The Earth system is the sum of our planet’s interacting physical, chemical, and biological processes. It includes the land, the oceans, the atmosphere and the frozen poles. It takes in the grand natural cycles through which vital elements like carbon, nitrogen, phosphorus and sulphur circulate around the planet; the movement of water between sea, sky, rivers and ice; the imperceptibly slow events deep beneath our feet that create and destroy continents and oceans.
Living things are part of the system too. For example, ocean plankton absorb carbon dioxide from the air to make their shells; after death their bodies sink to the seabed, where that carbon is locked up for millennia in layers of sediment that are slowly compressed to make limestone.
Humans are no exception. We aren’t an outside force disturbing the natural order of things – we’re an integral part of the Earth system ourselves. But as our societies have grown, the impact of our actions has increased too; we’re now among the main causes of environmental change.
For example, the amount of greenhouse gases in the atmosphere varies naturally, but levels of carbon dioxide and methane in the atmosphere are now higher than at any time in the last one million years, possibly 15 million. Our great dams are trapping gigatonnes of sediment; our farms are draining aquifers. In many places this is happening faster than they can refill; our appetite for land is clearing enormous swathes of forest. We are remaking the Earth system itself. But we don’t know where this will lead.