2.2 The second carbon trail, human influences
Industrial economies, first in Europe and North America and now globally, have made increasing demands on the Earth to meet their material needs. We meet increasing demands upon agriculture to feed a growing and more affluent population by bringing more land under cultivation and using it more intensively. We are also heavily reliant on the use of the energy stored in fossil fuels to provide the electricity, heating, lighting and transport that modern societies take for granted. As a by-product of meeting these needs, we have inadvertently changed, and are now knowingly changing, our atmosphere.
The global carbon cycle can help explain what happens when these activities release additional carbon dioxide into the atmosphere. The two main sources have been shown in Figure 6. The first is the burning of fossil fuels, such as coal, oil and gas. These are compounds composed largely of carbon and hydrogen. When they are burned to release energy, carbon combines with oxygen to produce carbon dioxide (CO2), and hydrogen combines with oxygen to produce water (H2O), usually in the form of steam. This process parallels the respiration by plants and animals illustrated in Figure 5. (Cement-making, which uses limestone or chalk, also produces carbon dioxide and makes a lesser but still significant contribution.)
Figure 7 shows global CO2 emissions from fossil fuel burning from 1850. Between 1950 and 2010 these emissions increased at a rate of about 500 million tonnes per year. However, the rate of increase has slowed since 2010 and the world may now be at, or close to, its peak rate of CO2 emissions from fossil fuels.
The second carbon dioxide source is land use change for agriculture and the spread of urban areas, which includes the burning of forests and grasslands and also the release of carbon from newly disturbed soil. Recent estimates (Global Carbon Project, 2023) suggest that fossil fuel burning is adding approximately 37 gigatonnes (37 000 million tonnes) of carbon dioxide to the atmosphere every year, with land use change contributing approximately 4 gigatonnes. The same estimates also suggest that around 40% to 50% of these carbon releases remain in the atmosphere for centuries or many thousands of years. The remainder is absorbed over a period of decades (or less) by the two carbon stores described earlier, namely by plants (mainly the forests of the northern hemisphere) and the oceans.
Box 2 Describing large and small numbers (and units)
37 gigatonnes can be written as 37 billion tonnes or 37 thousand million tonnes.
The prefix ‘giga’ and its symbol G (note that some symbols are capitalised and some are not) is an example of an internationally agreed set of terms to describe numbers and units. These form the Système international d’unités (international system of units) known as SI. The SI unit of length, for example, is the metre (m), and the SI unit for time is the second (s).
Example: What is 1000 metres in SI form? The prefix for one ‘thousand’ is ‘kilo’, so the answer is one kilometre (1 km in shortened form). One thousand metres, one kilometre and 1 km are alternative ways of describing the same thing.
There are several other different ways of writing large and small numbers, including fractions, decimals and powers of ten, which are shown here for reference (see Table 1).
Table 1 Examples of different ways of writing large and small numbers
| Prefix | Prefix name | Meaning | Number or fraction | Decimal | Power of ten |
| G | giga | billion or thousand million | 1 000 000 000 | 1 000 000 000 | 109 |
| M | mega | million | 1 000 000 | 1 000 000 | 106 |
| k | kilo | thousand | 1000 | 1000 | 103 |
| one | 1 | 1 | 100 | ||
| m | milli | thousandth | 1/1000 | 0.001 | 10−3 |
| µ | micro | millionth | 1/1000 000 | 0.000 001 | 10−6 |
| n | nano | billionth | 1/1000 000 000 | 0.000 000 001 | 10−9 |
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