All life is conjured from CO2. This is the original magic trick, from which everything else in the living world follows. At Earth’s surface, with mere sunlight and water, it is transformed into living matter through photosynthesis, leaving oxygen in its wake. This plant carbon then flows through animal bodies and ecosystems and back out into the oceans and air as CO2 once again. But some of this carbon slips the churn of the surface world altogether and passes into the Earth – as limestone, or as carbon-rich sludge, slumbering deep in the planet’s crust for hundreds of millions of years. If it isn’t buried, this plant stuff is quickly burned on Earth’s surface in the fires of metabolism, by animals, fungi, bacteria. In this way, life uses up 99.99 per cent of the oxygen produced by photosynthesis – and would use it all, if it weren’t for that infinitesimal leak of plant matter into the rocks. But it is from this leak into the rocks that the planet has been gifted its strange surplus of oxygen. In other words, the Earth’s breathable atmosphere is the legacy not of forests and swirls of plankton alive today but of the CO2 captured by life over all of our planet’s history and commended to Earth’s crust as fossil fuels.
If this was the end of the story, and CO2 was merely the fundamental substrate of all living things on Earth and the indirect source of its life-sustaining oxygen, that would be interesting enough. But it just so happens that this same unassuming molecule also critically modulates the temperature of the entire planet and the chemistry of the entire ocean. When this carbon chemistry goes awry, the living world is warped, the thermostat breaks, the oceans acidify and things die. This astounding significance of carbon dioxide to every component of the Earth system is why it’s not just another noisome industrial pollutant to regulate, like chlorofluorocarbons or lead. It is rather, as the oceanographer Roger Revelle wrote in 1985, ‘the most important substance in the biosphere’.
The most important substance in the biosphere is not one to be treated cavalierly. The movement of CO2 – as it billows from volcanoes, stirs into the air and oceans, swirls through eddies of life and soaks back into the rocks again – is what makes the Earth the Earth. This is called the carbon cycle, and life on Earth crucially depends on this global cycle maintaining a kind of delicate, if dynamic, balance. While CO2 perennially issues from volcanoes (at a hundredth the rate of human emissions) and living organisms exchange it in a ceaseless frenzy at the Earth’s surface, the planet is meanwhile constantly scrubbing it from the system at the same time, preventing climate catastrophe. Feedbacks that draw down CO2 – from the erosion of whole mountain chains to the sinking of blizzards of carbon-rich plankton to the bottom of the sea – serve to maintain a kind of planetary equilibrium. Most of the time. This is an unlikely, miraculous world we live on, and one that we recklessly take for granted.
