Earth energy system
Chapter 2 - Society
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Welcome to the Earth energy system page
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Earth is an energy-storing (photosynthetic) but also an energy-consuming (metabolic) system. The entropy of the outgoing entropy (infrared radiation) is about 22.66 times greater than the incoming entropy of sunlight.
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Core idea
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Entropy is the principle, energy is the pattern
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| Entropy |
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| One of the most important, yet least understood, concepts in all of physics. |
| https://www.youtube.com/live/DxL2HoqLbyA |
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The Earth gets a certain amount of energy from the sun daily. How much energy does the Earth radiate back into space relative to the amount that it gets from the sun?
For most of the Earth's history, it should be precisely the same amount of energy from the sun as Earth radiates into space. Because if we didn't do that, the Earth would get hotter, which would be a problem. (And it is becoming a big problem).
What are we really getting from the sun? The sun gives us a steady stream of low entropy, concentrated, bundled energy. This energy we get from the sun is more useful than the energy we give back. It's more compact, more clumped together.
Plants capture this energy and use it to grow and create sugars. Then, animals eat plants and use that energy to maintain their bodies and move around. Bigger animals get their energy by eating smaller animals, and so on.And each step of the way, the energy becomes more spread out. Ultimately, all the energy that reaches Earth from the sun is converted into thermal energy. When energy spreads like this, it is impossible to get it back. It's radiated back into space. This process is irreversible. The total amount of energy didn't change, but it became less usable.
The most common way to describe entropy is a disorder, which makes sense because it is associated with things becoming more mixed, random, and less ordered.
The best way to think about entropy is the energy's tendency to spread out. If the Earth were a closed system, the energy would spread out completely, meaning all life would cease, everything would decay and mix, and eventually, the Earth would reach the same temperature.
But luckily, Earth is not a closed system because we have the sun, which provides us with lots of low entropy photons. For each photon received from the sun, 22 photons are emitted. Everything that happens on Earth, plants growing, trees falling, herds stampeding, hurricanes and tornadoes, people eating, sleeping, and breathing, arises from the flow from low to high entropy. Everything happens when fewer, higher energy photons are converted into 22 times as many lower energy photons.
Without a source of concentrated energy and a way to discard the spread-out energy, life on Earth would not be possible. If the universe tends toward maximum entropy, then life offers a way to accelerate that natural tendency because it is spectacularly good at converting low entropy into high entropy. For example, the surface layer of seawater produces 30 to 680% more entropy when cyanobacteria and other organic matter are present than when they are not.
Life on Earth survives on the sun's low entropy, but where did the sun get its low entropy? The answer is the universe. If we know that the total entropy of the universe is increasing with time, then it was lower entropy yesterday and even lower entropy the day before that, all the way back to the Big Bang—the Past Hypothesis.
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Deep dive
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The transfer of heat energy
The Sun generates low entropy energy, transferring it through space to the Earth's atmosphere and surface. Some of this energy warms the atmosphere and surface as heat. There are three ways energy is transferred into and through the atmosphere:
Radiation
Radiation is the transfer of heat energy through space by electromagnetic radiation. If you have stood in front of a fireplace or near a campfire, you have felt the heat transfer known as radiation. The side of your body nearest the fire warms while your other side remains unaffected by the heat. Although you are surrounded by air, the air has nothing to do with this heat transfer. Heat lamps that keep food warm work in the same way.
Electromagnetic radiation is made of waves of different frequencies. The frequency is the number of instances a repeated event occurs over a set time. In other words, the frequency of electromagnetic radiation is the number of electromagnetic waves moving past a point each second. Most wavelengths of electromagnetic radiation are invisible. However, much of the electromagnetic radiation that reaches the Earth from the Sun is visible light.
Our brains interpret some of these frequencies as colours, including red, orange, yellow, green, blue, indigo, and violet. When the eye views all these different frequencies simultaneously, it is interpreted as white. Infrared and ultraviolet are two of the waves we can't see. Infrared has a lower frequency than red, and ultraviolet has a higher frequency than violet [more on electromagnetic radiation]. It is infrared radiation that produces a warm feeling in our bodies.
Most solar radiation is absorbed by the atmosphere, and much of what reaches the Earth's surface is radiated back into the atmosphere to become heat energy. Dark-coloured objects like asphalt absorb radiant energy faster than light-coloured objects. However, they also radiate their energy faster than lighter-coloured objects. (The variations in how Earth's surface absorbs heat from the Sun is called differential heating.)
