3.4: Earth's Energy Budget

Energy from the Exosphere

The Sun powers our solar system. In addition to the Sun, some planets have a significant source of internal heat. Venus is one such planet, releasing its heat through volcanism. Saturn produces more internal heat than it receives from the Sun, given its great distance from the star it orbits. Earth is also one of these planets. In Earth’s case, the radiation received from the Sun is the primary driver for the entire Earth system, including its climate.

When considering the energy that arrives from the Sun, it is worth studying the image below from NASA. Radiation from the Sun reaches the Earth system in a wide range of wavelengths spanning the entire electromagnetic spectrum. Much of this energy is blocked by the Earth’s atmosphere, allowing only radio waves, visible light, short-wave infrared radiation, and some amount of UV radiation to penetrate to the surface of the planet. This selective admission is important for the functioning of life on Earth as we see it today. But, it has not always been this way. The Earth’s atmosphere has evolved along with life, affecting life’s evolution and also being changed by life itself. The types of radiation that are admitted to the surface have also changed over time.

Energy from the Sun flows through the Earth system in its many forms and powers the hydrologic and the biogeochemical cycles that all make life possible on our planet. Of the radiation that enters the atmosphere, the shortwave (UV/Visible/infrared) radiation is most important. UV radiation gives you “sunburn” and is carcinogenic over time. Your eyes are evolved to detect the violet to red wavelengths of visible light. Infrared radiation is what you experience as heat. Some of this is reflected by clouds, a notoriously difficult factor to model, given their variability. The rest of it hits the Earth’s surface, being absorbed in places like land and ocean (low albedo/dark in color) and reflected in other places like ice and snowpack (high albedo/light in color).

Figure \(\PageIndex{1}\): Full spectrum of electromagnetic energy from the Sun. Most incident radiation from the Sun ranges from visible light through radio waves. Near infrared is shortwave radiation and far infrared is longwave radiation. Visible light drives most biological processes on the Earth. (NASA)

After the energy is absorbed, it can be re-radiated in the form of longwave infrared radiation. This is still in the form if heat, but has lower energy (longer wavelength). As this radiation attempts to leave the atmosphere, much of it is absorbed by greenhouse gases, such as water vapor, carbon dioxide, methane, and nitrogen dioxide. This trapping of heat by gases is what is referred to as the greenhouse effect. And, this effect is critical for life on Earth.

Figure \(\PageIndex{2}\): Earth’s energy budget (NASA)

Energy from Earth’s Interior

Unlike Mercury and Mars, who have a negative energy balance as they lose more heat over time than they create, Earth’s interior experiences a balance. Since the formation of all of the planets, they have been cooling over time. Whether a planet is able to balance this heat loss with its own heat production is a function of the composition of its interior. In Earth’s case (and for all terrestrial planets to varying degrees), it is producing much of its own interior heat through radioactive decay. The heat from this decay is caused by particles, emitted during decay of unstable isotopes, bouncing off of other particles. These collisions transform this kinetic energy into heat.

Figure \(\PageIndex{3}\): Cross-section of the Earth showing its main divisions and their approximate contributions to Earth’s total internal heat flow to the surface, and the dominant heat transport mechanisms within the Earth (Wikimedia, Bkilli\(_1\))

This heat is transported outward by conduction, convection, and advection, as seen in the figure below. Plate tectonics is driven by this internal heat, in addition to gravity.

Figure \(\PageIndex{4}\): The Earth system’s two main sources of energy and some of the transfers and flows of that energy, all of which contribute to biotic activity. Solid lines are modes of energy transfer and dissipation. Dotted lines are processes, or effects, created by these flows of energy. (Kleidon, (2012))

Through plate tectonics, this internal energy teams up with solar energy to help drive the biogeochemical cycling that provides important nutrients that sustain life, including carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur. Because of this, energy from the Sun is only part of the story of our planet’s geologic past. The combination of these sources of energy is critical for the function of Earth’s systems. You can see examples of some of these important interactions in the figure below.

Geological Sources of Important Elements for Life

Table \(\PageIndex{1}\): Table of important elements for life. All of these elements are ultimately sourced from within the Earth's interior. Plate tectonic processes, driven by Earth's interior heat, drive one end of the biogeochemical cycling of these nutrients while the Sun's incoming energy drives them from above.

Elemental Nutrient	Primordial Source on Earth	Important Life Processes
Carbon	Cosmogenetic, Mantle, Volcanism	Basic element of organic chemistry, important nutrient for photosynthesis of glucose
Hydrogen	Cosmogenetic, Mantle, Volcanism	Basic element of organic chemistry, important nutrient for photosynthesis of glucose
Nitrogen	Cosmogenetic ammonia, Earth’s atmosphere	Important for the creation of amino acids, enzymes
Oxygen	Cosmogenetic, Volcanism, Carbon Dioxide dissociation, Photodissociation in Atmosphere of Water	By-product of photosynthesis, Necessary for respiration and associated oxidation
Phosphorus	Phosphides in Earth’s core, Inorganic minerals such as apatite and fluorite, ocean-floor sediments	“Spine” of DNA molecule, important micronutrient that helps store energy in cells via ATP
Sulfur	Cosmogenetic, Stored in Earth’s interior, Volcanism	Allows for the synthesis of a greater variety of amino acids, Important nutrient for chemosynthesis