The variability in soil moisture is mainly governed by different rates of evaporation and precipitation, so that for example, severe drought can result in features such as hard, dry, cracked soil, while floods and landslides can be a consequence of very heavy rainfall. Less obvious perhaps is the fact that some areas of the Earth's oceans are significantly 'saltier' than others. Changes in the salinity of surface seawater are brought about by the addition or removal of freshwater, mainly through evaporation and precipitation, but also, in polar regions, by the freezing and melting of ice. Variability in soil moisture and ocean salinity is due to the continuous exchange of water between the oceans, the atmosphere and the land – the Earth's water cycle.

Terrestrial and atmospheric components of the water cycle

Soil moisture

Fact: 'About one-third of the Earth's land surface is desert'

The importance of estimating soil moisture in the root zone is paramount for improving short- and medium-term meteorological modelling, hydrological modelling, the monitoring of plant growth, as well as contributing to the forecasting of hazardous events such as floods.

The amount of water held in soil, is of course, crucial for primary production but it is also intrinsically linked to our weather and climate. This is because soil moisture is a key variable controlling the exchange of water and heat energy between the land the atmosphere. Precipitation, soil moisture, percolation, run-off, evaporation from the soil, and plant transpiration are all components of the terrestrial part of the water cycle. There is, therefore, a direct link between soil moisture and atmospheric humidity because dry soil contributes little or no moisture to the atmosphere and saturated soil contributes a lot. Moreover, since soil moisture is linked to evaporation it is also important in governing the distribution of heat flux from the land to the atmosphere so that areas of high soil moisture not only raise atmospheric humidity but also lower temperatures locally.

Sea-surface salinity and global circulation pattern

Ocean salinity

Fact: Between latitudes of 35°N and 35°S, the Earth receives more heat from the Sun than it loses to space. Poleward of these latitudes it loses more heat than it receives. The tropics would keep getting hotter and the poles would keep getting cooler if heat were not carried from the tropics by wind and ocean currents. Ocean currents are driven by temperature and salinity variations in the seawater.

Thermohaline circulation

In the surface waters of the oceans, temperature and salinity alone control the density of seawater – the colder and saltier the water, the denser it is. As water evaporates from the ocean, the salinity increases and the surface layer becomes denser. In contrast, precipitation results in reduced density, and stratification of the ocean. The processes of seawater freezing and melting are also responsible for increasing and decreasing the salinity of the polar oceans, respectively. As sea-ice forms during winter, the freezing process extracts fresh water in the form of ice, leaving behind dense, cold, salty surface water.

If the density of the surface layer of seawater is increased sufficiently, the water column becomes gravitationally unstable and the denser water sinks. This process is a key to the temperature- and salinity-driven global ocean circulation. This conveyor-belt-like circulation is an important component of the Earth's heat engine, and crucial in regulating the weather and climate.