Oceans, Hydrosphere, and the Cryosphere
Domain 8 examines ocean basins, water chemistry, circulation, waves and tides, groundwater, watersheds, and ice.
Core rules and concepts
- Oceans cover about 71% of Earth; salinity averages about 35‰. Evaporation and precipitation, river input, and ice formation modify salinity.
- Surface currents are wind-driven; thermohaline circulation is density-driven. Cold, salty water sinks and drives deep flow.
- Waves transfer energy, not water, across the ocean; tides are gravitational, mainly lunar and solar.
- Upwelling brings nutrient-rich deep water to the surface and supports fisheries.
- The water cycle stores water in oceans, ice, groundwater, and atmosphere; residence time = storage / flux.
- The cryosphere includes glaciers, ice sheets, sea ice, and permafrost. Melting land ice raises sea level more than floating sea ice.
Worked example
A lake holds 10,000 km³ of water and receives 200 km³/yr. Average residence time = 10,000/200 = 50 years. This assumes steady state and complete mixing. If outflow increases, residence time falls. The same logic applies to groundwater or atmospheric water.
Traps that cost marks
Do not call a tsunami a tide. Tides are predictable; tsunamis are seismic. Melting floating sea ice displaces little new water, but melting glaciers and ice sheets add water. Groundwater is not an infinite reservoir; pumping can exceed recharge. El Niño involves ocean–atmosphere coupling, not just warmer water.