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Water on EarthWater on Earth

Water covers about 71% of the Earth's surface and this has allowed life to develop.


link Hydration, water, and health

link Water in the atmosphere

link Hard water, descaling, and desalination

link Water and astrobiology

link Water and life

V Water in the mantle
V Salt water (seawater)

V Water and global warming

V Water cycle
V Water availability
V Water use

Water, water, every where,
Nor any drop to drink.

Coleridge, 1798     

English lake districtWater in the mantle

Water and ice have clearly been important in shaping the surface of the Earth due to erosion from rivers and glaciers (see left for the glacier-eroded U-shaped valleys in the Lake District of the UK). As well as adding to the beauty of the countryside, this increases the area of fertile areas for agriculture.


Water plays a crucial role in the mantle by reducing friction between moving rock and reducing the melting point of such rock and lowering its density, so encouraging the movement of the tectonic plates, seismic activity (earthquakes) and volcanic activity.

The tectonic plates and their movements, from the United States Geological Survey


The outer shell of the Earth (lithosphere)

consists of separate and distinct extensive rock layers (tectonic plates; see left), which float on the solid but fluid-like viscoelastic layer beneath (asthenosphere). The relative fluidity of the asthenosphere allows the tectonic plates to move (see red arrows left). The movement is caused by upswelling of the asthenosphere, as particularly noticed in the mid-Atlantic ridge; a deep rift valley that spreads about 2.5 cm per year. This motion is facilitated by water in and between the layers that reduces the friction. The water involved is drawn deep into the Earth (subduction) as one plate rides on top of another, taking many millennia to reappear in volcanic gasses.

Salt water (seawater)

Over time, water has reacted with the elements resulting in the minerals that make up the Earth's surface and the oceans' solutes [2270]. Most of the free water on Earth is salt water (seawater); an electrolyte solution that has a somewhat similar composition (if not total concentration) all over the planet. Natural seawater salinity varies and changes as a result of currents and mixing of water from different depths and densities, by rainfall and evaporation at the surface, by the freezing and melting of sea ice, and by the freshwater discharged from rivers and glaciers.To avoid even this small variation a 'standard' seawater has been defined and its thermodynamic properties described [1452].

hydrosphere salinity, temperature  and density, from http://en.wikipedia.org/wiki/World_Ocean_Atlas

The salinity [2474] of this IAPSO 'standard' seawater is 35.16504 g ˣ kg-1 exactly (1.16 molal ions; ionic strength 0.67 M; density 1.02346 g ˣ L-1; triple point 271.24 K, 521.56 Pa; melting point -1.91 °C; boiling point 100.56 °C; critical point 29.8 MPa, 680 K; osmotic pressure 2.576 MPa, surface tension 0.0728 N ˣ m-1 at 25 °C); containing many ionic species but mainly Na+ 10.781 g ˣ kg-1, Mg2+ 1.284 g ˣ kg-1, Ca2+ 0.412 g ˣ kg-1, K+ 0.399 g ˣ kg-1, Sr+ 0.008 g ˣ kg-1, Cl- 19.353 g ˣ kg-1, SO42- 2.712 g ˣ kg-1, HCO3- 0.105 g ˣ kg-1, Br- 0.067 g ˣ kg-1, CO32- 0.014 g ˣ kg-1, and B(OH)3 0.008 g ˣ kg-1 [1452]. This is just over one molal salt ion and almost four times the concentration of salt in our blood, meaning that it cannot be used as the sole source of water for drinking, as we cannot get rid of that concentration of salt once ingested. Atmospheric gas is naturally dissolved in seawater, typically about 7, 13 and 0.3 ml STP liter-1 (20 °C) for O2, N2 and Ar respectively at 10 m depth and about 5, 14 and 0.4 ml STP liter-1 (20 °C) for O2, N2 and Ar respectively at 5000 m depth [2897].

density of seawaterThe surface of seawater begins to freeze at −1.91  °C with the ice excluding the salt. a This ice floats on the surface and the salt that is "frozen out" adds to the salinity of the remaining liquid seawater just below it, making it denser and causing it to sink towards the bottom, carrying dissolved oxygen with it. This, together with changes in salinity and temperature (both changing density, see left) has a major influence on ocean currents and behavior. For example, high-density water in the North Atlantic sinks, carrying oxygenated water to great depth, which then flows back towards the equator.


