Science

Despite objections from some climate scientists, the most authoritative and widely accepted general viewpoint on global warming remains the report produced by the Intergovernmental Panel on Climate Change (IPCC) in 1990.9 The IPCC, a body of the world's leading climatologists which was set up in 1988, felt able to state in that report that they were certain that some global warming would occur due to human activities, should existing emissions trends continue (Houghton et al, 1990:xi). The basis for making such a claim was threefold: the naturally occurring greenhouse effect; the increases in concentrations associated with the greenhouse effect due to human activities; and an approximate correlation between increases in those concentrations and observed mean surface air temperature increases in the twentieth century.

The greenhouse effect is a natural phenomenon whereby certain gases in the atmosphere keep the earth's temperature significantly higher than it would otherwise be. It produces conditions at the earth's surface which are suitable for life. The main gases involved in the greenhouse effect are water vapour, carbon dioxide (CO2), chlorofluorocarbons (CFCs), methane (CH4) and nitrous oxide (N2O).10 These gases allow radiation to pass through from the sun, but they absorb the lower frequency, longer wavelength radiation from the earth's surface, thereby trapping heat in the atmosphere. It is estimated that without these gases, the earth's surface temperature would be 33 °C lower than at present (Nierenberg et al., 1989; Leggett, 1990b).

With the exception of water vapour, both the emissions from human activities and the atmospheric concentrations of these gases have increased significantly during the twentieth century. The increases in concentrations are shown in Figure 1.1 (from Houghton et al., 1990:xvi).

Lastly, the IPCC also produced figures which showed an increase in the global mean surface temperature over the previous one hundred years of between 0.3 and 0.6°C (Houghton et al, 1990:xii). Although the IPCC made clear their acceptance that this does not prove the global warming hypothesis, since it remains a warming within the natural variability of the climate system (1990:xxix), and in any case correlation does not prove causality, their inclusion of this (very) approximate correlation between increasing concentrations and observed warming indicates a belief that they may well be linked.

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Figure 1.11ncreases in atmospheric concentrations of major greenhouse gases

Source: J.T.Houghton, G.J.Jenkins and J.J Ephraums (1990) Climate Change: The IPCC Scientific Assessment, Cambridge:Cambridge University Press, xvi

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