The Earth’s climate system is fundamentally governed by a precise energy accounting known as the Earth’s Energy Budget. This budget tracks the flow of solar energy as it enters the planet’s atmosphere, interacts with the surface, and is ultimately radiated back into space. Maintaining a relatively stable global temperature over long periods requires a dynamic balance where the energy coming in nearly equals the energy going out. Any sustained disruption to this flow forces the planet’s temperature to adjust until a new equilibrium is established.
Incoming Energy The Solar Source
The Sun is the primary power source driving the Earth system. Energy arrives primarily as electromagnetic waves with relatively short wavelengths, known as shortwave radiation. This spectrum includes ultraviolet light, visible light, and near-infrared radiation.
Solar energy reaches the top of the atmosphere at a consistent rate, averaging approximately 340 watts per square meter globally. This incoming energy warms the planet, drives atmospheric and oceanic circulation, and fuels life through photosynthesis.
Atmospheric Processing Reflection Absorption and Scattering
As shortwave radiation travels through the atmosphere, a significant portion is redirected or absorbed before reaching the ground. About 29% of the incoming solar energy is reflected directly back to space by the atmosphere and the surface. This overall reflectivity is known as albedo, a measure that varies depending on the reflecting material.
Bright surfaces like clouds, snow, and ice have a high albedo and are effective at reflecting solar energy. Clouds account for a large share of the planetary albedo, reflecting radiation that would otherwise be absorbed by the surface.
The atmosphere also absorbs about 23% of the incoming shortwave radiation. Specific gases, such as oxygen and ozone in the upper atmosphere, effectively absorb harmful ultraviolet radiation. Atmospheric molecules like nitrogen and oxygen scatter the light in all directions, which is responsible for the blue appearance of the daytime sky. The remaining solar energy, approximately 48% of the initial input, passes through the atmosphere to be absorbed by the Earth’s land and ocean surfaces.
Outgoing Energy Thermal Radiation and the Greenhouse Effect
The shortwave energy absorbed by the land and oceans increases the temperature of these surfaces. The Earth’s surface re-emits this absorbed heat, but at much longer wavelengths than the incoming solar radiation. This outgoing energy is thermal infrared radiation, referred to as longwave radiation.
The longwave radiation emitted from the surface attempts to escape directly to space, but the atmosphere is not transparent to these wavelengths. Greenhouse gases efficiently absorb this thermal infrared energy. These gases include:
- Water vapor
- Carbon dioxide
- Methane
- Nitrous oxide
When these molecules absorb the energy, they re-emit the longwave radiation in all directions, both upward toward space and downward toward the surface.
This downward re-emission acts like a blanket, warming the Earth’s surface. Only about 10% of the longwave radiation emitted directly from the surface manages to pass through the atmosphere without being absorbed, escaping via the atmospheric window. The majority of the heat that ultimately leaves the planet is longwave radiation emitted from the higher, cooler layers of the atmosphere.
The Concept of Radiative Equilibrium
The Earth’s temperature remains relatively stable because the total energy absorbed by the planet is nearly balanced by the longwave energy radiated back to space. This condition is known as radiative equilibrium.
If the planet is in perfect radiative equilibrium, the global average temperature does not change. However, a perturbation, such as an increase in atmospheric greenhouse gases, reduces the efficiency with which longwave radiation can escape to space. This imbalance means that incoming energy temporarily exceeds outgoing energy, resulting in a net energy gain. Consequently, the global temperature must rise until the planet is warm enough to radiate sufficient thermal energy back into space to restore balance.