Earth is sometimes referred to as the “Blue Planet” because 71% of its surface is covered by water. With so much water, why do we worry about shortages?
Water is not naturally salty, so where did the salt in the oceans come from?
This salt has accumulated over the course of more than four billion years from the chemical weathering of rock.
The oceans are 3.5% salt. Most salt is from the weathering of rocks on land, but deep-sea vents also release ions into the water. There is so much salt in the oceans that if you removed it and spread it over the land, it would be 166 metres thick. That is the height of a 40-story building!
Most of Earth’s fresh water is frozen in the polar ice caps, sea ice, snow and glaciers. We refer to this frozen water as the cryosphere. Seasonal melting of glaciers and snow provides vital water for many rivers. However, as Earth warms, the cryosphere is shrinking and these freshwater reserves are flowing into the ocean. Global sea level is rising at an ever-increasing rate due to global warming. The 2019 IPCC report summarises the scientific evidence for these changes and estimates that sea level could rise 38 cm by 2100.
Groundwater may be a centimetre or a kilometre below the surface, located in sediments and porous rocks. Shallow groundwater may be continuous with rivers, lakes or oceans. Deeper layers may be trapped within rock formations or may eventually discharge into the sea. Some groundwater is highly saline, causing dryland salinity in Australia if the water table rises. Other groundwater is fresh and an important source of drinking water. For example, most of Western Australia’s drinking water comes from groundwater.
Water for Plants and Animals
Plants may access very shallow groundwater but tend to rely upon water trapped in the pore spaces of soil. Some animals are able to get all of their water from food, but most rely upon the scarcest type of water – that in lakes, swamps and rivers. Thus, the plants and animals on land rely on less than 1% of Earth’s water.
Although we tend to think of lakes as fresh water, this is not always the case. Australia’s famous Kati Thanda-Lake Eyre is a well-known salt-lake that covers up to 9500 km2 when it is full.
- Visualise the amount of water in each of Earth’s water stores in our video Representing Earth’s Water.
- Find out about water’s amazing properties in our Wonderful Water blog and explore our videos about water’s properties and specific heat.
- Our World Ocean Day blog post will tell you about our amazing oceans.
- Become a water expert with the USGS Water Science School.
- Geoscience Australia has a section of resources about groundwater and the Bureau of Meteorology provides many different types of information about groundwater resources.