The physical environment: atmosphere, hydrosphere, lithosphere and their interactions
This domain tests the Earth's physical environment as interacting spheres and the quantitative relationships between them. Expect water-balance and energy-budget reasoning alongside descriptive questions.
Atmosphere
- Dry air is about 78% nitrogen, 21% oxygen, 0.9% argon and 0.04% carbon dioxide; water vapour varies.
- Layers: troposphere (weather, lapse rate), stratosphere (ozone layer), mesosphere and thermosphere.
- Greenhouse gases absorb and re-emit infrared radiation; albedo controls reflected shortwave energy; circulation redistributes heat.
Hydrosphere
- The water cycle links precipitation, evapotranspiration, runoff and storage; residence times differ greatly between atmosphere, rivers, aquifers and oceans.
- A watershed is the area draining to a point; an aquifer is a permeable unit storing and transmitting groundwater.
- Porosity is storage capacity; permeability is the ability to transmit flow. Seawater intrusion follows overabstraction near coasts.
- The Alboran Sea and Gulf of Cadiz connect Atlantic and Mediterranean water masses; upwelling and currents control productivity.
Lithosphere and soils
- Weathering may be physical, chemical or biological; soil formation depends on climate, parent material, relief, organisms and time.
- Interactions include climate-driven erosion, vegetation stabilising slopes, karst dissolution and coastal sediment transport.
Worked example
A 500 km2 catchment receives 600 mm/yr; evapotranspiration is 400 mm/yr and storage is unchanged. Estimate runoff. Water balance: P = ET + R + change in storage, so R = 600 - 400 = 200 mm/yr. Volume = 0.2 m x 500 x 10^6 m2 = 1 x 10^8 m3/yr. Convert millimetres to metres before multiplying by area.
Traps that cost marks
Weather and climate are conflated; stratospheric ozone is confused with surface photochemical ozone; porosity and permeability are treated as synonyms; residence time is confused with renewal rate; millimetres over square kilometres are mishandled.
Key points
- The water balance is P = ET + R + storage change.
- Porosity stores water; permeability transmits it.
- Stratospheric ozone shields; tropospheric ozone pollutes.
- Soil formation depends on five interacting factors.
- Alboran and Cadiz water masses link Atlantic and Mediterranean systems.