Origin and Distribution of Water in the Ecosphere: Water and Prehistoric Life

The Eccentric Liquid

Water is the only inorganic liquid that occurs naturally on earth. It is also the only chemical compound that occurs naturally in all three physical states: solid, liquid and vapour. It existed on this planet long before any form of life evolved but, since life developed in water, the properties of the 'Universal Solvent' or 'Life's natural habitat' or 'Life's preferred habitat' came to exert a controlling influence over the many biochemical and physiological processes that are involved in the maintenance and perpetuation of living organisms. It is therefore in order to discuss briefly the occurrence of water on earth, its distribution and its controlling influence on the development of life.

The Hydrologic Cycle

At present we are left with several puzzles concerning the composition of the atmosphere and the quantity of water in the hydrosphere. It seems safe to assume that the presence of water in the liquid state can only date from a time when the temperature of the earth's crust had dropped to below the critical temperature of water, 374 °C. If all the water that now makes up the oceans had previously existed as a supercritical water atmosphere, then the pressure per square metre of earth surface would have been 25 MPa. During cooling to below the critical point, vast masses of water would have condensed onto the earth's surface and also penetrated deep into rock crevices. Some of this water would immediately have boiled off again, to be recondensed at a later time. The hydrologic evaporation–condensation cycle could thus have begun several billion years ago. It is therefore irrelevant whether the heat of the earth itself or solar radiation initiated the cycle. What is relevant, however, is the scale of the water movement. The total water content of the atmosphere is 6 x 108 ha m (1 ha m = 10000 m3. This is the amount of water which will cover an area of 1 ha to a depth of 1 m. Since the total annual precipitation is 225 x 108 ha m, the water in the atmosphere is turned over 37 times every year. This level of precipitation is equivalent to a water depth of 0.5 m averaged over the earth's surface. Such an averaging is of course meaningless, because the level of precipitation is quite non-uniform in time and space.

Although the hydrologic cycle, shown in Figure 1.1, is a continuum, its description usually begins with the oceans which cover 71% of the earth's surface. The heat of the sun causes water to evaporate. Under the influence of certain changes in temperature and/or pressure, the moisture condenses and returns to earth in the form of rain, hail, sleet or snow – collectively referred to as water of meteoric origin. It falls irregularly with respect to geographical location, with coastal areas receiving more.

Of the average rainfall, about 70% evaporates; the remainder appears as liquid water on or below the land surface. Some water evaporates in the air between the clouds and the land surface. The remaining losses are of two forms: direct evaporation from wet surfaces and transpiration through plants from their leaves and stems. The 30% of water not directly returned to the atmosphere constitutes the runoff and provides our potentially available freshwater supply. Actually, the proportion of the earth's total freshwater resources which participates in the hydrologic cycle does not exceed 0.003%; the remainder is locked up in the Antarctic ice cap. If melted, it would supply all the earth's rivers for 850 years.

Enormously large quantities of water participate in the cycle, as shown in Table 1.1. The oceans constitute by far the largest proportion of our water resources, with the Antarctic ice cap as the major freshwater reservoir. By comparison, all the other contributions are of a minor nature.

Available Water and Global Warming

The most important source of readily available, albeit recycled, fresh water is rain, the distribution of which is quite erratic. As a result of rainfall and percolation from the water table to the topsoil, the total volume of moisture in the soil is 25 000 km3. Plants normally grow on what is considered to be 'dry' land, but this is a misnomer, because even desert sand contains up to 15% of water. It appears that plant growth requires extractable water; thus, an ordinary tree withdraws and transpires about 190 1 per day. Groundwater is an increasingly important source of fresh water. Indeed, less than 3% of the earth's available fresh water occurs in streams and lakes, although the proportion is much higher in the UK. Groundwater hydrology is a relatively young, but rapidly growing, branch of science and technology, mainly as a result of the growth of research in botany, agriculture, chemistry, physics and meteorology.

Now that global warming is believed to cause a major future threat, the numbers in Table 1.1 assume a particular significance. If the Antarctic ice cap were to recede and become subject to partial melting, the water so produced would contribute to a rise in the sea level, in addition to any rise caused by thermal expansion of the oceans. Unfortunately such water which is now part of our freshwater resources, albeit locked up, would become useless for immediate utilisation.

Water and the Development of Life

Studies of the origin of water in the universe and the prehistoric changes that may have occurred in the composition of our atmosphere and hydrosphere make fascinating reading. The search for water has become an important aspect of space exploration. The existence of ice in many cold stars and meteorites is now firmly established. It is also believed that, next to hydrogen, oxygenated hydrogen is the most abundant chemical species in outer space. In principle, wherever ice exists in an extraterrestrial cold environment, there should also be evidence of water vapour, since ice has a finite sublimation pressure. Indeed, water vapour has been detected on our moon, on Mars, and moons of Jupiter and Saturn. However, during the past decade noncrystalline water has also been detected in the photosphere of the sun. By comparison with high-temperature emission spectra of very hot water, the infrared lines observed in sunspot spectra have been assigned to characteristic rotation and...