Introduction
The sun delivers about 1,361 watts of energy to every square metre of space facing it — enough to run a hair dryer continuously. Yet Earth reflects roughly 30% of that straight back to space before it even reaches the surface. The question of where the rest goes, and why Earth isn't either frozen solid or roasting like Venus, is what this lesson answers.
The sun is 150 million kilometres away, yet it is the ultimate source of nearly all energy that drives Earth's climate, weather, and life. The solar constant — the average amount of solar energy reaching the top of the atmosphere — is approximately 1,361 watts per square metre. Over time, this energy input must be balanced by an equal output of energy back to space, or Earth's temperature would continuously rise or fall. This balance between incoming solar energy and outgoing radiation is Earth's energy budget, and understanding it is the foundation of climate science.
But the energy budget is not as simple as: energy in = energy out. The distribution of solar energy across Earth's surface is profoundly unequal. The tropics receive direct solar radiation year-round and absorb vastly more energy than they emit; the poles, tilted away from the sun for much of the year, absorb little but still emit significant infrared radiation. This permanent inequity of the energy budget between tropics and poles is the engine that drives all atmospheric and ocean circulation: the global wind and current systems are fundamentally attempts by Earth's fluid envelopes to redistribute energy from energy-surplus tropical regions to energy-deficit polar regions. Without this redistribution, the tropics would be far hotter and the poles far colder than they are.
The energy budget also involves the atmosphere itself as an active participant. The atmosphere is not simply transparent to solar energy — it absorbs some, reflects some, and transforms some. Clouds, aerosols, and greenhouse gases all modify the flow of energy through the climate system. The most important concept is albedo: the fraction of incoming solar radiation that is reflected back to space without being absorbed. Earth's average albedo is about 0.30 — it reflects 30% of incoming solar energy. Changes in albedo (from ice sheets melting, cloud cover changing, or land use change) are among the most powerful feedbacks in the climate system.
Key Terms
The amount of solar radiation per unit area reaching the top of Earth's atmosphere at Earth's mean orbital distance: ~1,361 W m⁻². Not truly constant — varies slightly (~0.1%) over the 11-year solar cycle. The effective value for global average calculations is divided by 4 (to distribute over a sphere), giving ~340 W m⁻².
The fraction of incident solar radiation that a surface reflects. Fresh snow: 0.8–0.9 (reflects 80–90%). Ocean: 0.06 (reflects 6%). Forest: 0.1–0.2. Earth's average planetary albedo: ~0.30. High-albedo surfaces (ice, clouds) return more energy to space; low-albedo surfaces (ocean, forest) absorb more. Changes in albedo drive powerful climate feedbacks.
Incoming solar radiation at a given location and time. Varies with latitude (sun angle), season (tilt of Earth's axis), time of day, and atmospheric conditions. The tropics receive high insolation year-round; polar regions receive low insolation, especially in winter. Insolation is the primary control on surface temperature distribution.
All objects emit electromagnetic radiation based on their temperature, described by the Stefan-Boltzmann law (power ∝ T⁴) and Wien's displacement law (peak wavelength ∝ 1/T). The sun (~5,778 K) emits mostly visible and near-IR light (peak ~0.5 μm). Earth (~288 K) emits entirely in the infrared (peak ~10 μm). These two very different spectra allow the atmosphere to absorb Earth's outgoing IR selectively while being largely transparent to incoming solar visible light.
The equilibrium state in which outgoing longwave radiation (OLR) from Earth exactly equals incoming absorbed solar radiation. At equilibrium, Earth's temperature is stable. If more energy enters than leaves (positive imbalance), Earth warms. Current measured energy imbalance: ~0.3–0.9 W m⁻² due to increased greenhouse gases, driving ongoing warming.