Speaking up for CO2

By Oliver Morton

Chemicals are hard to get inspired by, in part because of the awful names they are given. The 19th-century convention of referring to them in terms of the proportions of their ingredients is off-putting to the inexpert. The sense of composed artificiality is surely one reason many people say they distrust chemicals in general and even claim they try to avoid them—though all the most wonderful substances in the world are chemicals.

Take carbon dioxide. Reduced to the 1:2 ratio of its components, it sounds unremarkable; add to that the knowledge that it is messing up the climate and who could like the stuff? But if the ancients had known of the great cycle whereby plants use carbon dioxide to store the energy in sunlight, and humans and other animals return it to the air, they would surely have put it up there with earth, air, fire and water. If it had been part of our sense of the world and our language, its cultural resonance would have echoed through the ages; it would be in Shakespeare and the Book of Common Prayer, Wordsworth and Goethe. And its role in the foundation of human history might be better appreciated.

For most of the thousands of years that humans talked, made tools, wore clothes and created art, they had no domesticated crops and therefore no agriculture, no cities and no civilisation. About 10,000 years ago that began to change; 5,000 years later agriculture prevailed everywhere from Peru to Japan.

It is natural to link this change to the end of the last ice age, which was at its peak 20,000 years ago and had mostly melted away 10,000 years later. But why did the ice age preclude the development of agriculture? Though pretty much everywhere was colder in the ice age than it is today, many subtropical places that are warm now were perfectly temperate back then, and the tropics were quite hot. And though the ice-age climate was dry, it was not so arid that crops everywhere would shrivel. The climate was certainly unstable by modern standards, juddering around in a way that primitive farmers would have found hard to deal with. But is that really all there was to it?

Rowan Sage, a Canadian plant physiologist, thinks not. He suggested in the 1990s that it was not the climate itself that kept the ice age free of agriculture. Instead, he points the finger at one cause of that cooling: the low carbon-dioxide level (that ice sheets reflect back a lot of sunlight is a big factor too). And since carbon dioxide is life to plants, the greenery suffers in its absence. Plants grown in labs under the low-carbon-dioxide conditions of the ice age are spindly, grotty things. The total mass of the plant life at the peak of the ice was about half what it is today. When changes in ocean circulation at the end of the ice age started to replenish the atmosphere’s stock of carbon dioxide, the plants went wild. Forests chased the ice-caps back towards the poles; the soils of the planet grew richer and darker, invigorated roots ran deeper and rains returned; the amount of grain produced by some wild grasses doubled.

If you were a hunter-gatherer, this Great Fertilisation was the dawn of a golden age. A more productive ecosystem meant you needed to travel less far to forage—it might even allow you to stay in one place all year. It also meant you could concentrate on a few productive species for most of your food (and because carbon-dioxide fertilisation has varying effects on different plants, it would produce some particularly productive winners). So such specialisation would bring about the selection, conscious or not, of strains that grow better under certain conditions—the beginnings of domestication, which, with sedentary settlement, is a necessary precursor to agriculture. At ice-age levels of carbon dioxide these might have been impossible.

When I first heard Sage’s idea, I expected it to change popular perceptions of prehistory. It hasn’t. But it has largely been ignored rather than refuted—and the principles it rests on can be seen at work today. Satellite pictures show the Earth is greener now than it was even a few decades ago—and that abundance of plant life is partly due to more carbon dioxide. Plants suck up about a quarter of the carbon dioxide humans emit into the atmosphere every year.

In the 1950s an American geochemist called Harrison Brown (who was, among other things, a mentor to John Holdren, Barack Obama’s science adviser) speculated that it might be worth deliberately doubling or tripling the planet’s carbon-dioxide level to feed its booming population. He concluded that it wouldn’t be a good idea—and that was without really appreciating what carbon dioxide does to the climate. The risks posed by disrupting the climate seem greatly to outweigh the benefits of more carbon dioxide for crops—and there are other ways to increase yield.

The carbon cycle is one of growth and decay, creation and destruction. But when we see the ruinous aspects of carbon dioxide at work, our imagination should at least find room to remember the beneficial ones—and to relish the richness of the world worth saving. If only our words made it a little easier.

Illustration Pete Gamlen