| Prior to the 1970s scientists sincerely believed that the earth was heading inexorably towards the beginning of the next Ice Age because temperatures in the 1940s 1950s and 1960s appeared to be falling. |
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| In the early 1980s however all this changed as temperatures were observed to be rising again. Theories began to emerge not long after that anthropogenic (human-induced) carbon emissions were in fact warming the atmosphere and that this was a serious problem. Twenty years on we are in a situation where the subject is as much a political one as it is scientific. |
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| Unfortunately the way that weather systems re-distribute energy around the world is extremely complex and frankly, though scientists have discovered a lot about the past, with so many variables they are still not able to make accurate predictions for the future. However here are some of the things that we do know. |
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| We are able to accurately infer how temperatures have changed in the past from ice core samples, lake and ocean sediments and tree rings. Examination of the isotope ratios in these samples gives a picture of climate over hundreds of thousands of years. We know for example that temperatures vary sufficiently every 100,000 years or so that much of Northern Hemisphere landmass is covered by ice sheets several kilometres thick; and that the last time this happened was just over 20,000 years ago. Between these glacial maximums are interglacial periods, one of which we are experiencing at the moment. Within the next few thousand years however this is due to end. |
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| It is generally accepted that these long-term fluctuations are caused by regular variations in the earth's orbit around the sun and by the fact that the earth actually wobbles on its axis. Luckily these changes are broadly predictable. But there are others that are not. Volcanic eruptions and variations in the sun's output also have an effect. |
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| Just over 1,000 years ago the earth was a little warmer than it is today; it was during this time that the Vikings began to colonise Greenland. However from about 1500 onwards average temperatures in the Northern Hemisphere dropped by a degree or two with the onset of the period commonly known as the 'Little Ice Age'. The coldest period was around the end of the seventeenth and beginning of the eighteenth century when there were many years that the River Thames in London froze over in winter; the last time this happened was in 1814. While in the United States during the winter of 1780 the whole of New York Harbour from Staten Island to Manhattan was frozen over for more than a month. |
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| Most scientists agree that the main cause of the 'Little Ice Age' was a fall in the energy output of the sun. It is still not clear why this reduction in solar activity happened. Astronomers observing the sun at the time noticed that between 1650 and 1700 hardly any sunspots, which are a major indicator of activity, were visible. |
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| During the seventeenth century there were also a number of very large volcanic eruptions in South America, South East Asia and the Japan, which probably contributed to the cooling. Large volcanic eruptions inject sulphur dioxide into the stratosphere, which reflects sunlight back into space. This process was witnessed in 1991 when nearly 20 million tonnes of sulphur dioxide were ejected by the eruption of Mount Pinatubo in the Philippines. The result was a reduction in global average temperatures by 0.4°C for 2 years before the aerosols finally dispersed. |
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| So what of the theory that the burning of massive amounts of hydrocarbon fuels, mainly oil, coal and natural gas by an overpopulated world is now possibly contributing to irreversible global warming? |
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| Despite what you may read in the press or hear from politicians and environmentalists the issue is by no means clear-cut. It is true that the world is getting warmer and that the level of carbon dioxide in the atmosphere is rising. The latest report from the Intergovernmental Panel on Climate Change (IPCC)* states that temperatures in eleven out of the last twelve years have risen to higher levels than have been seen in any of the years since records began in the middle of the nineteenth century. The IPCC also reported that the amount of carbon dioxide in the atmosphere is higher today than at any time in the last 650,000 years. |
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| But are these two phenomena related? And if so are we to blame? |
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| The average global temperature of the earth depends on a balance between the amount of energy that it receives from the sun and the amount that is radiated back into space. Temperature stability also requires that the energy received be equal to the energy lost. If more is received than lost the earth will warm, and if more is being lost than the received the earth will cool. Stability is generally restored by natural forces and a new temperature equilibrium is established. The atmosphere plays an important role in this process. If there were no atmosphere the earth would be very much colder because all the energy absorbed during the day would be radiated back into space at night. |
