3) Natural Climate Variability

Limitation of RF

• Does not include feedbacks (e.g., cloud changes, water vapour adjustments, or temperature responses).

• e.g. Doubling of CO₂ leads to an RF of +3.7 W/m², meaning Earth's energy budget is disturbed by 3.7 W/m² before any climate response occurs.

Define natural climate variability

Natural fluctuations in climate occurring without human influence, caused by internal or external processes

How do orbital variations cause climate variability?

Cycle	Definition	Timescale	Effect on Climate
Eccentricity	Changes in the shape of Earth’s orbit (circular to elliptical)	100,000 years	Affects the intensity of seasons and Ice Age timing. Low eccentricity (circular orbit) = warmer interglacial periods, High eccentricity (elliptical orbit) = stronger seasonality, colder Ice Ages.
Obliquity	Changes in the tilt of Earth's axis (22.1° to 24.5°)	41,000 years	Greater tilt → more extreme seasons; Lesser tilt → milder seasons. Current obliquity = 23.5°, moving towards 22° → ice sheets may expand.
Precession	Wobble of Earth’s axis (like a spinning top)	26,000 years	Changes timing of seasons, influencing glaciation.

How does solar variability cause climate variability?

• 11-year Solar Cycle: Changes in sunspot activity → small changes (~0.1%) in solar energy output.

• Grand Solar Minima/Maxima:

  • Maunder Minimum (~1645–1715): Reduced solar activity, associated with Little Ice Age.

• Effect: Minor contributor to recent climate change (small variations compared to GHG forcing).

How does volcanic activity cause climate variability?

• Large eruptions release aerosols & sulphur dioxide (SO₂) into the atmosphere.

• These form reflective sulphuric acid droplets, blocking sunlight and causing short-term cooling.

  • Cooling can last ~1–3 years.

  • e.g. Mount Pinatubo (1991): Injected 20 million tonnes of SO₂, reducing global temperatures by ~0.5°C for 1–2 years.

How does ENSO cause climate to vary?

Phase	Description	Effects
El Niño	Weakening of trade winds, warming of Pacific waters	Warmer global temperatures (~0.1–0.2°C temperature increase), droughts, heavy rainfall in some areas
La Niña	Strengthening of trade winds, cooling of Pacific waters	Cooler global temperatures (~0.1–0.2°C temperature decrease), increased hurricane activity

How do ocean circulation patterns cause climate variability?

System	Description	Climate Effect
ENSO (El Niño–Southern Oscillation)	Irregular 2–7 year oscillation of ocean temperatures across Pacific	El Niño: global warming spike, droughts/floods. La Niña: temporary global cooling.
Atlantic Multidecadal Oscillation (AMO)	Slow (~60–80 years) fluctuations of North Atlantic SSTs	Affects hurricane activity, rainfall patterns
Pacific Decadal Oscillation (PDO)	Longer-term Pacific SST changes	Alters precipitation and temperature across Pacific Rim

Define internal variability

Changes arising within the climate system (e.g., ENSO)

Define external forcing

Changes imposed from outside (e.g., solar changes, volcanic eruptions, orbital changes)

What evidence is there for past natural climate variability?

Source	Example
Ice cores	Measure past CO₂ levels, temperatures, dust
Sediment cores	Ocean sediments → SST proxies, productivity
Tree rings (Dendrochronology)	Record precipitation and temperature extremes
Pollen records	Reveal vegetation shifts linked to climate

These palaeoclimate records show how climate has changed naturally over millennia — and show that current warming is unusual in magnitude and speed.