1.2

• Where is most of Earth’s freshwater stored?

  In the cryosphere (glaciers and ice sheets) and as groundwater.
• How much freshwater is found in rivers, lakes, the atmosphere, or biosphere?

  Just 0.4% of global freshwater.
• What is the annual water transfer between oceans and atmosphere?

  400,000 km³ evaporates and 370,000 km³ falls as precipitation.
• How much water evaporates and precipitates between land and atmosphere?

  60,000 km³ evaporates and 90,000 km³ precipitates annually.
• What is the global surface runoff from land to ocean each year?

  Around 30,000 km³.
• How much of Earth’s total water is freshwater?

  Only 2.5%.
• What is meant by 'residence time' in the hydrological cycle?

  The time water stays in a store, ranging from a week to 10,000 years.
• Why is the global hydrological cycle considered a closed system?

  Because total inputs equal total outputs—water is neither added nor lost.
• What are the two key drivers of the global hydrological cycle?

  Solar energy and gravitational potential energy.
• What happens to evaporation and precipitation in a warming climate?

  Evaporation and atmospheric moisture increase, leading to more precipitation.
• What is the global water budget?

  The balance between inputs and outputs; oceans lose more than they gain, land gains more than it loses.
• What is fossil water?

  Ancient underground water stores found under polar areas and deserts.
• Why are the Tropics important to the hydrological cycle?

  High solar radiation causes evaporation; vapour rises at the ITCZ and forms clouds.
• What is the ITCZ, and why is it significant?

  The Inter-Tropical Convergence Zone is the biggest flux, moving water from oceans to land.
• Why are the polar regions important for water and heat transfer?

  They store 66% of freshwater and help drive thermohaline circulation.
• What drives the thermohaline circulation?

  Cold, dense polar water sinks, pulling in warmer water from the Tropics and circulating deep ocean currents.