Earth’s climate is fundamentally determined by its energy balance — the relationship between incoming solar radiation and outgoing thermal radiation. Identifying factors that disturb this balance, and distinguishing natural from human-caused factors, is central to understanding climate change.
Solar energy arrives at Earth primarily as short-wave visible light and UV radiation. Earth absorbs some and reflects some back (albedo). The absorbed energy warms the surface, which then re-emits energy as long-wave infrared radiation (heat). The atmosphere intercepts some of this outgoing radiation (greenhouse effect) before it escapes to space.
Energy balance equation (simplified):
$$\text{Absorbed solar energy} = \text{Outgoing long-wave radiation}$$
When this is in equilibrium, global temperature is stable. Anything that changes either side shifts global temperature.
Mechanism of effect:
- Large explosive eruptions (Volcanic Explosivity Index ≥ 5) inject sulfur dioxide (SO$_2$) and ash into the stratosphere
- SO$_2$ reacts with water vapour to form sulfate aerosols
- Aerosols scatter and absorb incoming solar radiation before it reaches the surface
- Net effect: temporary cooling (typically 0.1–0.5°C for 1–3 years)
Examples:
- Mt Pinatubo (1991): Caused ~0.5°C global cooling for 2 years
- Tambora (1815): Triggered the ‘Year Without a Summer’ (1816) — crop failures across Europe and North America
- Krakatoa (1883): Vivid sunsets worldwide; measurable cooling
Contrast with greenhouse warming:
- Volcanic cooling is temporary and short-term (years)
- Greenhouse warming from CO$_2$ is long-term (centuries to millennia)
Mechanism of effect:
- The Sun’s energy output varies due to changes in solar activity (sunspot cycles, ~11 years)
- Solar maximum: More sunspots → slightly higher solar irradiance → slight warming
- Solar minimum: Less sunspot activity → slightly cooler
Magnitude:
- The 11-year solar cycle causes only ~0.1% variation in solar irradiance
- This accounts for only a small fraction of observed 20th century warming
- Longer-term solar variability (Maunder Minimum, 1645–1715 — period of very low solar activity) may have contributed to the Little Ice Age
Key point: Solar variability does not explain the rapid warming since 1950 — satellite measurements show solar output has been flat or slightly declining since the 1980s, while temperatures have risen sharply.
Mechanism of effect:
- Human activities emit greenhouse gases (GHGs) that absorb and re-emit outgoing infrared radiation
- This reduces the rate at which Earth loses heat to space → planet warms until a new equilibrium at a higher temperature is reached
- This enhances the natural greenhouse effect → enhanced greenhouse effect
Key greenhouse gases from human activities:
| Gas | Human Sources | Global Warming Potential (100 yr) |
|---|---|---|
| Carbon dioxide (CO$_2$) | Fossil fuel combustion, cement, deforestation | 1 (baseline) |
| Methane (CH$_4$) | Agriculture (livestock, rice), waste, fossil fuel extraction | ~28 |
| Nitrous oxide (N$_2$O) | Agriculture (fertilisers), industrial processes | ~273 |
| Hydrofluorocarbons (HFCs) | Refrigerants, air conditioning | 1,000–14,000 |
| Sulfur hexafluoride (SF$_6$) | Electrical equipment | ~24,300 |
The CO$_2$ trajectory:
CO$_2$ concentration has risen from ~280 ppm (pre-industrial) to >420 ppm (2024), exceeding any level recorded in ice cores over the past 800,000 years.
| Factor | Type | Duration of Effect | Direction | Magnitude |
|---|---|---|---|---|
| Volcanic eruption | Natural | Years (temporary) | Cooling | 0.1–0.5°C |
| Solar variability (11 yr) | Natural | Years | Warming/cooling | <0.1°C |
| Milankovitch cycles | Natural | Thousands of years | Warming/cooling | ~5°C over millennia |
| GHG emissions | Anthropogenic | Centuries to millennia | Warming | 1–4°C by 2100 (IPCC) |
EXAM TIP: VCAA frequently asks students to distinguish between natural and anthropogenic climate forcings and to explain why human factors are driving current warming despite the existence of natural variability. Key point: the rate and timing of current warming matches the pattern of GHG emissions, not solar or volcanic activity.
