Systems ans Cycles

Water moves continuously between land, oceans and the atmosphere — driven by the Sun's energy.

1. Evaporation: heat from the Sun turns water from oceans, lakes and rivers into water vapour.
2. Transpiration: plants release water vapour from their leaves (sometimes lumped in as 'evapotranspiration').
3. Condensation: water vapour cools high up, condensing into droplets that form clouds.
4. Precipitation: rain, snow, hail — water returns to the ground.
5. Runoff: rainwater flows back into rivers, then to the sea.
6. Infiltration: some water sinks into the ground, becoming groundwater that eventually returns to surface water.

The total water on Earth is essentially constant; only its location changes. Human disruptions: damming rivers (changes flow patterns), urbanisation (paving stops infiltration), climate change (alters precipitation patterns).

Carbon constantly cycles between living things, the atmosphere, oceans and rocks.

Key processes:

• Photosynthesis: plants and algae take in CO₂ from air → make glucose. Removes CO₂.
• Respiration: animals AND plants release CO₂ from breaking down glucose. Adds CO₂.
• Decomposition: bacteria/fungi break down dead matter, releasing CO₂.
• Combustion: burning fuels releases CO₂. Includes fossil fuels (HUGE source of CO₂) and natural fires.
• Ocean absorption: oceans dissolve CO₂; some becomes carbonate rocks over millions of years.

In a STABLE pre-industrial world, photosynthesis ≈ respiration + decomposition. Atmospheric CO₂ stayed roughly constant.

Today, FOSSIL FUEL BURNING releases CO₂ that was locked away millions of years ago. The cycle can't reabsorb it fast enough → atmospheric CO₂ rising → greenhouse effect → climate change.

Deforestation also reduces CO₂ removal (fewer trees photosynthesising).

~78% of air is nitrogen (N₂) — but plants and animals can't use it directly because the N≡N triple bond is too strong to break.

Key transformations:

1. Nitrogen fixation: N₂ → nitrates. Done by LIGHTNING (high-energy spark splits N₂) and by NITROGEN-FIXING BACTERIA (e.g. in nodules on legume roots). Industrial: Haber process makes ammonia for fertilisers.
2. Plant uptake: plants absorb nitrates from soil through roots → use to make proteins and DNA.
3. Animal feeding: animals get nitrogen by eating plants (or other animals).
4. Decomposition: when organisms die, decomposers break down proteins → ammonia → nitrates (returning to soil).
5. Denitrification: some bacteria turn nitrates back into N₂ gas, returning it to the atmosphere.

Human disruptions: overuse of fertiliser → nitrate runoff into rivers → algal blooms → 'dead zones' where decomposing algae use up the oxygen. The Gulf of Mexico has a famous dead zone from Mississippi runoff.