4.1 Water systemsIB Environmental Systems and Societies HL: Revision notes

Movements of water in the hydrosphere are driven by solar radiation and gravity. The global hydrological cycle is a system of stores and flows.

Stores (share of Earth's water): oceans 96.5%, glaciers and ice caps 1.7%, groundwater 1.7%, surface freshwater 0.02%, atmosphere 0.001%, organisms 0.0001%.

Flows: transpiration, sublimation, evaporation, condensation, advection, precipitation, melting, freezing, surface run-off, infiltration, percolation, streamflow and groundwater flow.

Agriculture (irrigation, compaction), deforestation (less interception and transpiration, more run-off) and urbanization (impermeable surfaces, drains) alter flows and stores. The Aral Sea shows how diverting rivers for irrigation can cut inputs so that evaporation outputs exceed them and the lake shrinks.

A water body is in a steady state when inputs equal outputs; this is shown on a flow diagram. If outputs exceed inputs, the store shrinks.

Water has unique physical and chemical properties that support and sustain life:

• Polarity and hydrogen bonding make it an excellent solvent for metabolism and transport.
• Cohesion and adhesion allow transport in plants and capillary action.
• High specific heat capacity and latent heat keep temperatures stable in water bodies and organisms.
• Ice floats, because water is densest at about 4 °C, so lakes freeze from the top and life survives beneath.
• High buoyancy supports aquatic organisms.

The oceans act as a carbon sink: they absorb carbon dioxide from the atmosphere, about a quarter of human emissions, and sequester it.

• Short term: dissolved CO2 reacts with water to form carbonic acid, releasing H+ ions. This causes ocean acidification, lowering pH (about 0.1 units since the industrial revolution). Dissolved CO2 can be released again, especially as water warms.
• Longer term: carbon is taken up by phytoplankton into biomass. When organisms die, their remains sink and accumulate on the seabed, locking carbon away for long periods.

Water temperature varies with depth: warm water above, cold below. Differences in density restrict mixing, giving persistent stratification. It occurs in deeper lakes, coastal areas, enclosed seas and the open ocean. A thermocline is the transition layer between the warm mixed layer at the surface and the cooler water beneath it.

Global warming and salinity changes have increased the intensity of ocean stratification, reducing mixing of oxygen and nutrients.

Upwellings in oceans and freshwater bodies bring cold, nutrient-rich water to the surface, raising productivity (for example the Peruvian anchoveta fishery).

Thermohaline circulation is driven by differences in temperature and salinity, which cause differences in water density. Cold, salty water at high latitudes sinks, and surface water flows to replace it, forming the ocean conveyor belt that distributes heat around the world and so affects climate. For example, it helps keep north-west Europe milder than other places at the same latitude. Melting ice adds fresh water and could weaken the circulation.