How do solar variation, volcanic eruptions and ocean circulation drive natural climate change?
Explain how solar, volcanic and oceanic factors cause natural climate variability
A focused answer to the WACE Year 12 Earth and Environmental Science dot point on solar, volcanic and oceanic climate forcings. Covers solar output variation, volcanic aerosol cooling, ocean circulation and El Nino and La Nina, distinguishing short-term variability from long-term change, with Australian context.
Reviewed by: AI editorial process; not yet individually human-reviewed
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What this dot point is asking
SCSA wants you to explain these three natural forcings and to place them on the right timescale. A strong answer distinguishes brief variability, such as a volcanic cooling or an El Nino, from the slow orbital pacing of ice ages, and uses this to show why recent warming is not natural.
Solar variation
The Sun's energy output is not perfectly constant.
- It varies on an approximately 11-year cycle of sunspot activity, and over longer periods.
- More solar output slightly warms Earth; less slightly cools it.
- A prolonged reduction in solar activity coincided with cool conditions during part of the historical Little Ice Age.
The effect is real but small, and measurements show solar output has not increased in recent decades, so it cannot explain current warming.
Volcanic forcing
Large explosive eruptions affect climate mainly by adding aerosols, not gases.
- Sulfur dioxide injected high into the stratosphere forms tiny sulfate droplets that reflect incoming sunlight.
- This reduces the energy reaching the surface, causing global cooling of a fraction of a degree for one to three years.
- The 1991 eruption of Mount Pinatubo measurably cooled the planet for about two years before the aerosols settled out.
The effect is temporary because the aerosols fall out of the stratosphere within a few years.
Ocean circulation
The oceans store and move vast amounts of heat, so changes in circulation shift climate.
- Ocean currents redistribute heat between the equator and poles; changes in circulation can warm some regions and cool others.
- El Nino and La Nina are shifts in tropical Pacific temperatures and winds that alter weather worldwide. For Australia, El Nino tends to bring drier conditions and drought risk, while La Nina tends to bring wetter conditions and flooding.
- These ocean patterns drive year-to-year variability rather than a long-term trend.
Timescales matter
These forcings operate faster than the orbital cycles. Volcanic cooling lasts a year or two, El Nino cycles run over a few years, and solar variation runs over decades. They cause the bumps and wiggles in the climate record but, unlike the slow orbital pacing of ice ages, none of them explains the sustained warming of the past century. Showing that no natural forcing fits the recent trend is the bridge to anthropogenic climate change.
How forcings appear in the climate record
Each natural forcing leaves a recognisable signature in the temperature record, and being able to read these is what SCSA tests in data questions. A large explosive eruption shows up as a sharp dip of a fraction of a degree lasting a year or two, followed by recovery as the aerosols clear, so a brief downward spike that returns to trend points to volcanism. The solar cycle appears as a faint, regular oscillation of about 11 years superimposed on longer trends, too small to dominate. El Nino and La Nina appear as warm and cool excursions of a year or so that come and go without shifting the baseline. Against these short-lived wiggles, the past century shows a sustained, accelerating upward trend that none of the natural signatures matches, neither a one-off spike, a regular oscillation, nor a heat-shuffling excursion. Recognising that the recent trend has the wrong shape and duration for any natural forcing is the analytical skill that distinguishes natural variability (the bumps) from the human-driven trend (the rising line) in a climate graph.
Exam-style practice questions
Practice questions written in the style of SCSA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WACE 20227 marksExplain how solar variation, volcanic eruptions and ocean circulation each affect climate, and distinguish the timescale over which each operates.Show worked answer →
A 7 mark answer needs the mechanism and timescale for each forcing.
- Solar variation
- The Sun's output varies on an approximately 11-year sunspot cycle and over longer periods; more output slightly warms Earth, less slightly cools it (a prolonged solar minimum contributed to part of the Little Ice Age). Timescale: years to decades; the effect is small.
- Volcanic eruptions
- Large explosive eruptions inject sulfur dioxide into the stratosphere, where it forms sulfate aerosols that reflect sunlight, causing short-term global cooling (Pinatubo, 1991, cooled the planet for about two years). Timescale: one to three years, because aerosols settle out.
- Ocean circulation
- Currents redistribute heat between equator and poles, and El Nino and La Nina shift tropical Pacific temperatures and winds, altering weather (El Nino tends to dry Australia, La Nina to wet it). Timescale: months to a few years; redistributes heat without adding net energy.
Markers reward the correct mechanism and an appropriate timescale for each, especially aerosol cooling for volcanoes and heat-redistribution (not net energy) for the ocean.
WACE 20206 marksUse your knowledge of natural climate forcings to explain why none of solar variation, volcanic eruptions or ocean circulation can account for the sustained global warming of the past century.Show worked answer →
A 6 mark answer must exclude each natural forcing with a reason.
- Solar
- Measurements show solar output has not increased over recent decades, so the Sun cannot be driving the sustained rise; if anything its small variations would not produce a century-long trend.
- Volcanic
- Volcanic aerosols cause brief cooling, not warming, and the effect fades within a few years, so eruptions cannot explain sustained warming (they would, if anything, cause cooling spikes).
- Ocean circulation
- El Nino, La Nina and current changes redistribute existing heat, producing year-to-year variability, but add no net energy to the system over the long term, so they cannot create a sustained upward trend.
- Conclusion
- Each natural forcing acts on too short a timescale or in the wrong direction, leaving the rising greenhouse gas concentration as the cause that fits the sustained warming.
Markers reward excluding all three with the correct reason (no solar rise, volcanic cooling and brief, ocean redistributes not adds energy) and the anthropogenic conclusion.