The effects of elevated carbon dioxide levels on global temperature and other properties

So far we have examined the global cycling of carbon without paying attention to the role CO2 plays in the Earth's climate. Although CO2 is a minor component of the atmosphere (see Section 3.2), it plays a vital role in the Earth's radiation balance and hence in controlling the climate. This is illustrated in Fig. 7.12a, which shows the wavelength emission spectrum of the Sun and the Earth, at their effective radiating temperatures of about 5700°C and -23°C respectively.

Wavelength (um)

Fig. 7.12 (a) Black body radiation spectra for the Sun (6000K) and (not on the same scale) the Earth (250K). (b) Absorption spectrum produced by the principal absorbing gases. After Spedding (1974). Reprinted by permission of Oxford University Press.

Figure 7.12b illustrates how this emitted radiation is absorbed by various atmospheric gases. As Figure 7.12 shows, much of the UV radiation impinging on the atmosphere is absorbed by O3 molecules in the stratosphere, and this explains the concern that human-induced decreases in stratospheric O3 may lead to larger amounts of harmful UV radiation reaching the Earth's surface (see Section 3.10). Much of the remainder of the solar energy passes through the atmosphere without major absorption.

Turning now to the Earth's emission spectrum, it is the CO2 absorption band centred around 15 mm which is particularly important here. This, together with other absorption bands due to water molecules, means that the atmosphere is considerably warmer (mean temperature about 15°C) than the effective emission temperature of the Earth (-23°C; Fig. 7.12 a). The combined effect of the atmosphere's transparency to most of the incoming solar radiation, and the absorption of much of the Earth's emitted radiation by water and CO2 molecules in the atmosphere, is often referred to as the 'greenhouse effect' —by analogy to the role played by the glass of a garden greenhouse.

From the above discussion, it is easy to see why elevated concentrations of CO2 in the atmosphere resulting from fossil fuel burning are likely to lead to a warmer climate. However, close inspection of Fig. 7.12 indicates that there is sufficient CO2 in the pre-industrial atmosphere for the 15 mm band to be absorbing almost 100% of the energy in that wavelength range coming from the Earth. Although the CO2 absorption band will broaden as CO2 concentrations rise, a major effect is for more of the absorption to occur lower in the atmosphere with less at higher altitudes. The result is that the lower layers warm, whereas higher up there is cooling.

Highly sophisticated mathematical models are used to predict the details of the temperature changes to be expected from rising levels of atmospheric CO2. The results of one rather straightforward model are shown in Fig. 7.13. The model confirms the simple prediction made above; the lower atmosphere warms by about 3°C for a doubling of atmospheric CO2 (although the distribution of the increase varies considerably with latitude), with a concomitant decrease in temperatures aloft. Figure 7.11 shows that, for many fossil fuel consumption scenarios, such a doubling might occur some time in the second half of this century.

Although CO2 is the most important of the anthropogenic greenhouse gases, it is not the only one of significance. Figure 7.14 shows, for the period 1980-90, the relative contributions of various gases to the change in the total greenhouse gas forcing over that decade. Just over half the effect was due to CO2 but other gases, including methane (CH4), nitrous oxide (N2O) and chlorofluorocarbons (CFCs) also contributed substantially to the total effect. In the case of these other gases, although the absolute amounts entering the atmosphere were small compared with CO2, their contributions to the greenhouse effect were proportionately large due to their absorption of energy being in parts of the Earth's emission spectrum (Fig. 7.12) which are not saturated. To illustrate this we should note that on a molecule-for-molecule basis methane is about 21 times more effective at absorbing energy than CO2, and CFC-11 is 12 000 times more effective. The

Fig. 7.13 Latitude-height distribution of the change in the zonal mean temperature (K) in response to a doubling of atmospheric CO2 content. Shaded area identifies decreases in temperature above about 15 km. After Manabe and Wetherald (1980).
Fig. 7.14 The contribution from each of the anthropogenic greenhouse gases to change in radiative forcing from 1980 to 1990. After IPCC (1990). With permission of the Intergovernmental Panel on Climate Change.

conclusion has to be that an understanding of the cycles of the non-CO2 greenhouse gases is in total as important as knowledge of the cycling of CO2.

So far we have concentrated only on greenhouse-gas-induced temperature changes. However, other climatological changes—for example, in the distribution of rainfall—may be more important in a practical sense than temperature increase per se. Computer models of likely changes in climate as a result of increase in CO2 and other gases indicate that global average water vapour, evaporation and rainfall are projected to increase, whereas at the regional scale, both increases and decreases in rainfall are likely. It is the net result of all these changes in rainfall, atmospheric water vapour and evaporation that will determine the agriculturally important property of soil-water content. Water is vital to crop growth and so it is hugely important to be able to predict changes in amounts of soil water, which can be either beneficial or harmful to crop yields. The social, economic and political consequences of such changes and geographical shifts are likely to be considerable.

