It was not until the end of the 1 950s that scientists found the first evidence that man-made gases were beginning to create a global catastrophe by causing average temperatures to rise-and not until the late 1980s that definite proof emerged of how serious the situation was becoming.3
The scientific conclusions are well enough known for a mere summary here to suffice. The most important of these gases, as most people now know, is carbon dioxide, produced by burning carbon based substances such as oil and coal to obtain energyalthough gases such as methane and nitrous oxide also have to be taken into consideration. The concentration of these gases in the atmosphere is measured in terms of parts of carbon dioxide equivalent per million, or ppm. In pre-industrial times it was 280 ppm; it now stands at 385 ppm and is rising by about 2.1 ppm a year. So far the change has been sufficient to raise the average temperature of the Earth by about 0.8 degrees celsius, and if emissions continue at their present rate, there will be further temperature rises of about 0.2 degrees a decade. This is if other things remain as at present. But there are various feedback mechanisms produced by rising temperatures that would lead to accelerated change-the melting of ice caps, the release of carbon dioxide in the sea or of methane from arctic tundra, the desertification of forests. There is no final scientific consensus as to the temperatures (the "tipping points") that would cause these feedback mechanisms to take effect, but it was widely accepted in 2007 that some would be likely to set in if the temperature rose 2 degrees above the pre-industrial level-about 1 .2 degrees above that at present (that does not preclude some of these mechanisms setting in earlier, as, for instance NASA's James Hanson argued in April 2008).4 To avoid that point being reached, carbon concentrations have to be kept down-the IPCC argues to between 445 and 490 ppm but even 400 ppm might push the temperature up to the 2 degrees threshold. 5
308
The Runaway System