What natural processes drive climate change and how do we know past climates?
Explain natural causes of climate change and the proxy evidence for past climates
A focused answer to the WACE Year 12 Earth and Environmental Science dot point on natural climate change. Covers Milankovitch cycles, solar and volcanic forcing, feedbacks, and proxy records from ice cores, sediments and tree rings, with examples.
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 the natural drivers of climate change and the proxy evidence used to study them. A strong answer links each driver to how it changes energy balance and names the proxy that records each kind of change. This sets up the human-caused change examined in the next dot point.
Earth's energy balance
Climate reflects the balance between energy arriving from the Sun and energy radiated back to space. Anything that changes incoming solar energy, how much is reflected, or how much heat is trapped by the atmosphere acts as a climate forcing. Natural forcings have driven ice ages and warm periods long before humans.
Natural drivers of climate change
- Milankovitch cycles: slow, regular changes in the shape of Earth's orbit, the tilt of its axis and the wobble (precession) of that axis alter how sunlight is distributed across the planet over tens of thousands of years. These cycles pace the ice ages.
- Solar variation: the Sun's output changes slightly over time, altering incoming energy.
- Volcanic eruptions: large eruptions inject ash and sulfate aerosols into the upper atmosphere, reflecting sunlight and causing short-term cooling for a few years.
- Greenhouse gas changes: natural shifts in carbon dioxide and methane, for example from oceans warming and releasing gas, change how much heat is trapped.
- Plate tectonics: over very long timescales, moving continents alter ocean currents and weathering, affecting climate.
Feedbacks amplify change
Feedbacks can magnify a small forcing.
- Ice-albedo feedback: warming melts reflective ice, exposing darker land and ocean that absorb more heat, causing further warming. Cooling works the reverse way.
- Water vapour feedback: warmer air holds more water vapour, itself a greenhouse gas, adding warming.
- Carbon feedbacks: warming oceans release dissolved carbon dioxide, adding more warming.
These feedbacks explain how modest orbital changes can trigger full glacial cycles.
Proxy evidence for past climates
Because instruments only record the last century or two, scientists reconstruct earlier climates from proxies.
- Ice cores from Antarctica and Greenland trap ancient air bubbles, giving direct samples of past atmospheric carbon dioxide and methane, while the ice chemistry indicates past temperature. Cores extend records back hundreds of thousands of years.
- Ocean sediment cores contain the shells of microorganisms whose chemistry records past ocean temperature, plus layers that reveal past conditions.
- Tree rings record year-by-year growth, with ring width reflecting temperature and rainfall.
- Pollen preserved in lake and bog sediments shows which plants grew, indicating past climate.
- Coral and speleothems (cave deposits) add further dated records.
Multiple proxies that agree give confidence in a reconstruction.