Feedback to natural greenhouse gases water vapour

The most abundant greenhouse gas in the atmosphere is water vapour, but humans are not directly increasing its amount. However, it is anticipated that atmospheric increases in other greenhouse gases will lead to global warming, which will in turn lead to increased water vapour in the atmosphere because of increased evaporative capacity. Therefore, increase in water vapour is viewed as a feedback from the increases in the anthropogenically produced greenhouse gases CO2, CH4, N2O and CFCs, rather than as an anthropogenically generated greenhouse gas.

2.4.4 Future increases in greenhouse gases

There is no doubt that there have been significant and even alarming increases in greenhouse gas concentrations. How they may change in the future is highly uncertain, and this is one of the most difficult problems in studying possible future climatic change, because changes in these gases greatly depend upon changes in the future economic and political activities of all nations. This is particularly true of those countries or regions, such as the USA, western Europe and China, that are or will be largely responsible for most of the future emissions of these gases. Predicting economic and environmental policy development on a global scale is a most daunting task.

Numerous scenarios of possible future increases of greenhouse gases have been constructed. They are based on different assumptions of future human activities, such as economic growth, technological advances, and human responses to environmental or socioeconomic constraints (Jager, 1988; Houghton et al., 1990, 1992, 1996; German Bundestag EnquĂȘte Commission, 1991). Thus, it should be understood that these scenarios of future greenhouse gas emissions are highly uncertain and become more so as the length of time of the projection increases.

Four scenarios were developed for the first Report of the Intergovernmental Panel on Climate Change (IPCC) (Houghton et al., 1990). These scenarios assumed identical population increases and economic development scenarios, but different technological development and environmental controls. Individual scenarios using changes in concentrations of CO2, CH4, N2O and CFCs were developed. The scenarios varied from Business as Usual (BAU) with very little environmental control to scenario C with high levels of controls. The rate of increase of greenhouse gases decreased with increasing environmental control.

The IPCC 1992 Supplement (Houghton et al., 1992) provided a more detailed set of scenarios that have been frequently used in a number of global change contexts. Six alternative scenarios (IS92a-f) to the year 2100 were constructed based on different quantitative assumptions about population growth, economic growth and energy supplies within different world sectors, i.e. developing and developed countries (Fig. 2.5a). The IS92a and IS92b scenarios were more or less updates of the scenarios presented in the 1990 IPCC report (Houghton et al., 1990) and form 'middle-of-the-road' projections (Leggett et al., 1992). The other scenarios assumed rates of change in emissions of greenhouse gases that encompassed a large total range. For example, gigatons (109 tons) of carbon (GtC) emitted by 2100 ranged from 4.6 (IS92c) to 35.8 (IS92e) and significantly departed from an actual 1990 value of 7.4 GtC year-1. The IS92a 'middle-of-the-road' scenario assumed an emission of 20.3 GtC year-1 (Fig. 2.5a).

IS92e IS92f

IS92e IS92f

2000 2020 2040 2060 2080 2100 2000 2020 2040 2060 2080 2100

Year Year

Fig. 2.5. (a) Total anthropogenic CO2 emissions under the IS92 emission scenarios; (b) the resulting atmospheric CO2 concentrations using a carbon cycle model. (Source: Houghton et al., 1996.)

2000 2020 2040 2060 2080 2100 2000 2020 2040 2060 2080 2100

Year Year

Fig. 2.5. (a) Total anthropogenic CO2 emissions under the IS92 emission scenarios; (b) the resulting atmospheric CO2 concentrations using a carbon cycle model. (Source: Houghton et al., 1996.)

When given a particular quantity of emissions, the calculation of future concentrations of greenhouse gases in the atmosphere is determined by modelling the processes that transform and remove the relevant gases from the atmosphere. Future concentrations of CO2, for example, are quantified using carbon cycle models that simulate exchange of CO2 among atmosphere, oceans and biosphere. The CO2 concentrations corresponding to the emissions scenarios listed above range from about 480 ppmv to well over 1000 ppmv, with about 710 ppmv for IS92a (Fig. 2.5b).

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