1 Concepts of system and mass balance

Concepts of System and Mass Balance

Global Water Cycle

• There is a fixed amount of water in Earth's atmosphere system.

• Water is stored in places and moved between them through various flows at different speeds.

• Global water stores include: atmosphere, ocean, soil, rivers, lakes, reservoirs, and vegetation.

• Inputs: Water flowing into a store.

• Outputs: Water flowing out of a store.

• The global water cycle is a closed system, meaning it does not gain or lose water from outside, so the total amount of water remains the same over time.

• Water can change states: it can be vapor, liquid, or solid.

• Cryosphere: The part of Earth's water stored as ice.

• Mass balance means the total water amount is kept the same globally, even if its location changes.

• Locally (in a drainage basin), the amount of water coming in equals the amount going out plus any changes in storage.

• Water moves between stores and can change state or move physically.

• Water vapor in the atmosphere falls as rain or snow and can evaporate back or flow towards bodies of water, either above ground or underground, due to gravity.

• If water cannot soak into the ground, it flows over land towards rivers, lakes, or seas.

Global Water Storage

• Oceans: Hold the majority of Earth's water in liquid form.

• Cryosphere: Holds water mostly in solid form; includes icebergs.

• Land: Contains a small amount of liquid water in rivers, streams, lakes, and groundwater (known as "blue water").

• Water is also stored in vegetation and soil, called "green water" (the invisible part of the water cycle).

• Vapor transport in the atmosphere is linked to temperature; clouds can have droplets or ice crystals.

Changes in Water Storage

• Short-term and Long-term Changes: Water storage can vary naturally over different time periods.

• Seasonal changes in the cryosphere, like ice buildup and melting, cause water flow changes (e.g., Alps, Andes, Tibetan Plateau).

• Steady-state equilibrium: the system maintains balance over the long term despite changes in inputs and outputs each month or season.

• Arctic Sea Pack Ice reduction: reduced by up to 50% in summer, reforming in winter. Evidence suggests this size reduction may be permanent due to climate change.

Long-term Changes in Cryosphere Storage

• Historical fluctuations: Ice ages and warmer periods have led to large changes in Earth’s ice coverage over millions of years.

• Cryogenian ice ages (around 650-750 million years ago) involved significant ice storage.

• During Paleocene and early Eocene epochs (65-35 million years ago), polar areas likely had no ice caps, leading to high global sea levels.

• Milankovitch cycles: These are long-term processes that affect Earth's orbit and climate, resulting in warming and cooling:

  • Every 100,000 years, Earth's shape changes from circular to oval.

  • A cycle repeats every 41,000 years, affecting the angle of Earth's tilt between 22° and 24.5°.

  • A cycle every 22,000 years involves a wobble of Earth’s axis.

Changing Processes and Water Transfers

• Plate tectonics: Have affected ice storage, like the formation of Antarctica.

• Seasonal Changes in Rainfall: Wind pattern shifts lead to uneven rainfall.

• Biggest impacts: India, Pakistan, China, East Africa, Australia.

• Example: In Mangalore, India, two-thirds of the annual rainfall occurs in just three months.

Climate Variability

• El Niño Southern Oscillation (ENSO): Occurs every 3-7 years and lasts about 18 months.

• Affects global rainfall patterns:

  • Dryer conditions in Southeast Asia, India, and Australia.

  • Increased rain in east-central Pacific areas, including California.

Long-Term Changes in Rainfall and Aridity

• Climate change is expected to change rainfall patterns.

• Areas with heavy rain may see even more rain, while dry regions may experience less.

• High-risk areas for being drier: desert edges in Australia, China, USA, and the Sahel region.

• The Sahel has shown signs of long-term rainfall reduction recently.

Risks of Melting Ice Sheets

• Climate change is causing ice sheets like those in Greenland and Antarctica to melt.

• These ice sheets contain more than 99% of Earth's fresh water ice.

• Antarctica is losing more ice than it gains (70 billion tonnes/year).

• The complete melting of the Greenland Ice Sheet could raise sea levels by 6 meters; for the Antarctic Ice Sheet, it could rise by 60 meters.