Causes of climate change: natural and anthropogenic

OCR J383 Paper 2 tests climate change causes and effects. A common exam question asks students to evaluate the relative importance of natural vs human causes — always be prepared to assess both sides.

Natural causes of climate change

1. Milankovitch cycles (orbital changes)

The Earth's orbit around the Sun changes over tens of thousands of years:

• Eccentricity (shape of orbit): changes from circular to elliptical over ~100,000 years — affects how much solar energy Earth receives.
• Axial tilt (obliquity): changes between 22.1° and 24.5° over ~41,000 years — affects the intensity of seasons.
• Precession (wobble of Earth's axis): ~26,000 years — affects which hemisphere points toward the Sun at perihelion.

These cycles correlate well with ice-age cycles — natural causes of long-term climate change.

2. Solar output variation

• The Sun's energy output varies in 11-year sunspot cycles and over longer periods.
• Periods of low solar output (e.g. the Maunder Minimum, c.1645–1715) correspond to the "Little Ice Age" in Europe.
• Solar variation cannot explain the rapid warming since 1950 — solar output has actually decreased slightly since then.

3. Volcanic eruptions

• Large eruptions emit sulphur dioxide (SO₂) into the stratosphere.
• SO₂ forms sulphate aerosols which reflect sunlight → short-term cooling effect.
• Example: Pinatubo (1991) caused global average temperature to drop by ~0.5°C for 1–2 years.
• Note: volcanic eruptions cause cooling, not warming, in the short term.

Anthropogenic (human) causes

The enhanced greenhouse effect

Natural greenhouse effect: water vapour, CO₂ and methane trap some solar radiation in the atmosphere — essential for life.

Enhanced greenhouse effect: human activities increase greenhouse gas concentrations → more radiation trapped → global warming.

Gas	Sources	Relative impact
CO₂	Fossil fuel combustion; deforestation; cement production	Largest contribution
Methane (CH₄)	Agriculture (cattle, rice paddies); landfill; natural gas leaks	28× more potent than CO₂ (weight for weight)
Nitrous oxide (N₂O)	Fertilisers; agriculture	265× more potent than CO₂
CFCs	Now banned but long-lived in atmosphere	Ozone depletion + greenhouse effect

Deforestation

• Forests are carbon sinks — they absorb CO₂.
• When trees are burned or left to rot, stored carbon is released as CO₂.
• Reduced transpiration → less water vapour cycling; can change local climate.
• Amazon loses c.10,000 km² per year (deforestation for agriculture, cattle ranching, soya).

Scientific consensus vs denial

• 97% of climate scientists agree that current warming is primarily human-caused.
• Ice cores, ocean sediment cores, tree rings and direct measurements (since 1850s) all show unprecedented recent warming.
• Recent warming rate (~1.1°C since pre-industrial) cannot be explained by natural causes alone — particularly since solar output has been stable or declining.

Common OCR exam mistakes

1. Saying volcanic eruptions cause global warming — they cause short-term cooling (sulphate aerosols blocking sunlight).
2. Confusing the natural greenhouse effect (beneficial) with the enhanced greenhouse effect (problematic).
3. Forgetting that methane is far more potent than CO₂ per unit — even though CO₂ is the largest contributor by volume.
4. Only describing causes without evaluating their relative importance — OCR rewards assessment of which cause is most significant.