The temperature of maximum temperature decreases with increasing salt concentration (see below. psu are units on the Practical Salinity Scale b) until the water freezes at about -1.3  °C at a density about 1015 kg ˣ m-3.


Maximu density of salt water, psu are units on the Practical Salinity Scale based on conductivity

There has been recent concern over the acidification of the oceans and the consequential loss of dissolved carbonate from before the industrial revolution to the present (see below for 1751-1994) due to higher and rising atmospheric CO2 [2221].Estimated change in annual mean sea surface pH (and carbonate, mouse over) between the pre-industrial period and the present day from the Global Ocean Data Analysis Project

Ocean acidification [2222]

If this continues then further carbon dioxide dissolution will alter the pH [2474] and carbonate chemistry of seawater and so reduce marine calcifying organisms.


CO2 + CaCO3 (solid) + H2O = Ca2+(dissolved) + 2 HCO3-


The relationship between atmospheric CO2, global warming and ocean acidification [2222] is very complicated with several feedback and feed-forward controls and different modeling approaches giving rise to different potential outcomes. It is likely that continued increases in CO2 together with the related warming of the oceans will have substantial negative effects on marine ecosystems over the next 50 or so years [2352].


The salt composition causes seawater to behave like pure water under extra pressure and so many of the anomalies are reduced or lost [2179], particularly at higher salinity. For example, no maximum density is apparent as the seawater density increases as the temperature is lowered towards its freezing point.


seawater does have some industrial uses such as in cooling power plants, and for conversion into drinking water.

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Water and global warming

The maximum water in the air varies with temperature, at 1 atm

Water is the main absorber of the sunlight in the atmosphere; without it, the Earth would be in a permanent ice-age. The 13 trillion tons of water in the atmosphere (~0.33% by weight; compare CO2 ~0.04% ) is responsible for about 70% of all atmospheric absorption of radiation. This is mainly in the infrared region where water shows its strongest absorption. It contributes 50 % to 60 % to the Greenhouse effect; more than twice that due to animal and anthropogenically associated carbon dioxide, ensuring a warm habitable planet, but it operates a negative feedback effect, due to the cloud formation that reflects the sunlight away (see Quicktime movie) and the oceanic export of latent heat, to attenuate global warming. Note that, without this Greenhouse effect, the average temperature on Earth would be well below the freezing point.


Clouds both reflect sunlight away from the Earth and trap the heat that radiates from the Earth's surface, resulting in a complex relationship between global temperature and cloud cover. The water content of the atmosphere varies about 100-fold between the hot and humid tropics and the cold and dry polar ice deserts (see right and Quicktime movie). Clouds have a complex and controversial relationship in global warming as increased cloud cover insulates the planet so contributing to global warming by preventing heat loss, but also reflects back sunlight into space, so cooling the planet; with relative effects dependent upon the type and height of the clouds. If increasing water vapor leads to warmer temperatures, more water vapor will be incorporated into the atmosphere and warming and water absorption will increase in a spiraling cycle. By contrast, if increased cloud cover leads to cooling then the system will be stabilized. Changes in other gasses, such as CO2 will feed into these effects [2222]. On balance, it appears that increased atmospheric CO2 is feeding into a global warming spike in an otherwise cooling Earth.


             Cloud cover                   September 2014           Water vapor without clouds

Atmospheric vapor content (cm of water equivalent) from NASA satellite observations; Imagery by Reto Stockli, NASA's Earth Observatory

Cloud cover from NASA satellite observations; Imagery by Reto Stockli, NASA's Earth Observatory


Cloud condensation nuclei changes over the industrial age, from [2424]


The current knowledge concerning ice formation within clouds has been outlined [2477]. Cloud condensation nuclei (aerosol particles, cloud seeds; such as sea salt, carbon black and 'dust') are particles with a dry diameter larger than 70 nm, which form cloud droplets at 0.22 % supersaturation. They are seen to have increased substantially during the industrial age [2424]. Over half of the atmosphere's cloud condensation nuclei are formed in situ from highly oxidized molecules, such as sulfuric acid, binding ammonia [2584].


Polar ice affects our climate as its white surface has a high aldebo (reflectance) of up to about twelve times that of the open ocean. Any reductions of sea ice or the darkening of the ice caps, due to lack of fresh snow, exerts positive feedbacks in global warming.

