Met5: The Earth's Energy Budget (v1.0)

 The key reference for this section is the center for science education.

The earth’s energy budget represents the balance between the amount of energy incoming to earth from the sun and the amount of energy outgoing from earth back to space. For thermodynamic equilibrium the two should balance each other. The energy budget provides a way to account for all the energy entering and leaving the earth system.

The diagram below shows how the energy reaching Earth from the sun is absorbed, reflected and released by Earth’s atmosphere and surfaces.

Most of the energy from the sun reaches Earth in the form of visible and infrared radiation. Just over half of this incoming solar energy reaches the ground. The rest is reflected away by clouds or ice or gets absorbed by the atmosphere.

The solar energy that makes it to the ground warms earth’s surface. The warm ground and oceans then release this heat back into the atmosphere in the form of infrared or thermal radiation. The amount of radiation emitted depends on temperature. Clouds and the gases that make up Earth’s atmosphere absorb much of this infrared radiation and emits it as radiation both into space and back to Earth. Did you know that Earth’s surface gets almost twice as much radiation from the atmosphere as it does from direct sunlight? That is because the sun heats the surface only during the day while the atmosphere and any heat it holds is present all the time. 

Earth’s surface is covered by things like water, soil, rocks, forests, snow, and sand. Different surface characteristics have different ways of affecting solar energy reaching our planet. Some surfaces are more reflective than others, characterized by the surface’s albedo. Albedo refers to the amount of energy reflected by a surface and is measured on a scale from zero to one (or sometimes as a percentage). Dark colored surfaces like oceans and forests reflect very little while light colored parts like snow and ice reflect almost all of the solar energy that reaches them.

The diagram below shows the albedo across the earth.

When incoming and outgoing energy fluxes are in balance, Earth is in equilibrium, and the climate system remains stable. However, human-induced changes (in the composition of the atmosphere such as increased greenhouse gases or melting sea ice) have disrupted the balance. Since at least 1970, measurements show a warming imbalance leading to global warming. The previous essays should have established the key forces governing this temperature.