Biogeochemical cycles

Burning fossil fuels releases carbon that was stored underground for millions of years, adding extra CO₂ to the atmosphere faster than sinks can remove it.

Cold water can hold more dissolved gas than warm water, so colder oceans absorb more CO₂ from the atmosphere.

Energy flows through ecosystems in one direction and is lost as heat. Matter is recycled repeatedly through biogeochemical cycles.

Major carbon stores include the atmosphere, living biomass, soils, oceans, and rocks/fossil fuels.

Matter cycles; energy flows through and is lost as heat.

Most carbon in a tree is stored in woody biomass, especially the trunk, branches, and roots.

Deforestation reduces carbon uptake because fewer trees photosynthesise, and it can release stored carbon if biomass is burned or decomposes.

Stores: oceans and soils (also atmosphere/biomass/rocks). Flow: photosynthesis (also respiration/decomposition/combustion).

The biological pump is the process where phytoplankton fix carbon by photosynthesis and carbon-rich material sinks to deeper water when organisms die or produce waste, storing carbon long-term.

Combustion oxidises carbon in wood and releases it rapidly to the atmosphere as CO₂.

A carbon store is a place where carbon is held for a period of time, such as the atmosphere, oceans, soils, biomass, or fossil fuels.

A biogeochemical cycle is the movement of elements (matter) between living organisms and the physical environment.

Decomposers break down dead organic matter and respire, releasing carbon back as CO₂, and methane may form in low-oxygen conditions.

A carbon sink is a store that absorbs more CO₂ than it releases over a given time period.

Key carbon flows include photosynthesis, respiration, decomposition, feeding, and combustion.

Ploughing/disturbing soil can increase decomposition and release CO₂ (source). No-till, cover crops, or adding compost can increase soil organic matter (sink).

A transfer moves matter without changing its form (e.g., water flowing from river to ocean). A transformation changes matter into a new form (e.g., photosynthesis turning CO₂ into glucose).

Carbon can leave the ocean when surface water warms and releases dissolved CO₂, or during upwelling when CO₂-rich deep water rises and CO₂ escapes to the air.

Cold temperatures and waterlogged or dry conditions slow decomposition, allowing more carbon to remain in soils as organic matter.

Oceans absorb CO₂ and reduce the amount in the atmosphere, but dissolved CO₂ forms carbonic acid and lowers pH, harming shell-forming organisms and food webs.

A carbon sink absorbs more CO₂ than it releases. A carbon source releases more CO₂ than it absorbs.

Fossil fuel carbon can be stored for millions of years, so it moves into and out of the active cycle extremely slowly compared with carbon in living biomass.

Oceans absorb only part of human emissions, and increased uptake causes ocean acidification, so atmospheric CO₂ can still rise while marine ecosystems are harmed.

Ocean acidification is a decrease in ocean pH caused by the ocean absorbing excess atmospheric CO₂, forming carbonic acid in seawater.

You must name at least one carbon store (e.g., atmosphere, ocean, soil) and at least one flow/process moving carbon between stores (e.g., photosynthesis, respiration).

A growing forest is a carbon sink because photosynthesis removes CO₂ from the air. Burning fossil fuels is a carbon source because combustion releases CO₂.

Photosynthesis transfers carbon from the atmosphere into biomass, while respiration releases carbon dioxide back to the atmosphere.

Fossil fuel combustion increases carbon sources by releasing long-term stored carbon, so atmospheric CO₂ rises because natural sinks cannot absorb the extra carbon fast enough.

Say CO₂ “dissolves into seawater at the surface”, rather than saying it “flows in as bubbles”.

Deforestation increases atmospheric CO₂ because stored carbon is released by burning or decomposition, and fewer trees remain to absorb CO₂ by photosynthesis.