found that since about 1979 to 1991 the ozone levels reduced by 5-6%. Although this may not sound like a very large amount, it has caused a 12% increase in UV levels in New Zealand between 1990 and 1999. Apart from the human factors which have added to the natural greenhouse gases there are a further number of reasons as to why global warming could be occurring. Plates with glacial features can be found near sea level in Africa which suggests a large shifting of the plates over a period of time. It is highly unlikely that any glaciers ever extended this far from the poles and there is little evidence to support it. Shifting tectonic plates also cause volcanoes to form along with underwater ridges. The speed at which plates move could have an effect on our climate, for instance when the plates are moving rapidly there is likely to be an increase in the rate at which volcanoes both form and erupt. Erupting volcanoes produces greenhouse gases such as CO2, H 2O, SO 2 and N 2 which add to global warming. A secondary effect is that ashes from the eruption which vaporises in the upper atmosphere forming a small dense layer of haze that absorbs and reflects back for many years an amount of incoming radiation from the earth. After the 1991 eruption of Mount Pinatubo scientists predicted a 0.5C fall in global temperatures and in the following three years this happened. Underwater ridges can have an effect also, they can displace large amounts of sea water thus causing sea level rising, along with releasing a whole host of greenhouse gases. The oceans themselves are absorbers of CO2 and have a large part to play in stopping global warming getting out of control. Recent studies, as late as 2005 in Germany have shown that biomass carbon in the oceans represents a way of long term storage. It was shown some years earlier that adding small iron or sulphate particles to the ocean can stimulate the growth of plankton, whereas the natural oceans supply of this has been decreasing over recent times. The increased productivity of the oceans through this may have a positive effect on reducing global warming, as it can be shown that between 10,000 and 100,000 carbon atoms can be "sunk" for each iron atom added to the ocean. However the method should be used with care, as increased carbon levels in the ocean lead to it becoming increasingly acidic. A rise in sea level would mean that more land is covered, and vegetation which is important for the removal of CO2 from the atmosphere. A rise in sea level could be caused by water being displaced by underwater ridges, the melting of glaciers from the increased temperature or also from the increased energy in the atmosphere causing the oceans to expand. Naturally large parts of the earth are rich in vegetation which converts CO2 to O 2; however deforestation is happening on a large scale worldwide everyday which means that even though we have an increasing amount of CO2 being released into the atmosphere the amount of vegetation available to convert it back to O 2 is being reduced. At current rates, if deforestation continued then the "world's rain forests will vanish within 100 years"2. Soil is a good storage for carbon, about twice as good as storage above ground. Carbon levels in the ground have lowered over recent years are more has been released to above ground storage. In grassy areas and unploughed fields the storage of carbon means it can remain un-oxidised for many years however with fields being dug for development and agricultural systems being put in place much of the carbon has been released. When it becomes oxidised as CO2 it becomes a greenhouse gas adding to the global warming effect. The changing of the earths orbit is also believed to have some effect on global temperatures. The effect, known as Milankovitch theory can be summed up as that as the earth moves through space, "three separate cyclic movements combine to produce variations in the amount of solar energy that falls on the earth"3. At present we are in a period that the closest time to the sun is January, and this alters around every 100,000 years. This ties in well with evidence found that suggests that ice sheets peak around every 100,000 years or so. Smaller peaks were noticed at around 23,000 and 11,000 years which are also significant in the Milankovitch theory, however levels of CO2 at this time are considerably lower than today and therefore the CO2 cannot be the single reason for climate change. Variation in the solar output can also have an effect on our climate. The sun's output has been linked by astronomers to sun spots, which peak cyclically every 11 years. Although it may sound small, there is a 0.1% different between periods of maximum and minimum output. Ozone levels increase at the peaks, absorbing the extra UV radiation but this has a further effect of warming the stratosphere and thus creating temperature gradients in troposphere, which seem to have a circulatory effect on the Hadley cell for instance. There is little firm evidence to support the fluctuation in solar output with climate change because there is simply not enough data available at present and it unlikely there will such data available for many years to come. From the small area I have been able to cover about this vast subject, it is obvious to see that the earth is undergoing a constant climate change. The earth was once been much warmer and similarly once much colder than it is today. During the last 200 years the earth surface temperature has steadily risen and there is firm evidence unlike before of this. Actual recording of the climate rather than geological studies can give us much more confidence of the change we are undergoing, and in a developed world it is something to be concerned about. 