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Water cycleWater cycle

An outline of the water cycle on Earth is thought to have been first recognized and observed by Leonardo da Vinci (1452-1519) [2494]. Now, it can be partially represented by the diagram right [2357]. Water has a lifetime in the atmosphere of about a week, on average, before precipitation. 20% of this precipitation over land evaporates before it reaches the ground. Of the remaining 80% about a fifth is run-off into the river system. Most of the remainder goes into the soil and vegetation and 3/4 of this is returned to the atmosphere by transpiration with the remainder seeping to the river system or evaporating from the soil.


Transpiration is the major route for the return of water to the atmosphere over land with plants releasing about 300 molecules of water for every CO2 molecule captured during photosynthesis [2357].

Some seawater gets subducted, between tectonic plates, hundreds of kilometers down into the mantle taking with it small amounts of organic carbon and taking many millennia to reappear. Reactions occur under these extreme conditions (900 °C, 5.0 GPa) and materials like oil and diamonds may be formed [2453].

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Water availability

Nordaustlandet ice capOnly 2.5% of the water on Earth is fresh water (< ~0.5 g ˣ kg-1 salt in widely varying proportions; fresh water is not necessarily potable water). Most of that fresh water is frozen in the ice caps of Antarctica, Greenland, and Svalbard. Most of the remainder is inaccessible. Less than 1% of the world's fresh water is in lakes, rivers, reservoirs and accessible underground sources (~1014 m3) that can be used for drinking, cooking, hygiene, industry, and agriculture. Over half the worlds population has access to piped water on premises but about 10% still have to use surface water and other poor sources. Although small as a percentage this 10% represents over 700,000,000 people. The map below shows the blue water (fresh surface water and groundwater) scarcity throughout the world [2603].


The number of months per year in which blue water need was not met; 1996-2005 from [2603]


There have been great recent improvements in India and China but much of the rural areas of Africa are poorly served and not improving year on year [2174]. Few people these days consider fresh water to be abundant and generally they believe that there are likely to be shortages in the future. We need adequate, safe and affordable water to lead healthy lives and to build secure livelihoods [2176]. It will be challenging to achieve this end for the poorest in society.

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Water use

Water is required for drinking (~3 L ˣ day-1 ˣ person-1), cooking (~10 L ˣ day-1 ˣ person-1), sanitation (~20 L ˣ day-1 ˣ person-1) and washing (~15 L ˣ day-1 ˣ person-1). In western cities, we are wasteful in our water usage, using 6-10 times these necessary amounts, whereas in rural third-world countries they get by on 10 L or less per day. The global domestic average is 184 L ˣ day-1 ˣ person-1 (455 km3 ˣ year-1 ) and the global industrial average is 300 L ˣ day-1 ˣ person-1 (739 km3 ˣ year-1 ) [2570]. Only about 1.7 % of the domestic water waste is reused, mainly for irrigation, with the rest being wasted through evapotranspiration or the sewerage system. Recent approaches to urban water management have been reviewed [2570]. The water we use domestically forms a very small amount of the fresh water required for modern life. As examples [2175], a liter of beer requires 300 liters (including growing the barley; a liter of milk requires 1000 liters (including farming the cows); a cup of tea requires 120 liters, a kg of potatoes requires 950 liters; a pair of leather shoes requires about 10,000 liters; a kg of beef requires up to 70,000 liters, and even a 2-gram microchip requires 32 liters. As water supplies become more expensive and subject to greater demand, ways are constantly being sought to reduce these industrial water usage figures.


Water plays a central role in the world's religions as it is necessary for life and is a cleanser of body and spirit.

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a Sea ice consists of pure water ice plus air pockets and very salty liquid water inclusions that gradually drain away with time. It differs from glacial ice, which does not contain any salt water inclusions. [Back]


b A full chemical analysis of seawater is too time-consuming for routine use. The Practical Salinity Scale 1978 has been developed so that routine salinity determination may be simply made for a comparison between different studies [2484]. The Practical Salinity Scale defines salinity in terms of the conductivity ratio of a sample to that of a standard solution of 32.4356 g of KCl at 15 °C in a 1 kg solution. A sample of seawater at 15 °C with a conductivity equal to this KCl solution has a salinity of exactly 35 practical salinity units (psu). Other conductivities are associated with salinities using empirical relationships, roughly equivalent to g ˣ kg-1 solution. [Back]




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