Understanding the distinction between the natural greenhouse effect and the enhanced greenhouse effect is fundamental to VCE Environmental Science.
| Gas | Concentration (pre-industrial) | Natural Sources |
|---|---|---|
| Water vapour (H$_2$O) | Variable, ~1–4% | Evaporation, transpiration |
| CO$_2$ | ~280 ppm | Volcanic outgassing, respiration, decomposition |
| CH$_4$ | ~0.72 ppm | Wetlands, ocean sediments, termites |
| N$_2$O | ~0.27 ppm | Soil bacteria, ocean |
| O$_3$ | Trace | Photochemical reactions |
Without the natural greenhouse effect: global average temperature ≈ -18°C
With the natural greenhouse effect: global average temperature ≈ +15°C
Net warming from natural GHE: ~33°C
This warming is essential for liquid water and all life on Earth.
The enhanced greenhouse effect occurs when human activities increase the concentration of greenhouse gases beyond natural background levels, intensifying the greenhouse mechanism and causing additional warming.
| Feature | Natural Greenhouse Effect | Enhanced Greenhouse Effect |
|---|---|---|
| Cause | Earth’s natural atmospheric composition | Human emissions of GHGs |
| CO$_2$ concentration | ~280 ppm (Holocene average) | >420 ppm (2024) |
| Temperature effect | ~+33°C above no-atmosphere baseline | Additional ~+1.1°C above pre-industrial (so far) |
| Rate of change | Gradual (millennia) | Rapid (decades–centuries) |
| Human role | None — entirely natural | Caused by human activities |
| Trend | Relatively stable during Holocene | Increasing — warming is accelerating |
| Primary driver | Water vapour, CO$_2$, CH$_4$ in natural proportions | Excess CO$_2$, CH$_4$, N$_2$O, F-gases from human sources |
| Ecological role | Enables life on Earth | Threatens biodiversity and human civilisation |
| System | Natural GHE Effect | Enhanced GHE Effect |
|---|---|---|
| Atmosphere | Maintains temperatures for life | Increasing average temperatures; more extreme events |
| Biosphere | Enables photosynthesis and growth | Species range shifts; bleaching; phenological mismatches |
| Hydrosphere | Maintains liquid water oceans | Sea level rise; ocean acidification; changed precipitation |
| Lithosphere | Enables soil formation and productivity | Desertification; permafrost thaw; coastal erosion |
Misconception 1: “The greenhouse effect is bad.”
Reality: The natural greenhouse effect is essential for life. Only the enhanced greenhouse effect is problematic.
Misconception 2: “Greenhouse gases trap all heat.”
Reality: Greenhouse gases slow the escape of heat to space — Earth still loses heat, just at a rate that produces a higher equilibrium temperature.
Misconception 3: “More CO$_2$ is always better for plants.”
Reality: While elevated CO$_2$ can initially boost plant growth (CO$_2$ fertilisation effect), the negative effects of associated climate change (drought, heat stress, reduced soil moisture) typically outweigh this benefit.
EXAM TIP: A common VCAA error is confusing the mechanism of the natural greenhouse effect with the cause of the enhanced effect. The mechanism is the same — GHG absorption of IR — but the CAUSE of the enhanced effect is human emissions. Always address both mechanism and cause.
