The current concentration in the atmosphere of carbon dioxide, or CO 2, at about 360 parts per million (ppm), is 10 per cent higher than 30 years ago, and an estimated 30 per cent more than 1750. The gas has been pilloried in recent years as one of the greater evils of our time, a foul pollutant and the cause of all our global warming problems.
But CO2 is far from being the only culprit. Although it is by far the most abundant "greenhouse gas" there are others, albeit present in smaller quantities, which are more efficient at trapping the long-wave radiation which might otherwise escape from Earth.
CO2 is believed to account for some 64 per cent of the global warming being experienced; the remaining 36 per cent is accounted for by other gases, the four most important of which are nitrous oxide, methane, ozone and the CFCs.
The atmosphere contains only about 0.3 ppm of nitrous oxide, and it is increasing in quantity by about 1 per cent every three years because of the extensive use of nitrogen-based fertilisers.
But the "greenhouse effect" of nitrous oxide quite belies its sparseness; it is about 200 times more effective as a greenhouse trap than CO2. Methane, currently 1.7 ppm, is also becoming more prevalent because of more extensive animal farming, and the release of natural gas through exploration; it is estimated that if current trends continue, concentrations in the year 2050 will be about 80 per cent higher than at present.
Ozone is deceptive. Because of the scare about the "ozone hole", we have come to look on ozone as a friendly agent. And so it is - in the right place some 20 miles or so above our heads. In the lower regions of the atmosphere, however, ozone acts as a very powerful greenhouse gas.
And finally we have CFCs, the man-made chlorofluorocarbons which destroy the ozone in the stratosphere, but happily coexist with it lower down. It has been calculated that a single molecule of a CFC has the same greenhouse effect as 10,000 molecules of CO2; but happily, thanks to internationally co-ordinated action, concentrations of chloro fluorocarbons are decreasing.
Because all these gases are so much better than carbon dioxide at interfering with outgoing radiation from the Earth, scientists believe that before long, and despite their much smaller quantities, their combined effect will be as great as that of CO2.
Control of these "minor constituents" of the atmosphere will be just as important in the years to come as any measures to curtail production of CO2 itself.