Another potentially important consequence of global warming would be a global rise in sealevel. This would come about in part due to thermal expansion of seawater and also as a result of melting of glaciers and small ice-caps. Calculations of the magnitude of sealevel rise have considerable uncertainty, but a figure of about half a metre for a doubling of atmospheric CO2 is the current best estimate. If it occurs, this would have very significant effects in many countries that have centres of population close to the sea or on low-lying land. Further, there is a possibility that warming might eventually lead to the melting of a large mass of grounded ice, for example, the west Antarctic ice sheet. Such an event could produce a more substantial rise in sealevel (several metres), but, even if the temperature rise is great enough to melt the ice, it is estimated that it would take several hundred years for this to occur.

A further impact of rising levels of CO2 is on the chemistry of the oceans, in particular their pH. Increase in atmospheric CO2 will lead to a slight lowering of pH of the surface oceans (Section 6.4.4) as a result of extra amounts of the gas crossing from the atmosphere into the oceans. The resulting lowering in pH can be calculated, as can the potential for this increased acidity leading to enhanced calcium carbonate dissolution, for example in corals. Figure. 7.15 shows the pH of surface seawater as a function of temperature calculated for three different concentrations of atmospheric CO2. A pCO2 of 280 ppm corresponds to the situation in pre-industrial times with a seawater pH value of just below 8.2. A pCO2 of 354 ppm, corresponds to 1992 and a seawater pH of just under 8.1 (i.e. a drop in pH of about 0.1 unit from the pre-industrial value). Finally, a pCO2 of 750 ppm is given as a reasonable estimate of the likely concentration at the close of this century, with a corresponding seawater pH of just under 7.8 (i.e. a drop of 0.4 units from the pre-industrial level). Also shown are pH values derived from measurements made in the Atlantic Ocean surface waters in summer; they cluster in the range 8.0-8.3. What is clear is that by 2100, or whenever the atmospheric pCO 2 reaches double its current value, the pH of surface seawater will be far outside the range of values currently experienced by organisms living in the surface oceans. The effects of such a change on the biology of the oceans and on

15 20

Temperature (°C)

15 20

Temperature (°C)

Fig. 7.15 Equilibrium surface seawater pH for various atmospheric pCO2 concentrations assuming no change in alkalinity. Also shown are summertime pH values derived from measurements in the north Atlantic ocean. Courtesy of Doug Wallace, University of Kiel.

associated factors such as CO2 uptake and trace gas emissions (e.g. dimethyl sulphide in Section 7.3) are essentially unknown.

As mentioned above, decreased seawater pH may seriously affect marine organisms that secrete a CaCO3 skeleton (see Section 6.4.4), particularly reef forming corals (e.g. the Australian Great Barrier Reef), since the process is sensitive to the acidity of the seawater. This is illustrated in Fig. 7.16 where the saturation index (W) (see Section 6.4.4) for aragonite, the main reef-forming CaCO3 mineral today, is plotted against atmospheric pCO2. The shaded area shows the calculated values of W at water temperatures of 25 and 30°C (the range within which reef-building corals live) as a function of the atmospheric pCO2. It is apparent that as pCO2 increases the degree of aragonite supersaturation decreases. Since it is supersaturation that enables coral-forming organisms to synthesize their aragonitic (CaCO3) skeletons, any decrease in W will potentially lead to inhibited growth or death of the organisms, and so to die-back of coral reefs. The seriousness of this problem is not well established and is currently a topic of active research.

Given the potentially serious consequences for humans of the climatic changes and other environmental changes discussed above, it is not surprising that emissions of greenhouse gases are now subject to control and regulation through international conventions such as the Kyoto Protocol. Such international action has only come about because the scientific case for action has become compelling. However, as we have seen in this section there still remain many uncertainties in

Fig. 7.16 Effect of rising atmospheric pCO2 (dark stippled band), on the saturation index (W) of aragonite (CaCO3). After Buddemeier et al. (1998), courtesy of Joan Kleypas.

our understanding of the global carbon cycle and the climatic consequences of changes to it. Predicting such effects into the future is subject to further unknowns, some of these being scientific others being sociological or political. The task of environmental scientists is to reduce the uncertainties in the science so that society can make the serious social changes needed to ameliorate the effects of human-induced climate change in the most cost effective and least socially disruptive way possible.

0 0

Post a comment