The Biosphere: Where Life Lives

Scientists describe the Earth as a set of overlapping systems:

• the lithosphere — all the rock, stone and soil;
• the hydrosphere — all the water: oceans, rivers, dams, groundwater and ice;
• the atmosphere — the blanket of air (mostly nitrogen and oxygen) held around the Earth by gravity.

The biosphere is the part of the Earth in which living things are found. It is not a fourth layer stacked on top of the others — it is the thin zone where rock, water and air meet and where the conditions for life exist.

Life needs a fairly narrow set of conditions: liquid water, a usable source of energy (almost always sunlight), gases to breathe, nutrients from soil or water, and temperatures that are not extreme. Only a thin shell around the Earth supplies all of these at once.

Worked example — a rock pool at Kalk Bay. Look into one pool and you can see three spheres at once: the rock the pool is cut into (lithosphere), the seawater filling it (hydrosphere), and the air above it (atmosphere). The limpets, sea anemones and seaweed living in the pool are the biosphere — life sitting exactly where the three overlap.

The biosphere reaches roughly 10 km down into the deepest ocean trenches and about 10 km up into the atmosphere, so it is about 20 km thick. Compared with the Earth's radius of about 6 400 km, that is astonishingly thin.