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Global distribution & size major stores of water

Water on Planet Earth
Water found on or close to the Earth’s surface is called the hydrosphere. 2 The Hydrosphere is defined as a discontinuous layer of water at or near the earth's surface. It includes all liquid and frozen surface water, groundwater, and atmospheric water vapour.
It is an essential resource for human survival, but its distribution and availability vary within countries and across the planet.  Only 2.5% of the water on the Earth is freshwater, the rest is found in the oceans.  Unfortunately, a lot of the freshwater is “locked up” in ice caps and glaciers so is unavailable for human use.  This poses problems for people as freshwater is a scarce resource.

World water stores

About 71 percent of the Earth's surface is water-covered, and the oceans hold about 96.5 percent of all Earth's water. Water also exists in the air as water vapour, in rivers and lakes, in icecaps and glaciers, in the ground as soil moisture and in aquifers, and even in biological systems. 3
The amount of water on planet Earth has been estimated at 1,386,000,000 (1.386 billion) cubic kilometres (km3) by NASA. 3
The second issue with water availability is that it is in continuous movement across the globe.  It moves in a cycle called the hydrological cycle. Global freshwater supplies are affected by three major factors;
1. Geology – This affects where water is stored and the location of aquifers and groundwater
2. Climate – influences the availability of rainfall, snowfall and rates of evaporation.  Climate can vary over time, with wetter and drier periods, hotter and colder periods.  This can affect water availability.
3. Rivers – which move or transfer water across river basins

The distribution of this water can be seen in the pie charts and the divided bar graphs.  Be careful, there is a slight difference, as the final pie chart shows “accessible fresh water” whilst the final bar chart shows surface water and other freshwater. This classification leads to differences in the proportions quoted.

Pie charts water resources

Atmospheric stores of water
The atmosphere is the layer of gasses surrounding our planet.  The atmosphere has water held in the air as gas, clouds, and precipitation.  The Amount of water found in the atmosphere is only 12,900km3, 8% of all easily accessible freshwater.  Most of this is found in the gaseous state, water vapour, which is a colourless and odourless gas.  Water vapour is produced from evaporation or liquid water or from the sublimation of ice.  Visible water can be seen in clouds - visible masses of water droplets or ice crystals suspended in the atmosphere.
Atmospheric water is very important despite it being one of the smallest stores of water on planet Earth.  It is important because;
1. It is a greenhouse gas that absorbs, scatters and reflects incoming solar energy and outgoing terrestrial energy, so plays a role in modifying our climate
2. It redistributes water around the globe
3. It helps to “clean” the air, removing impurities when it rains

Warmer air can hold more water vapour that cooler air. For a 1°C increase in the air temperature, the atmospheric water content should increase by about 7%. 4 This means that water vapour levels in air will vary according to temperature around the globe, with high levels at the Equator (where there is available water for evaporation) and lowest levels at the Poles.

