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Future energy demand and supply
Future energy demand and supply

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3.1 Fossil-fuel burning and global warming

The amount of carbon dioxide produced by combustion varies from fuel to fuel, depending on its ratio of carbon to hydrogen. Natural gas produces the least carbon dioxide. Burning 1 t of natural gas (e.g. methane, CH4) in a power station releases about 2.75 t of CO2; burning 1 t of petrol (that contains hexadecane, C16H34) in a car engine releases over 3 t of CO2. Coal produces about 20% more, since coal contains between 60% (lignite) and 90% (bituminous) of carbon by mass.

  • Why is the mass of CO2 released greater than that of the fuel burned?

  • Carbon combines with oxygen in proportion to the relative atomic mass of the two elements (C = 12; O = 16) and the relative numbers of atoms in the molecule, so that every 12 t of carbon combines with 32 t of oxygen to produce 44 t of CO2, i.e. burning 1 t of carbon produces 44/12 = 3.67 t of CO2.

Burning fossil fuels emits 5×109 t of carbon into the atmosphere every year.

  • How does the yearly amount of carbon released through burning fossil fuels compare, as a percentage, with that fixed by land plants?

  • Figure 9 showed that 60×109 t of carbon is fixed by plants each year. The carbon flux from burning fossil fuels is approximately 8% of this.

Until the 1960s, the atmosphere was considered to be large enough to absorb large quantities of carbon-based gases and dilute them to harmless levels. It has become clear that this is not the case. Rapid increase in carbon-based atmospheric gases interferes with the energy budget of the Earth itself by enhancing the 'greenhouse effect' (Box 2).

Box 2 The greenhouse effect

All objects emit electromagnetic radiation at wavelengths that depend on their temperature: the hotter an object is, the shorter the wavelength of radiation that it emits. The temperature of the Sun's surface is over 5500 °C, so it emits short-wavelength radiation (Figure 11) in the ultraviolet, visible and near-infrared parts of the spectrum. Much of the Sun's energy output is in the form of visible light.

Figure 11 The origin of the natural greenhouse effect shown schematically. Incoming short-wavelength energy from the Sun is absorbed by atmospheric oxygen, ozone and water vapour but most visible wavelengths reach the Earth's surface. Outgoing longer wavelengths radiated from the Earth are largely absorbed by atmospheric gases, causing a heating effect. Note: The upwards sequence of gases does not represent any vertical zonation. Pale yellow indicates where wavelengths are absorbed, or are neither emitted by the Sun nor the Earth's surface. Each gas has a number of absorption bands, but, of course, these only result in absorption of the wavelengths emitted by the Sun and Earth.

Figure 7 summarised what happens to solar radiation received by the Earth. The crucial feature for climate is what happens to energy that is reemitted. Because the Earth's surface temperature on average is about 16 °C, it emits radiation at much longer wavelengths than those of solar radiation.

Atmospheric gases absorb radiation at specific wavelengths that depend on the composition of each gas and on its concentration. Figure 11 shows schematically those parts of the electromagnetic spectrum absorbed by six important atmospheric gases. Nitrogen, which makes up 80% of the atmosphere, is an exception and is omitted; it does not absorb any infrared radiation. Dark-red bars signify absorption of incoming solar radiation, whereas the black bars show absorption of long-wave energy emitted from the Earth's surface.

Water vapour, carbon dioxide, methane, nitrous oxide (N2 O) and ozone (O3) each absorb wavelengths in part of the long-wave infrared range emitted by the Earth. Taken together, atmospheric gases can absorb most wavelengths of terrestrial radiation, but water vapour and carbon dioxide make the most significant contribution, depending on their concentrations. The energy that they absorb raises atmospheric temperature. Almost all the mass of the atmosphere lies within 30 km of the Earth's surface, with half comprising the lowermost 6 km. So the heating mostly affects the lowest atmospheric levels (with the exception of ozone, which heats up the stratosphere). As a result, the Earth's surface is some 33 °C warmer than it would be if it had no atmosphere. This phenomenon is the greenhouse effect, without which the Earth would be in a permanently ice-bound state.

There is a natural moderating effect to this 'greenhouse' heating. The warmer it is, the more the atmosphere eventually radiates long-wave radiation to space. Conversely, a cooler atmosphere radiates less, and absorbs more energy emitted by the surface and heats up. Before industrial greenhouse-gas emissions began, atmospheric heat gains and losses were roughly balanced at an average global temperature of 15 °C. The problem with these emissions is that this balanced temperature rises as concentrations of these gases increase; an enhanced greenhouse effect.

Some gases, such as methane, are far more potent absorbers of long-wavelength radiation than carbon dioxide and water. Each year, 108 t of methane is released into the atmosphere from natural gas venting at oil well heads, from leaking gas pipelines and during coal mining. One kilogram of methane released into the atmosphere can potentially cause eleven times more warming than 1 kg of CO2. But methane reacts with oxygen in a matter of years to form carbon dioxide, some of which dissolves in rainfall and in ocean water. Although the carbon cycle tends to achieve a balance, that balance fluctuates as CO2 is added by volcanoes and the burning of biomass and fossil fuels, and reduced by various transfers of carbon to long-term storage. Each gas therefore has its own warming potential, based on its potency and atmospheric lifetime. The greatest concern about global warming focuses on the large volumes of atmospheric CO2 added through human activities, although there are fears that methane may be released from the vast natural stores of gas hydrate in ocean floor sediments.