WHEN Julius Caesar encountered setbacks during his Roman Conquests, he had a stratagem that always worked. In 55 BC, for example, according to De Bello Gallico, "Doubling the number of hostages he had previously demanded from the Britons, Caesar ordered them delivered on the Continent".
And Inspector Renault of Casablanca employed a remarkably similar expedient when under inconvenient pressure from the persistent Major Strasser. "Realising the importance of the case", he tells the German, "my men are rounding up twice the usual number of suspects."
Now it is not unknown for meteorologists to use precisely these techniques. In what one might call the "early days" of global warming, attention was concentrated almost exclusively on the possible consequences of increasing amounts of carbon dioxide, or CO2, in the global atmosphere.
It soon became apparent, however, that CO2 was not the only culprit. Other gases, although, present in smaller quantities, were much better than carbon dioxide at trapping the longwave radiation from the Earth. The four most obvious suspects were nitrous oxide, methane, ozone and the man made CFCs.
There are 350 pars per million (ppm) of carbon dioxide in the Earth's atmosphere, compared with only 1.7 ppm of methane, 0.3 parts per million of nitrous oxide, and equally tenuous traces of the other two. But nitrous oxide is about 200 times more effective as a greenhouse trap than CO2 and one molecule of the most commonly used CFCs has the same greenhouse effect as 10,000 molecules of CO2.
Although these gases existed in much smaller quantities, it was evident the combined effect of any increase in their concentrations was as great as that of the increasing carbon dioxide on its own. They were duly rounded up and taken account of in the computer models for predicting future climate.
But by the early 1990s, things were still not going right. The models had one embarrassing flaw in that they were unable to reproduce the fluctuations in the Earth's temperature that had taken place over the last century or so. So what did the modellers do? Naturally they rounded up another suspect.
This time they incorporated sulphate aerosols into their calculations, industrial pollutants that reflect some of the sun's radiation back to space. With this addition, and with the appropriate initial data made available, the models were now able to mirror almost exactly the global temperature pattern from 1860 to the present day; which, of course, made their prediction for the future very much more plausible.