Earth's energy budget and past climates: HSC Earth and Environmental Science Module 7

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.