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Oliver Morton

The Music of Science

The fleeting warmth of the embers in a fire embodies millions of years of life on Earth

Oliver Morton | January/February 2014

Because my father was born at Christmas time, his parents named him Noel. But that was not the name by which they came to know him. As a baby he would stare, rapt, into the fire in the grate of their house in a Welsh mining valley, off in a world of his own. His mother took to calling him "Joseph the dreamer", and from then on his family always called him Joe.

A love of looking into a hearth is something I have inherited from him. As I write this in my own sitting room, a log is crackling on a bed of slower-burning coals in a fireplace not dissimilar to the one my dad grew up with. The house would be warm without it, but I find it one of the more enjoyable duties of winter, once a week or so, to walk up the road to the filling station, load a battered old rucksack with solid fuel, and lug it back home. It makes the northern hemisphere's turning away from the sun a bit more palatable. And, in a simple, reassuringly domestic way, it celebrates what makes the planet quite so special.

The coal and the wood are, basically, the same thing: reduced carbon. Reduced, here, does not mean lessened. It means that the carbon is stuck to hydrogen atoms as well as (or, if thoroughly reduced, instead of) oxygen atoms. The carbon molecules that fuel cars and power stations and, indeed, people—the ones that we eat as food and of which we are made—are all reduced. All this reduction is the result of photosynthesis, far and away the most successful of the ways life has found of sticking hydrogen onto carbon atoms. In the case of the log on my fire, the photosynthesis took place in the past decade or so. For the coal it was a few hundred million years earlier.

The fire, which I have just poked into a little more activity, is that process in reverse: the reduced carbon is being torn from its hydrogen by voracious oxygen. Oxidation of some sort is the fate of all the reduced carbon that humans use. It is what we do with the fuel in our cars, the food in our bellies, the gas in our boilers and the coal in our grates. Whether in the biological guise of respiration, in the spectacle of fire or through the slower depredations of rust and tarnish, there is an oxidation to match almost every reduction. The two processes complement each other; what one does, the other undoes.

The most important oxidations, as the name suggests, are done by oxygen. This is why the photosynthesis done by plants and some bacteria is so central to the life of the planet. To reduce carbon, plants need energy and hydrogen. The energy comes from the sun. The hydrogen comes from water. When the hydrogen is added to carbon, the other part of the water—oxygen—is released to go its own way. The wood burning in the fireplace recombines with oxygen to make carbon dioxide. Almost everything that photosynthesis reduces is re-oxidised this way in pretty short order, allowing energy received from the sun to power the planet's biosphere.

Given this, though, a question arises: why is there so much oxygen around? If photosynthesis produces oxygen at the same rate as it reduces carbon, and if oxygen is a voracious oxidiser, why isn't it all used up almost as fast as the plants churn it out? 

The answer is the coal in my grate, glowing red under the hissing wood. The reason we have oxygen to breathe—and to burn—is that there are fossil fuels in the ground. While almost all organic matter produced by photosynthesis is indeed quickly re-oxidised, every year a tiny fraction of a percent ends up in a place where the atmosphere's oxygen cannot reach it, such as a stagnant marsh, or the floor of a poorly aerated sea. When reduced carbon is buried away like this, oxygen builds up in the atmosphere. Over the planet's life, enough carbon has been buried in the sediments of the crust—in coal seams, in oil- and gasfields, in shales and slates—to account for the hundreds of trillions of tonnes of oxygen in the atmosphere.

A planet that did not store reduced carbon in its sediments would not have high levels of oxygen in its atmosphere. The long-term storage of carbon is the gift that coal, like other fossil fuels, brings to life's table, or to its grate. The chemistry of coal is not that different from wood, but its history is vastly longer—and by staying unoxidised over that time, it contributes to the great pool of oxygen which my fire is now, in an ever so tiny way, depleting. The fire is fleeting. The reason that there can be fire is as big and old as the continents.

I don't know the details of the coal's history, its specific whens and wheres. Such things are not marked on the bags sold on garage forecourts, any more than the basin the oil came from is displayed on the pumps. And I don't know what my infant father dreamed of as he watched the embers in the grate. But I do know what I see: the long abyss of time, the slow cycles of the Earth, and a burning stone that is a comfort, a threat—and part of the essence of life.

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