Earth's Water by the Numbers

Scientists estimate that Earth holds about 1,386 million cubic kilometres of water (332.5 million cubic miles). That counts everything: the oceans, ice, water underground, lakes, rivers, the vapour in the air and even the water inside living things. Gathered into one ball, it would be only about 1,385 kilometres across, small next to the planet itself.

The figures on this page come from one widely used estimate, published by the hydrologist Igor Shiklomanov and shared by the US Geological Survey (USGS). They describe Earth at a single moment. Water never stops moving between these stores, and the percentages are rounded, so they do not add up to exactly 100.

About 96.5 per cent of all Earth's water is in the oceans, seas and bays. Add the salty water found underground and in salt lakes, and roughly 97.5 per cent of the planet's water is too salty to drink or use on crops without treatment.

That leaves only about 2.5 per cent as fresh water. Even this small share is mostly out of reach:

• 68.7 per cent of fresh water is frozen in ice caps, glaciers and permanent snow, mainly in Antarctica and Greenland.
• 30.1 per cent is groundwater, held in the cracks and spaces of soil and rock.
• Only a little more than 1.2 per cent is surface water and other fresh water, such as lakes, rivers, swamps, frozen ground, soil moisture and the air.

Freshwater lakes and rivers together hold about 93,100 cubic kilometres. That is roughly 1/150 of one per cent of all Earth's water. Rivers alone hold about 2,120 cubic kilometres, around 0.0002 per cent of the total.

Yet rivers and lakes supply most of the water people use every day, for drinking, farming and industry. They can do this because they are constantly topped up by rain, snowmelt and groundwater seeping into riverbeds. A small store can still carry a lot of water if water flows through it quickly.

The average time a water molecule spends in one store before moving on is called its residence time. It varies hugely from store to store. Typical ranges from the University of California Museum of Paleontology's Understanding Global Change project are:

Store	Typical residence time
Living things	hours to days
Atmosphere	5 to 15 days (about 9 on average)
Soil moisture	1 to 2 months
Rivers and streams	2 to 6 months
Valley glaciers	20 to 100 years
Freshwater lakes	50 to 100 years
Shallow groundwater	100 to 200 years
Oceans and inland seas	3,000 to 3,500 years
Deep groundwater	3,000 to 10,000 years
Ice caps	10,000 to 200,000 years

The air holds only about 0.001 per cent of Earth's water, but each molecule stays there for only about nine days before it falls as rain or snow. So the atmosphere is a tiny store with a very fast flow.

Residence times help explain why some water sources recover quickly and others do not. A river that runs low in a dry season can refill within months once the rain returns. Water in a deep aquifer may have soaked in thousands of years ago, so it is replaced extremely slowly. If people pump it out faster than rain can top it up, the store shrinks.

The same idea applies to pollution: a polluted river can be flushed clean fairly quickly, but polluted deep groundwater can stay that way for a very long time.