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| The regulating role of the atmosphere has been compared to the glass in a greenhouse, which gave rise to the expression 'greenhouse effect'. The earth's atmosphere keeps the earth warm in the same way that glass keeps a greenhouse warm by allowing in short-wave radiation and preventing long wave radiation from escaping. However the composition of the atmosphere is important since different gases react in different ways. Nitrogen and oxygen, which make up 99% of the atmosphere, hardly absorb any of the sun's radiation at any wavelength. But within the remaining 1% are gases like carbon dioxide methane and water vapour that are powerful greenhouse gases that prevent long wave thermal radiation from escaping back into space. |
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| Water vapour is by far the most important greenhouse gas accounting for at least 90% of the total greenhouse effect. It exists in the atmosphere in uncondensed and condensed states; when water vapour condenses it forms clouds. Most of us will have experienced how much colder it can be on a clear night than a cloudy one. Clouds provide a blanket that absorbs energy radiating off the land while on a clear night there is less to prevent that energy from escaping into space. But even when there are no clouds there is still always water vapour in the atmosphere absorbing radiation. Ironically clouds have the opposite effect on the incoming short wave radiation; they reflect the sun's energy back into space during daytime. |
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| Carbon dioxide accounts for most of the rest of the total greenhouse effect with other gases making up the balance. It may come as a surprise to many to discover how little of the carbon dioxide in the atmosphere is actually produced by burning fossil fuels. Oceans are by far the largest source; carbon dioxide dissolves directly in seawater especially at low temperatures and is carried to the ocean depths by sinking currents. Eventually it is released back into the atmosphere when it rises to the surface again, hundreds of years later in warmer tropical waters. The oceans store about 50 times more carbon dioxide than the atmosphere, and also produce about 20 times as much carbon dioxide each year as the burning of fossil fuels. Others sources of carbon dioxide include the decomposition of vegetable and organic material, and volcanoes, again in quantities many times more significant than produced by fossil fuels which only account for 3% of the total. |
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| If less than 10% of the greenhouse effect is caused by carbon dioxide and less than 5% of this comes from burning fossil fuels why do scientists believe that we are causing climate change? |
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| In theory it is true to say that, all other things being equal, increasing the level of greenhouse gases in the atmosphere will cause the earth to warm up. However ice core records for the past 350,000 years also show that carbon dioxide in the atmosphere tends to rise as a result of temperature increases. The additional carbon dioxide comes from the oceans because oceans release carbon dioxide as they warm. So the observed rise in carbon dioxide in the atmosphere today may be the result of rising temperatures not the cause of it. It is a classic chicken and egg situation. |
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| A small number scientists who dismiss the anthropogenic carbon emissions argument, assert that it is the output of the sun and our position and angle of orbit relative to the sun that mainly determines global climate. Clearly this is true with respect to the advance and retreat of continental ice sheets during ice ages and when sunspot activity decreased during the 'Little Ice Age'. However very small scale changes in the sun's output, as advocated by these scientists, are not yet recognised by the mainstream scientific community as influencing shorter-term variations. |
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| If these 'carbon sceptics' were to come up with a credible theory and prove that the increase in temperature had very little to do with our carbon emissions, the climate change industry would probably disintegrate overnight. But in the absence of such proof it looks more likely that anthropogenic carbon emissions are a major contributing factor to Climate Change in the 21st century. |
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| However small in overall terms our contribution to greenhouse gases may be - something in the region of less than one half of one percent - it may only take a small amount to shift the balance. If 20 million tonnes of sulphur dioxide ejected by the eruption of Mount Pinatubo in 1991 cooled the earth by 0.4°C, then the 30,000 million tonnes of carbon dioxide pumped into the atmosphere every year by humans could also make a difference. |
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| *Summary for Policymakers, February 2007 |
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