Kinds and amounts of water
The hydrosphere is the water part of Earth. About 97.5% is salt water in the oceans. Only 2.5% is fresh. Of that fresh water roughly 69% is ice (glaciers, ice caps), 30% is groundwater and about 1% is in lakes, rivers, soil and air. So the water we can easily reach is a very small share.
Sea water properties and pollution
Sea water has salinity, the amount of dissolved salt, about 35 g per kg on average. It is higher where evaporation is strong and rain is low, and lower near river mouths and melting ice. Salt makes sea water heavier and gives it a lower freezing point than fresh water.
Pollution of the sea comes from oil spills, plastic, sewage and farm chemicals. It harms fish and corals and spreads in currents.
Surface ocean currents and their effects
Surface currents are large streams of sea water driven mostly by steady winds and shaped by Earth's spin and by coastlines. Warm currents carry heat from the tropics towards the poles and warm the coast. Cold currents flow towards the equator and cool the coast. Currents change climate, bring fish-rich water and help ships.
River networks
A river system has a source, many small tributaries that join the main stream, and a mouth where it meets the sea or a lake. The land that gives water to one river is its drainage basin. Rivers carry water, soil and nutrients, and shape valleys and plains.
Glaciers
A glacier is a very large body of ice that slowly flows downhill under its own weight. Mountain glaciers feed rivers in summer. Ice sheets in Greenland and Antarctica hold most of the world's fresh water.
Shrinking polar ice
As the planet warms, polar ice and glaciers melt faster. Melting land ice adds water to the sea, so sea level rises. Floating sea ice, when it melts, does not raise sea level much (like an ice cube in a full glass), but it exposes dark water that absorbs more heat. Warmer sea water also expands, adding to the rise.
Try it
In the 3D, count the blocks of fresh water. Then use the salinity slider and predict: does the beaker get saltier or fresher? At home: float an egg in plain water, then add spoons of salt. The egg rises as the water becomes saltier. Also, fill a glass with ice and water to the top, and see that it does not overflow as the ice melts.
Key formulas and definitions
- Sea water 97.5% + fresh water 2.5% = 100%
- Fresh water: ice about 69%, groundwater about 30%, other about 1%
- Average salinity of sea water is about 35 g per kg (35 per mille)
- Drainage basin = land that sends water to one river
Worked examples
1. Out of 1000 litres of Earth's water, how many litres are fresh?
2.5% of 1000 = 25 litres are fresh. Of these about 17 litres are ice and 7.5 litres groundwater.
2. A kilogram of sea water has 35 g salt. How much salt is in 200 kg?
35 g x 200 = 7000 g = 7 kg of salt.
3. Why is the sea near a river mouth less salty?
Fresh river water mixes in and dilutes the salt, so salinity falls.
Common mistakes
- Thinking most fresh water is in rivers. Most is frozen in ice, then underground.
- Saying all ocean water has the same salt. Salinity changes with evaporation, rain and river inflow.
- Thinking melting floating sea ice and melting land ice raise the sea equally.
- Mixing up warm and cold currents. Warm ones come from the tropics, cold ones from near the poles.