Living and breathing

<strong>An Ocean of Air: a natural history of the atmosphere</strong>

Gabrielle Walker <em>Blooms

It’s been 16 years since I last set foot in a science lab, and reading this book had a sobering effect. How much we forget. If you were to stop people at random in the street and ask them what proportion of the air is carbon dioxide, what would they say - 10 per cent, 1 per cent, perhaps 0.1 per cent?

They would be miles off but then so, rather embarrasingly, is Gabrielle Walker. The figure, according to her book, is 0.004 per cent. At this level, the earth would be frozen not starting to burn. The true figure is just under 0.04 per cent.

Walker has set herself an ambitious task - to unite stories of human research into air pressure, the gases that make up air, the movement of the wind, the ozone layer, ionosphere and outer space, in a history of our atmospheric discovery. She fails. It is a flawed premise that all these different atmospheric layers will fit together as neatly in a book as they do in the universe. And some are more interesting than others.

The first 100 pages, from Torricelli and his "ocean of air" to the slow discovery of oxygen and carbon dioxide and the first murmurings about climate change, are a blend of science writing and historical anecdote that is hard to fault. But what follows on the workings of wind and the upper layers of the atmosphere is as dull as those long-forgotten Tuesday afternoons locked in double physics, which no amount of vignettes featuring Christopher Columbus or James Lovelock can illuminate.

And yet, Walker's account of half a dozen scholars and their inspired hunches, painstaking experiments, wrong turns and dazzling discoveries is like a good detective story where you already know whodunnit. The pleasure is in watching the various imaginative leaps necessary to crack the mystery, all in an age before microscopes or computers.

The most impressive and affecting stories are those of Joseph Priestley and Antoine Lavoisier, two intellectuals working in the latter half of the 18th century in Birmingham and Paris. Priestley discovered that if you heat a lump of mercury ore and then collect the gas, a candle exposed to this "pure air" will flare up and burn brighter and longer than with common air. Lavoisier went on to calculate that this "oxy-gene" makes up one-fifth of the air and is used by humans for respiration. But both these dedicated rationalists were to suffer for their belief in reason and plain speaking. Priestley was forced to flee to America after his house, containing his laboratory and 40 years of research, was burned down by an angry mob; Lavoisier was denounced during the Terror by an old scientific rival and, after a show trial, guillotined.

Sixty years later, an Irish professor at the Royal Institution, John Tyndall, noticed that carbon dioxide is, in Walker's words, "a monster absorber of infrared light". The upshot, as we are now painfully aware, is that the energy becomes trapped in the atmosphere and warms the planet. Without it, the earth would be permanently frozen; but with too much of it, we begin to overheat. Although Tyndall did not predict global warming, he guessed that even small changes in carbon-dioxide levels might account for "all the mutations of climate which the researches of geologists reveal".

His work was carried on by two Swedes, Svante Arrhenius and Arvid Högbom. In 1896, Högbom noticed that CO2 levels were on the rise, and put this down to man-made emissions from the increasing amounts of coal being burnt. Aware that a reduction in CO2 had caused an ice age, Arrhenius realised that an increase in the gas would produce warming, although the danger at this time appeared remote. Nevertheless, he estimated that, by doubling the CO2 level, temperatures would increase by 5°C, which is uncannily close to what many of today's models suggest.

As well as this engrossing back story of climate change, Walker has written half a good book on the discovery of oxygen and carbon dioxide. There is a strong case for getting out the Stanley knife and then sending the first 100 pages to every secondary school pupil.