Cryospheric stores of water
The Cryosphere includes all of the frozen water in the Earth's system. This means that frozen parts of ocean, glaciers, and ice sheets are all parts of the cryosphere.  Frozen water can be found in the following forms;
Sea ice – this is ice that floats at the surface of our seas and oceans.  The extent of this ice varies between summer and winter, growing in the winter and shrinking in the summer.  The Arctic is an example of an area of extensive sea ice, and its extent and thickness has reduced in recent decades.  Whilst sea ice has no impact on changing sea levels as it melts, scientists are concerned about its reduction.  This is because sea ice reflects a lot of solar energy back to space as it has a high albedo (a measure of surface reflectivity).  Without this ice, more solar energy is ABSORBED in these regions rather than reflected. Sea ice tends to freeze at temperatures slightly below 0°C because of the salinity of sea water.
Ice Shelves are another type of sea ice – they are platforms of ice where ice sheets and glaciers extend out over the oceans from the land.  They are common in Antarctica and when the separate from the main ice sheet they become ice bergs.  In 2017 a giant section of the Larsen C ice shelf in the Antarctic peninsula broke off, unleashing a 5,000 km2 iceberg – about a quarter of the size of Wales.  5
Ice sheets – these are a mass of glacial land ice extending more than 50,000km2. The Antarctic ice sheet is one of the two polar ice caps of the Earth. It covers about 98% of the Antarctic continent and is the largest single mass of ice on Earth. It covers an area of almost 14 million square kilometres (5.4 million square miles) and contains 26.5 million cubic kilometres (6,400,000 cubic miles) of ice. 6  The Greenland ice sheet (is a vast body of ice covering 1,710,000 square kilometres (660,000 sq mi), roughly 80% of the surface of Greenland. Together, Antarctica and Greenland ice contains more than 99% of the freshwater ice on planet Earth.  If the Greenland Ice Sheet melted, scientists estimate that sea level would rise about 6 meters (20 feet). If the Antarctic Ice Sheet melted, sea level would rise by about 60 meters (200 feet). 8
Permafrost – this is soil, rock or sediment that is frozen for more than two consecutive years.  The thickness of this layer varies from just a few metres to over a kilometre in Northern regions of Canada and Russia.  The permafrost generally was created during previous ice ages and is now under threat of melting due to global warming.  This has the potential for a positive feedback loop as melting permafrost releases methane, a highly potent greenhouse gas.
Ice caps – these are smaller than ice sheets and are a mass of ice that covers less than 50,000 km3 of land area (usually covering a highland area).  They are found in mountainous areas such as the Himalaya and the Rockies, and are the source areas for many of the worlds valley glaciers.  These glaciers occupy many of the world’s valleys and their meltwater is significant as a water source for millions of people.

Terrestrial or land-based water
Terrestrial water may be considered as falling into four broad classes:

1. Surface water – the free-flowing water of streams and rivers, and the water of ponds and lakes:
2. Ground water – the water held in saturated strata below:
3. Soil water – the water held in association with air in unsaturated superficial layers of the earth.
4. Biological water – all of the water stored in plant and animal matter on Earth. 
The lithosphere is the solid outer section of Earth, including Earth's crust as well as the underlying cool dense and rigid upper part of mantle.  This is the zone we are concerned with when considering terrestrial water.
Only 3%  of the World's water is freshwater and 79% is unfortunately locked up in ice sheets and glaciers.  Another 20%
of freshwater is found in the GROUND and needs to be accessed by drilling into the earth.
Rivers and lakes are the most accessible water in the terrestrial system. 

Oceanic water
The Oceans are by far the biggest store of water.  They dominate the amount of water available, 97% of all water is found in our oceans.  Ocean water contains dissolved salts and ocean pH is 8.14 but is falling, linked to the increase in atmospheric carbon and ocean acidification.  The world's oceans contain enough water to fill a cube with edges over 1000 kilometres (621 miles) in length. The oceans contain 326 million cubic miles (1.332 billion cubic kilometers), according to a recent study from the U.S. Geological Survey..


1 - https://en.wikipedia.org/wiki/Water_cycle accessed 23rd September 2018
2 – “Geography for A-level and AS”, page 4, Skinner et al, Hodder Education, 2016
3 - https://water.usgs.gov/edu/earthhowmuch.html accessed 23rd September 2018
4 - https://www.e-education.psu.edu/earth103/node/558 accessed 23rd September 2018

5 - https://www.theguardian.com/world/video/2017/jul/12/vast-iceberg-splits-from-antarctic-ice-shelf-video-explainer

6 - https://en.wikipedia.org/wiki/Antarctic_ice_sheet  accessed 23rd September 2018

7 - https://en.wikipedia.org/wiki/Greenland_ice_sheet  accessed 23rd September 2018

8 - https://nsidc.org/cryosphere/quickfacts/icesheets.html
 

 

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