How does the balance of incoming and outgoing radiation control Earth's climate, and what natural factors have changed it through time?
Investigate the Earth's energy budget and the natural drivers of long-term climate change, including but not limited to solar variation, Milankovitch cycles, volcanism and changes in atmospheric composition
A focused answer to the HSC Earth and Environmental Science Module 7 dot point on the energy budget and natural climate change. Incoming and outgoing radiation, albedo, Milankovitch cycles, solar variation and volcanism, with Australian context.
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What this dot point is asking
NESA wants you to explain Earth's energy budget, the balance between energy received from the Sun and energy radiated back to space, and the natural factors that have changed climate over geological time. You need albedo and the balance idea, plus the natural drivers: Milankovitch cycles, solar variation, volcanism and changing atmospheric composition. This distinguishes natural change from the human-driven change covered elsewhere in the module.
The answer
Earth's climate is set by its energy budget. Incoming solar radiation warms the planet; the planet radiates energy back to space as infrared. When these balance, temperature is steady. When something shifts the balance, the climate warms or cools until balance is restored at a new temperature. Over Earth's history, natural factors have repeatedly shifted this balance.
The energy budget and albedo
About a third of incoming sunlight is reflected straight back to space, mainly by clouds, ice and bright surfaces; the rest is absorbed and later re-emitted as infrared. The fraction reflected is the albedo. Bright surfaces such as ice and snow have a high albedo and cool the planet; dark surfaces such as ocean and forest have a low albedo and absorb more energy. Greenhouse gases slow the escape of outgoing infrared, raising surface temperature. Climate change happens whenever incoming or outgoing energy changes, whether through albedo, the Sun, or greenhouse gases.
Milankovitch cycles
Over tens of thousands of years, regular changes in Earth's orbit alter how much sunlight reaches different latitudes and seasons. The three Milankovitch cycles are eccentricity (the shape of the orbit, about 100,000 years), obliquity (the tilt of the axis, about 41,000 years) and precession (the wobble of the axis, about 23,000 years). These do not change the total solar energy much, but they redistribute it and are the pacemaker of the ice ages, triggering the glacial and interglacial cycles recorded in ice cores and ocean sediments.
Solar variation
The Sun's output varies on shorter timescales, including the roughly 11-year sunspot cycle and longer fluctuations. The cool period of the Little Ice Age coincided with reduced solar activity (the Maunder Minimum). Solar variation is a genuine natural driver, but measurements show it is far too small to explain the rapid warming of recent decades, which is why scientists attribute that warming to greenhouse gases rather than the Sun.
Volcanism
Large volcanic eruptions inject sulfate aerosols and dust into the stratosphere. These reflect sunlight (raising albedo) and cause short-term cooling lasting a few years; the 1991 eruption of Mount Pinatubo cooled global temperatures measurably. Over much longer times, volcanic carbon dioxide is part of the slow geological carbon cycle and can warm climate. Volcanism therefore acts in both directions depending on timescale.
Changing atmospheric composition
Over millions of years the composition of the atmosphere has changed, altering the greenhouse effect. The rise of oxygen produced by early photosynthesis, and long-term changes in carbon dioxide driven by weathering, volcanism and the burial of carbon, have moved Earth between warm greenhouse states and cold icehouse states. Australia's own ancient glacial deposits record times when the continent, then part of Gondwana, sat near the South Pole in a cold phase.
Try this
Q1. Explain how albedo influences the Earth's energy budget, using ice as an example. [3 marks]
- Cue. High-albedo ice reflects most incoming sunlight, reducing absorbed energy and cooling; melting ice lowers albedo, increasing absorption and warming.
Q2. Describe how Milankovitch cycles drive the glacial and interglacial cycles. [4 marks]
- Cue. Eccentricity, obliquity and precession periodically alter the distribution of sunlight by latitude and season, pacing the advance and retreat of ice sheets recorded in ice and sediment cores.
Exam-style practice questions
Practice questions written in the style of NESA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
2024 HSC1 marksChanges in global surface temperature are associated with three major volcanic eruptions (Mt Agung, El Chichon and Mt Pinatubo). Which statement correctly links these eruptions to changes in global surface temperature? A. Effusive eruptions produced large ash clouds that spread globally. B. Sulfur dioxide emissions formed sulfuric acid aerosols in the stratosphere. C. Large amounts of carbon dioxide were released which caused cooling. D. One volcano only affected local climate but the combined activity affected the whole planet.Show worked answer →
The correct answer is B: sulfur dioxide emissions formed sulfuric acid aerosols in the stratosphere.
Large explosive eruptions inject sulfur dioxide high into the stratosphere, where it reacts to form sulfuric acid aerosols. These tiny droplets reflect incoming solar radiation back to space, reducing the energy reaching the surface and causing a short-term fall in global temperature, the dips seen on the graph after each eruption.
A is wrong because these are explosive, not effusive, eruptions; C is wrong because carbon dioxide is a warming gas, not a cooling one; D contradicts the data, which show each major eruption produced a global temperature response.
2021 HSC1 marksThe eruption of Mt Pinatubo in 1991 released about 20 million tonnes of sulfur dioxide into the atmosphere. The 0.5 degrees C fall in average global temperature which lasted throughout 1991 to 1993 was most likely due to the A. amount of ash carried around the Earth. B. production of solid material in the lower atmosphere. C. production of sulfuric acid aerosols in the lower atmosphere. D. stratospheric winds which carried sulfuric acid aerosols around the Earth.Show worked answer →
The correct answer is D: stratospheric winds which carried sulfuric acid aerosols around the Earth.
The sulfur dioxide from Pinatubo reached the stratosphere and formed sulfuric acid aerosols. Stratospheric winds then spread these aerosols around the globe, where they reflected sunlight and produced a worldwide cooling of about 0.5 degrees C for two years.
The key distinction from option C is location: the cooling effect depends on aerosols high in the stratosphere being distributed globally, not on material in the lower atmosphere (which would wash out quickly). Ash (A) settles out too fast to cause years of global cooling.
2024 HSC1 marksWhich of the following features of Earth's orbit around the Sun causes the most rapid variations in climate? A. Tilt of Earth's axis (obliquity). B. Wobble of Earth's axis (precession). C. Shape of Earth's orbit (eccentricity). D. Earth's distance from the Sun (proximity).Show worked answer →
The correct answer is B: wobble of Earth's axis (precession).
The Milankovitch cycles operate over different periods: eccentricity (the shape of the orbit) cycles over about 100,000 years, obliquity (axial tilt) over about 41,000 years, and precession (the wobble of the axis) over about 19,000 to 26,000 years. Precession has the shortest period, so it drives the most rapid of these orbital variations in climate.
C (eccentricity) is the slowest cycle; A (obliquity) is intermediate; D is not an independent Milankovitch cycle, since Earth's distance from the Sun is governed by the eccentricity of the orbit.