Optional Themes: Freshwater, Oceans, Extreme Environments, Food and Urban
What the optional themes test
SL candidates study two optional themes and HL candidates three, chosen from freshwater, oceans and coastal margins, extreme environments, geophysical hazards, leisure/tourism/sport, food and health, and urban environments. Each theme is examined through processes, spatial patterns, impacts and management, so answers must use that theme's own vocabulary rather than generic 'sustainability' language.
Theme logic
- Freshwater: the drainage basin is an open system. Storm hydrographs respond to urbanisation, soil saturation, slope, vegetation and drainage density - lag time shortens and peak discharge rises as infiltration falls.
- Oceans and coastal margins: sediment cells and longshore drift shape coasts; coral bleaching depends on local heat thresholds (roughly 1 °C above the summer maximum sustained for weeks, with mortality rising near 2 °C).
- Extreme environments: cold and arid margins recover slowly, so fragility matters more than productivity.
- Food and health: food security is not the same as food supply; the double burden of undernutrition and obesity can coexist within one country.
- Urban environments: urbanisation is the proportion living in cities; urban growth is the absolute increase.
Worked example
A 25 km² basin receives 40 mm of rain; runoff coefficient 0.35. Volume = 0.04 m × 25,000,000 m² = 1,000,000 m³. Runoff = 0.35 × 1,000,000 = 350,000 m³. Reasoning: convert millimetres to metres and km² to m² before multiplying, and state units. If the basin were urbanised, the coefficient would rise and the hydrograph peak would arrive sooner and higher.
Traps
Confusing weathering, erosion, transport and deposition. Treating the concentric zone model as valid everywhere. Assuming all reefs bleach at the same temperature. Answering with global generalities instead of the theme's specific processes.