3.1 Fossil fuels
So what are the principle energy systems used by humanity at present, and how sustainable are they?
Until quite recently, human energy requirements were modest and our supplies came either from harnessing natural processes such as the growth of plants, which provided wood for heating and food to energise human or animal muscles, or from the power of water and wind, used to drive simple machinery.
But the nineteenth and twentieth centuries saw a massive increase in global energy use, based mainly on burning cheap and plentiful fossil fuels: first coal, then oil and natural gas. These fossil fuels now supply over 80 per cent of the world's current energy consumption (see Box 1).
Box 1: World Primary Energy Consumption
The population of the world rose nearly four-fold during the twentieth century, from 1.6 billion in 1900 to approximately 6.1 billion in 2000. However, world primary energy use increased at a much faster rate. Between 1900 and 2000, it rose more than 10-fold (Figure 9).
For most of the nineteenth century the world's principal fuel was firewood (or other forms of traditional 'bioenergy'), but coal use was rising fast and by the beginning of the twentieth century it had replaced wood as the dominant energy source. During the 1920s, oil in turn began to challenge coal and by the 1970s had overtaken it as the leading contributor to world supplies. By then, natural gas was also making a very substantial contribution, with nuclear energy and hydro power also supplying smaller but significant amounts.
As Figure 10 shows, total world primary energy use in 2000 was an estimated 424 million million million joules, i.e. 424 exajoules, equivalent to some 10,100 million tonnes of oil.
Total consumption in 2000 was 424 exajoules, equivalent to just over 10,000 million tonnes of oil. The average rate of consumption was some 13.4 million million watts (13.4 terawatts). Note that the actual amounts of electricity produced by nuclear and hydro power were almost the same, but due to a statistical convention in the definition of primary energy, the nuclear contribution is multiplied by a factor of 3.
By the year 2000, oil was still the largest single contributor to world supplies, providing about 35 per cent of primary energy, with gas and coal supplying roughly equal shares at around 21–22 per cent, nuclear providing nearly 7 per cent and hydropower 2 per cent. In 2000, traditional biofuels still supplied an estimated 11 per cent, while more modern forms of 'bioenergy' provided around 2 per cent, with other 'new renewables' like wind power contributing a very small (though rapidly growing) fraction of world demand,
On average, world primary energy use per person in 2000 was about 70 thousand million joules (70 gigajoules), including non-commercial bioenergy. This is equivalent to about one and two-thirds tonnes of oil per person per year, or about 5 litres (just over one Imperial gallon) of oil per day.
But this average conceals major differences between the inhabitants of different regions. As Figure 12 illustrates, North Americans annually consume the equivalent of about 8 tonnes of oil per head (about 20 litres per day), whereas residents of Europe and the former Soviet Union consume about half that amount, and the inhabitants of the rest of the world use only about one-tenth.
World consumption per person has shown almost no growth over the past 20 years. North American consumption per capita is more than twice that of Europe and the former Soviet Union, and almost 10 times the level in the Rest of the World. Note that these figures include only commercially traded fuels (i.e. they exclude traditional biofuels).
Fossil fuels are extremely attractive as energy sources. They are highly concentrated, enabling large amounts of energy to be stored in relatively small volumes. They are relatively easy to distribute, especially oil and gas which are fluids.
During the twentieth century, these unique advantages enabled the development of increasingly sophisticated and effective technologies for transforming fossil fuel energy into useful heat, light and motion; these ranged from the oil lamp to the steam engine and the internal combustion engine. Today, at the beginning of the twenty-first century, fossil fuel-based systems reign supreme, supplying the great majority of the world's energy.
The fossil fuels we use today originated in the growth and decay of plants and marine organisms that existed on the earth millions of years ago. Coal was formed when dead trees and other vegetation became submerged under water and were subsequently compressed, in geological processes lasting millions of years, into concentrated solid layers below the earth's surface. Oil and associated natural gas originally consisted of the remains of countless billions of marine organisms that slowly accreted into layers beneath the earth's oceans and were gradually transformed, through geological forces acting over aeons of time, into the liquid and gaseous reserves we access today by drilling into the earth's crust. The fossil fuels are composed mainly of carbon and hydrogen, which is why they are called hydrocarbons.
