Evaporation is nature's desalination plant, and the ocean runs most of it
When seawater evaporates, salts and particles stay behind, so every cloud is freshly distilled water. The ocean holds about 97 per cent of Earth's water, supplies 86 per cent of global evaporation and receives 78 per cent of all precipitation, making the sea the engine room of the planet's plumbing.
Earth's total stock of water stays roughly constant, but its split between ice, fresh water, seawater and vapour shifts with the climate. Water moves between these reservoirs through evaporation, transpiration from plants, condensation, precipitation, sublimation of snow and ice straight into vapour, infiltration into soil, runoff over land and flow underground. Along the way it switches between liquid, solid and gas.
Sunlight powers the whole system. Evaporation absorbs heat, the latent heat of vaporisation, and condensation or freezing releases it again, so moving water also moves energy; ocean currents carry heat from the tropics towards the poles. Clouds form because water molecules are lighter than oxygen and nitrogen molecules, so moist air rises. Higher up, lower pressure cools it until vapour condenses into droplets held aloft by updrafts; the same process at ground level makes fog.
Once water falls, its paths diverge. Snow can lock up in glaciers and ice caps for thousands of years. Rain may run into rivers, fill lakes, soak down to recharge aquifers that hold water for long periods, or be drawn up by roots and breathed out again. Some groundwater bursts out as springs, and beneath river valleys surface and underground water mix constantly in the hyporheic zone. Advection, the sideways movement of vapour through the air, is what lets ocean water rain on land; atmospheric rivers are a dramatic example. Plants and land surfaces together return around 505,000 cubic kilometres of water to the air each year.
Flowing water also sculpts continents through weathering, erosion and deposition, and ferries minerals around the globe. Human activity is now altering the cycle. Deforestation, urban sprawl and groundwater pumping change how water moves across land, while climate change is intensifying the cycle, shifting rainfall patterns and making extreme weather more frequent, with knock-on effects for farming, ecosystems and water supplies.
Source: Water cycle
