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A major element of the Industrial Revolution was the switch from the use of sustainable fuels to the use of coal (and later, oil) as a source of power. Between the middle of the 19th and the middle of the 20th century the burning of fossil fuels, together with extensive deforestation, added about 90 gigatonnes (Gt) of carbon dioxide (CO₂) to the atmosphere and more has been added since. The concentration of CO₂ in the atmosphere before the Industrial Revolution (measured in gas trapped in ice cores) was about 280 ppm, a fairly typical interglacial ‘peak’ (Figure 2.26a), but this had risen to around 370 ppm by the turn of the millennium (Figure 2.26b) and in May 2013 reached 400 ppm for the first time in at least the last 800 000 years.

Figure 2.26 Atmospheric concentrations of CO₂during the past 420 000 years and since 1850. (a) Concentrations of CO₂ in gas trapped in ice cores from Vostok, Antarctica. Transitions between glacial and warm epochs, and peaks in CO₂, occurred around 335 000, 245 000, 135 000 and 18 000 years ago. (b) Atmospheric concentrations of the greenhouse gases CO₂ (green), methane (CH₄, brown) and nitrous oxide (N₂O, blue) determined from ice core data (dots) and from direct atmospheric measurements (lines) since the mid‐18th century. BP, before present; ppb, parts per billion; ppm, parts per million.

Source: (a) After Petit et al. (1999) and Stauffer (2000). (b) After IPCC (2014).

Solar radiation incident on the earth’s atmosphere is in part reflected, in part absorbed, and part is transmitted through to the earth’s surface, which absorbs and is warmed by it. Some of this absorbed energy is radiated back to the atmosphere where atmospheric gases, mainly water vapour and CO₂, absorb about 70% of it. It is this trapped reradiated energy that heats the atmosphere in what is called the ‘greenhouse effect’. The greenhouse effect was of course part of the normal environment before the Industrial Revolution and was responsible for some of the environmental warmth before industrial activity started to enhance it. At that time, the greater proportion of the greenhouse effect was due to atmospheric water vapour.

CO₂ – but not only CO₂

In addition to the enhancement of greenhouse effects by CO₂ emissions, other trace gases have increased markedly in the atmosphere, particularly methane (CH₄) and nitrous oxide (N₂O) (Figure 2.27) and to a smaller extent the chlorofluorocarbons (CFCs, e.g. trichlorofluoromethane (CCl₃F) and dichlorodifluoromethane (CCl₂F₂)) and some other minor contributors. Each greenhouse gas has a global warming potential (usually expressed as ‘equivalents of CO₂’) that depends on how long it stays in the atmosphere and how strongly it absorbs energy. Thus, CH₄ and N₂O have global warming potentials some 30 and 300 times that of CO₂ over a 100‐year period (they persist in the atmosphere for around 10 or 100 years, respectively, compared with thousands of years for CO₂, but absorb energy much more efficiently). Together, these gases contribute about 35% to enhancing the greenhouse effect, compared with 65% by CO₂ (Figure 2.26). The increase in CH₄ is mainly of microbial origin in intensive agriculture on anaerobic soils (especially increased rice production) and in the digestive process of ruminants (a cow produces approximately 40 litres of CH₄ each day). N₂O is emitted during agricultural and industrial production and the combustion of fossil fuels and solid waste. The effect of the CFCs from refrigerants, aerosol propellants and so on was potentially great (their global warming potentials are thousands or tens of thousands greater than CO₂), but international agreements, mainly to counteract damage to the ozone layer, have strongly moderated increases in their concentrations. However, the rate of increase in annual greenhouse gas emissions has accelerated since the turn of the millennium (Figure 2.27).

Figure 2.27 Total annual anthropogenic greenhouse gas (GHG) emissions from 1970 to 2010 converted to gigatonne equivalents of CO₂ per year. FOLU, forestry and other land use change.

Source: IPCC (2014).

It is possible to draw up a balance sheet of how the CO₂ produced by human activities translates into changes in concentration in the atmosphere. Human activities have released more than 2000 Gt CO₂ since 1750, but the increase in atmospheric CO₂ accounts for only 40% of this (IPCC, 2014). The oceans absorb an estimated 30% of CO₂ released by human activities. Furthermore, recent analyses indicate that terrestrial vegetation has been ‘fertilised’ by the increased atmospheric CO₂, so that a considerable amount of extra carbon has been locked up in vegetation biomass. And more is to be found as soil carbon. This softening of the blow by the oceans and terrestrial vegetation notwithstanding, however, atmospheric CO₂ and the greenhouse effect are increasing.

The most profound effect of anthropogenic CO₂ emissions, global warming, is dealt with in the next section. In addition, ocean acidification is another worrying consequence.

ocean acidification

A large proportion of anthropogenic CO₂ is absorbed by the oceans, thus far reducing seawater pH by 0.1 units since the Industrial Revolution (equivalent to a 30% increase in acidity) as well as reducing carbonate ion concentrations. We have already seen that pH is a condition with significant influences on the success of organisms, but the fact that many parts of the ocean are also becoming undersaturated with calcium carbonate minerals is expected to have profound consequences for calcifying species such as corals, molluscs, sea urchins and calcareous plankton. On the other hand, photosynthetic production in the oceans is likely to benefit from higher CO₂ concentrations.