By climate we mean the time averages of the many variables describing the condition of the atmosphere: the temperature, purity and humidity of the air, the rainfall, the strength of the winds and storms, and the clouds, mists and fogs. All these are constantly changing, and we can describe it by taking averages for a local region or for the whole world.
It is useful to distinguish between the climate and the weather. The weather is what we experience from day to day; it is always changing and impossible to predict more than a few days in advance. This is because the atmosphere is what is called a chaotic system; very small changes can have large effects. Climate refers to averages of the weather taken over periods of several years. These averages smooth out the fluctuations of the weather, and are what we are concerned with here. It is not always easy to detect long-term changes in a fluctuating quantity, and this accounts for the difficulty of studying climate change.
During the last two million years studies of sediments, ice cores and cave deposits have shown that there have been a succession of ice ages due to changes of the earth's orbit. In the colder periods most of Europe was covered by ice sheets up to 3 km thick. We are now in a relatively warm period, when there are large temperature differences between the hot and humid equator and the ice-bound poles. This temperature difference of about 70°C accounts for much of our climate, as warm air from the equator moves towards the poles, stirring up hurricanes, typhoons and monsoons. Superposed on the larger changes have been shorter climatic fluctuations that have strongly affected human history (Maslin 2004).
The changes in the climate over the last millennium have been found by studying tree rings, ice cores and corals. The results are consistent, which confirms their accuracy. During the last forty years more extensive data have been obtained by instruments carried aloft by balloons and by satellites. The most important long-term effects are changes in the average temperature and in the sea level. The average temperature has remained much the same for the first 900 years of the millennium, and since then has risen on the average by about 0.005°C per year. In the same period the sea level has risen by between 4 and 14 cm, the uncertainty being due to the rising or sinking of the land.
Climate is one of the determining features of civilisation, so any change in the climate can have momentous consequences. The importance of climate for civilisation is discussed in Section 7.2.
It is becoming increasingly evident that our present energy policies may be having a disastrous effect on the world climate. Already these have been estimated to cause 160,000 deaths per year due to heat waves, flooding and crop damage (Physics World, July 2007, p. 25). This is attributed to the carbon dioxide and other gases that are released when fossil fuels are burned. The chemical reaction that releases heat is the combination of the carbon in the fuel with oxygen in the air to produce carbon dioxide, and this is released into the atmosphere. The only way to avoid this is by a variety of chemical processes referred to as carbon sequestration described in Section 2.1. This process is so expensive that for the foreseeable future all the carbon dioxide from burning fossil fuels will be emitted into the atmosphere. As described in Section 7.3, there is increasing evidence that this is seriously affecting climate throughout the world.
Fossil fuel power stations also release many poisonous substances into the atmosphere that eventually fall to earth as acid rain, killing trees and changing the ecology of rivers and lakes. An IAEA study estimates the emissions from a coal power station to be (in kg/MWh) 830 for carbon dioxide, 2.16 for nitrogen oxides, 0.6 for sulphur dioxide and 0.1 for particulates containing radioactivity (Nuclear Issues 26, No. 8).
The effects of these emissions into the atmosphere are responsible for global warming, as described in Section 7.4. The effects of global warming are described in Section 7.5 and the possibility of catastrophic changes in Section 7.6.
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