Coal was the fuel that powered the industrial revolution. Its combustion produces relatively large amounts of carbon dioxide (CO2) compared with other fuels. It also results in particulates (soot), and sulphur dioxide emissions, though these can be reduced by various techniques. The use of coal in UK homes and industry has now been largely superseded by natural gas, but it is still used for electricity generation. Huge world-wide coal reserves remain, enough for several hundred years' use at current rates.
Oil is the world's leading energy source. Its high energy density and convenience of use are particularly advantageous in the transport sector, where it is the dominant fuel. Oil combustion produces less CO2 per unit of energy released than burning coal, but more CO2 than burning natural gas. Proven world oil reserves are sufficient for about 40 years of use at current rates.
Natural gas combustion produces significantly lower CO2 emissions per unit of energy than the combustion of other fossil fuels. Emissions are also free from sulphur dioxide or particulates. The relative cleanliness and convenience of natural gas have made it the preferred fuel for heating and, increasingly, for electricity generation in Western Europe. Proven world gas reserves are sufficient for about 60 years of use at current rates.
Since the fossil fuels were created in specific circumstances where the geological conditions were favourable, the largest deposits of oil, gas and coal tend to be concentrated in particular regions of the globe (see Figure 16a, b, and c) – although less appreciable deposits are remarkably widespread. The majority of the world's oil reserves are located in the Middle East and North Africa, while the majority of our natural gas reserves are split roughly equally between the Middle East/North Africa and the former Soviet Union (BP, 2002). Although coal deposits are rather more evenly spread throughout the world, three-quarters of world coal reserves are concentrated in just four countries: Australia, China, South Africa and the United States of America (United Nations, 2000; BP, 2002) (Figure 16c).
Although human society now consumes fossil fuels at a prodigious rate, the amounts of coal, oil and gas that remain are still very large. One simple way of assessing the size of reserves is called the reserves/production (R/P) ratio – the number of years the reserves would last if use continued at the current rate.
Coal has the largest R/P ratio. Present estimates suggest the world has more than 200 years' worth of coal left at current use rates. For oil, current R/P estimates suggest a lifetime of about 40 years at current rates. For gas, the R/P ratio is somewhat higher, at around 60 years (BP, 2002) (Figure 17).
Fossil fuel reserves/production ratios need to be interpreted with great caution, however. They do not take into account the discovery of new proven reserves, or technological developments that enable more fuel to be extracted from deposits or improve the economic viability of 'difficult' deposits.
Despite these developments, it seems likely that, at least in the case of oil from conventional sources, world production will reach a peak in the first decade of this century. From then on, although vast quantities of conventional oil will still remain, the resource will be on a declining curve (United Nations, 2000; Campbell and Laherrere, 1998). This seems likely to lead to increased instability and potential for conflict as the twenty-first century proceeds.
The massive use by our society of coal, oil and gas has, literally, fuelled enormous increases in material prosperity – at least for the majority in the industrialised countries. But it has also had numerous adverse consequences. As already mentioned, these include air and water pollution, mining accidents, fires and explosions on oil or gas rigs, conflicts over access to fuel resources and, perhaps most profoundly, the global climate change that is likely to be the result of increasing atmospheric carbon dioxide concentrations caused by fossil fuel combustion (see Box 2).
Box 2: The Greenhouse Effect and Global Climate Change
The greenhouse effect in its natural form has existed on the planet for hundreds of millions of years and is essential in maintaining the Earth's surface at a temperature suitable for life. Without it, we would all freeze.
The sun's radiant energy, as it falls on the earth, warms its surface. The earth in turn re-radiates heat energy back into space in the form of infra-red radiation. The temperature of the earth establishes itself at an equilibrium level at which the incoming energy from the sun exactly balances the outgoing infra-red radiation.
