Fossil fuel combustion fundamentally disrupts the natural carbon cycle by transferring carbon from a long-term geological store into the active atmosphere–biosphere–ocean cycle, at a rate far exceeding natural processes.
Under natural conditions, carbon cycles through four reservoirs:
| Reservoir | Carbon Stored (Gt C) | Exchange Rate |
|---|---|---|
| Atmosphere | ~865 | Fast (decades) |
| Biosphere (land) | ~2,600 (living + soil) | Moderate (years–centuries) |
| Ocean (surface + deep) | ~38,000 | Fast (surface) to very slow (deep) |
| Lithosphere (rock, fossil fuels) | ~66,000,000 | Extremely slow (millions of years) |
Natural fluxes between these reservoirs (photosynthesis, respiration, decomposition, weathering) are roughly in balance over geological time.
Fossil fuels represent carbon that was removed from the active cycle and stored in geological formations over hundreds of millions of years. Burning them releases this ancient carbon in decades:
$$\text{C (fossil fuel)} + O_2 \rightarrow CO_2 + \text{energy (heat)}$$
This is not a natural flux — it is a one-way transfer from the geological reservoir to the atmosphere.
Current human emissions from fossil fuels: approximately 37 billion tonnes of CO$_2$ per year (2023).
Of this, the atmosphere absorbs approximately 45%; oceans absorb ~25–30%; land biosphere ~25%.
Result: Atmospheric CO$_2$ has risen from ~280 ppm (pre-industrial) to >420 ppm (2024) — a 50% increase.
Oceans absorb ~25–30% of anthropogenic CO$_2$. When CO$_2$ dissolves in seawater:
$$CO_2 + H_2O \rightarrow H_2CO_3 \rightarrow H^+ + HCO_3^-$$
The release of H$^+$ ions increases ocean acidity. Since 1850, ocean pH has fallen from ~8.2 to ~8.1 — a 26% increase in acidity (pH is a logarithmic scale).
Consequences of ocean acidification:
- Reduced ability of organisms to form calcium carbonate (CaCO$_3$) shells and skeletons
- Affects corals, oysters, mussels, sea urchins, pteropods (sea butterflies)
- Weakens coral reef structure (lower calcification rate; increased dissolution)
- Disrupts marine food webs
Increased CO$_2$ stimulates plant growth (CO$_2$ fertilisation effect), slightly increasing the terrestrial carbon sink.
However:
- Increased temperatures, droughts and fire frequency (driven by climate change) reduce forest carbon storage
- Deforestation (also associated with fossil fuel era development) removes carbon sinks
- Net effect: terrestrial carbon sink is growing but not fast enough to offset emissions
Fossil fuel combustion also emits:
- Sulfur dioxide (SO$_2$): Causes acid rain — damages soils and aquatic ecosystems
- Nitrogen oxides (NO$_x$): Contribute to acid rain and tropospheric ozone
- Particulates and soot: Absorb solar radiation; deposit on snow/ice, reducing albedo
| Time Scale | Consequence |
|---|---|
| Decades | Rising atmospheric CO$_2$; ocean surface acidification; global warming |
| Centuries | Slow deep ocean absorption of CO$_2$; continued warming from committed emissions |
| Millennia | ~20% of today’s CO$_2$ pulse remains in atmosphere for >10,000 years |
| Millions of years | Eventual removal by silicate rock weathering and carbonate sedimentation — but this is far too slow to help on human timescales |
The natural carbon cycle has mechanisms to absorb extra CO$_2$ (ocean, vegetation), but they operate on timescales of decades to centuries. Human emissions are adding CO$_2$ much faster than these natural buffers can absorb it.
This creates a rising stock of CO$_2$ in the atmosphere — analogous to filling a bathtub faster than water drains out.
EXAM TIP: When explaining consequences for the carbon cycle, clearly distinguish between: (1) direct atmospheric concentration increase; (2) ocean acidification; (3) feedback effects on other cycle components. VCAA may present a diagram of the carbon cycle and ask you to annotate the changes caused by fossil fuel combustion.
