1: Earth's Energy Balance

Section 1: Fundamentals: 1: Earth's Energy Balance

Earth's climate system is fundamentally governed by the Earth's Energy Balance – the delicate equilibrium between the energy the planet receives from the Sun and the energy it radiates back out to space. This continuous flow of energy drives all atmospheric and oceanic circulation, weather patterns, and ultimately determines our global climate.

The primary energy source is solar radiation, arriving predominantly as shortwave radiation (including visible light and ultraviolet). The amount of solar energy reaching the top of Earth's atmosphere per unit area per unit time is termed the Solar Constant (approximately 1361 W/m²), though it exhibits slight variations. Not all this incoming energy is absorbed.

A significant portion is immediately reflected back into space. The fraction of incoming solar radiation reflected by a surface or system is its albedo. Earth's global average albedo is about 0.3 (or 30%), primarily due to reflection from clouds, ice sheets, snow, and light-colored land surfaces. The remaining ~70% of incoming solar radiation is absorbed by the Earth system: approximately ~48% by the Earth's surface (land and oceans) and ~22% by the atmosphere and clouds.

The absorbed energy heats the planet. Like any warm object, Earth then emits energy as longwave (infrared) radiation. However, the atmosphere is not transparent to this outgoing longwave radiation. Certain atmospheric gases, known as Greenhouse Gases (GHGs) – primarily water vapor (H₂O), carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) – absorb most of this outgoing infrared radiation. These GHGs subsequently re-radiate energy in all directions, including back downwards towards the Earth's surface. This process, the Greenhouse Effect, is a natural and essential phenomenon; without it, Earth's average surface temperature would be about -18°C (0°F), rather than the habitable +15°C (59°F).

Energy Equilibrium: For Earth's average temperature to remain stable over time, the total incoming solar energy absorbed by the system must equal the total outgoing longwave radiation emitted to space ($Energy In = Energy Out$). This is the state of radiative equilibrium at the top of the atmosphere.

Disequilibrium and Climate Change: Human activities, especially the burning of fossil fuels and deforestation, have significantly increased atmospheric concentrations of CO₂, CH₄, and other GHGs since the Industrial Revolution. This enhances the natural greenhouse effect, causing the atmosphere to absorb and re-radiate more longwave energy back towards the surface. This creates an energy imbalance – temporarily, more energy enters the system than leaves ($Energy In > Energy Out$). This excess energy accumulates, primarily heating the oceans (ocean heat content) and melting ice, and warming the land surface and atmosphere, manifesting as global climate change.