If the earth had no atmosphere, its surface temperature would be approximately minus 18°C, well below the freezing point of water. However our atmosphere, whilst largely transparent to incoming solar radiation in the visible part of the spectrum, is partially opaque to outgoing infra-red radiation. It behaves in this way because, in addition to its main constituents, nitrogen and oxygen, it also contains very small quantities of 'greenhouse gases'. Put simply, these enable the atmosphere to act like the panes of glass in a greenhouse, allowing the sun to enter but inhibiting the outflow of heat, so keeping the earth's surface considerably warmer than it would otherwise be. The average surface temperature of the earth is in fact around 15°C, some 33°C warmer than it would be without the greenhouse effect.
The most important greenhouse gases are water vapour, carbon dioxide and methane, though other gases such as the chlorofluorocarbons (CFCs) also play significant but lesser roles.
Water vapour evaporating from the oceans plays a major part in maintaining the natural greenhouse effect, but human activities have very little influence on the vast processes through which water cycles between the oceans and the atmosphere.
Carbon dioxide (CO2) is also primarily generated by natural processes. These include the process of respiration, in which organisms 'breathe out' carbon dioxide; and the emissions of CO2 that occur when organisms die and the carbon compounds of which they are composed decay. But since the industrial revolution, the burning of fossil fuels by humanity has been adding substantial quantities of CO2 to our atmosphere. The fossil fuels are essentially compounds of carbon and hydrogen. Coal consists mostly of carbon, the chemical symbol for which is C. Natural gas, the chemical name for which is methane, consists of carbon and hydrogen. Each carbon atom is surrounded by four hydrogen atoms, so in chemical shorthand its symbol is CH4. Oil is a more complex mixture of many different hydrocarbon molecules. When any of these fuels is burned, carbon dioxide is produced, along with water.
The concentration of CO2 in the atmosphere in pre-industrial times was around 280 parts per million by volume (ppmv) but levels have been steadily rising since then, reaching some 360 ppmv in 2000.
The other main greenhouse gas, methane, is given off naturally when vegetation decays in the absence of oxygen – for example, under water. However various human activities, including increasing rice cultivation, which causes methane emissions from paddy fields, and leaks of fossil methane from natural gas distribution systems, have caused the levels of methane in the atmosphere to increase sharply. Concentrations have risen from about 750 parts per billion by volume (ppbv) in pre-industrial times to around 1750 ppbv in 2000.
These additional emissions of carbon dioxide and methane are the main causes of the so-called anthropogenic – that is, human-induced – greenhouse effect. Unlike the operation of the natural greenhouse effect, which is benign, the anthropogenic greenhouse effect is almost certainly the cause of a global warming trend that could have very serious consequences for humanity. Though a small minority dissents, the majority of scientists now believe that the anthropogenic effect, acting to enhance the natural processes, has already caused the mean surface temperature of the earth to rise by about 0.6°C during the twentieth century (Intergovernmental Panel on Climate Change, 2001). Moreover, if steps are not taken to limit greenhouse gas emissions, atmospheric CO2 levels will probably rise by 2100 to between 540 and 970 ppmv (depending on the assumptions made). These levels would be between two and three times the pre-industrial CO2 concentration, and would be likely to lead to rises in the earth's mean surface temperature of between 1.4 and 5.8°C by the end of the century. Such temperature rises would be unprecedented since the ending of the last major Ice Age, more than 10,000 years ago.
These temperature rises would be very likely to result in significant changes to the earth's climate system. Such changes would probably include more intense rainfall, more tropical cyclones, or long periods of drought, all of which would disrupt agriculture. Moreover, ecosystems might be damaged with some species unable to adapt quickly enough to such rapid changes in climate.
In addition, due to thermal expansion of the oceans, sea levels would be expected to rise by around 0.5 metres during the twenty-first century, sufficient to submerge some low-lying areas and islands. In the longer term, further sea level rises would result if the Antarctic ice sheets were to melt significantly.