(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.
2.9.2 Global warming
We started this chapter discussing temperature, moved through a number of other environmental conditions to pollutants, and now return to temperature because of the effects of those pollutants on global temperatures. The globally averaged combined land and ocean surface temperature has increased by about 0.85°C from 1880 to 2012 (Figure 2.28a). We have already witnessed melting of arctic ice and rises in sea level (Figure 2.28b) (related to thermal expansion and the input of ice meltwater) and can expect further melting of the ice caps, a consequent rising of sea level and significant shifts in the pattern of global climates and changes to the distribution of species.
Figure 2.28 Annual land and ocean surface temperature anomalies and sea‐level changes. (a) Globally averaged combined annual land and ocean surface temperature anomalies from 1850 relative to the average over the period 1986–2005. Colours indicate different datasets. (b) Globally averaged annual sea‐level changes from 1900 relative to the average over the period 1986–2005. Colours indicate different datasets that have been aligned to have the same value in 1993. Uncertainties are indicated by shading.
Source: IPCC (2014).
Predictions of the extent of global warming resulting from the enhanced greenhouse effect come from two sources: (i) trends detected in measured datasets, including the width of tree rings, sea‐level records and measures of the rate of retreat of glaciers; and (ii) predictions based on sophisticated computer models that simulate the world’s climate according to a variety of possible mitigation scenarios. The latter range from the best case, where there is a concerted and international political drive to minimise the temperature rise by the use and development of efficient technologies (e.g. switching to renewable energy and bioenergy, carbon capture and geological storage) to the worst case where very little is done and the expected outcome is close to the no‐mitigation baseline scenario.
global distribution of climate change
Global warming so far has not been evenly distributed over the surface of the earth, and neither will it be in the future. Northern high latitudes are expected to change more rapidly than the tropics, land areas will change more rapidly than the oceans, and small islands and coastal regions will be particularly prone to associated rises in sea level.
We