Conduction
Conduction transfers heat energy from one substance to another or within a substance. Have you ever left a metal spoon in a pot of soup being heated on a stove? After a short time, the handle of the spoon will become hot. This is due to the transfer of heat energy from molecule to molecule or from atom to atom. Another example is when objects are welded together; the metal becomes hot (the orange-red glow) by transferring heat from an arc. Conduction is a very effective method of heat transfer in metals. However, air conducts heat poorly.
Convection
Convection is the transfer of heat energy in a fluid. In the kitchen, this type of heating is most commonly seen as the circulation that develops in a boiling liquid. Air in the atmosphere acts as a fluid. The Sun's radiation strikes the Earth's surface, thus warming it. As the surface's temperature rises due to conduction, heat energy is released into the atmosphere, forming a warmer bubble of air than the surrounding air. This bubble of air rises into the atmosphere. As it rises, the bubble cools, moving its heat into the surrounding atmosphere. As the hot air mass rises, it is replaced by the surrounding cooler, more dense air, which we feel as wind. These movements of air masses can be small in a particular region, such as local cumulus clouds or large cycles in the troposphere, covering large sections of the Earth. The large cycles are called convection currents and are responsible for many weather patterns in the troposphere.
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Low entropy energy from our Sun
Solar energy
The sun is our best energy supplier. Solar energy reaching Earth is 9000 times greater than our total energy needs.
This solar energy comes in the form of light, and visible light also includes infrared and ultraviolet radiation, which is invisible. Light created by nuclear fusion in the sun takes eight minutes to reach the Earth. This radiation has a high intensity.
- Incoming sunlight:
- The Sun sends energy to Earth mainly in the form of visible and ultraviolet light. On average, Earth receives about 340 watts per square meter (W/m²). This is called the solar constant averaged over Earth's surface area in Watt.
- Outgoing infrared radiation:
- Earth radiates energy back into space as infrared light. The outgoing energy also averages around 340 W/m², keeping Earth’s energy budget roughly in balance.
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The entropy of solar energy
The sun provides Earth with low-entropy energy in the form of visible light photons. This low-entropy input allows the Earth to maintain a state of disequilibrium and enables Earth system processes to perform work.*The Earth absorbs high-energy, low-entropy visible light photons from the Sun
- These photons are converted into various forms of energy (chemical, kinetic, and thermal) through Earth system processes
- The Earth then radiates this energy back into space as lower-energy, higher-entropy infrared photons
When considering the entire Earth, for every visible photon received, the Earth radiates about twenty infrared photons back into space.
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Useful energy
This entropy increase occurs because the Earth effectively converts high-energy (visible) photons from the Sun into many low-energy (infrared) photons, increasing the disorder of the universe. The process of absorption and re-emission at a lower temperature is fundamentally irreversible, leading to entropy generation.
The entropy differential is crucial for maintaining Earth's energy balance and supporting life:
- It allows the Earth to maintain a state of disequilibrium, enabling various Earth system processes to perform work
- As long as the entropy decrease associated with life on Earth is less than the net entropy creation rate, the Second Law of Thermodynamics is not violated
- The low-entropy input from the Sun provides the "free energy" necessary for driving dynamics and converting energy from one form to another within Earth systems
In conclusion, the low-entropy nature of incoming solar radiation, combined with the Earth's ability to radiate higher-entropy energy back into space, creates a thermodynamic gradient that powers the complex processes and life forms on our planet.
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The figures
Energy:
- Incoming Energy (from sunlight): 340 W/m²
- Outgoing Energy (as infrared radiation): 340 W/m²
The energy in and out are balanced on average to keep Earth's temperature stable.
Entropy:
- Incoming Entropy (from sunlight): ~0.059 W/K·m²
- Outgoing Entropy (as infrared radiation): ~1.33 W/K·m²
The entropy of the outgoing entropy (infrared radiation) is about 22.66 times greater than the incoming entropy of sunlight. This significant increase reflects how energy spreads out and becomes more disordered (higher entropy) as it cools down from the Sun’s temperature (~5778 K / ~ 5500 C) to Earth’s effective radiating temperature (~255 K / ~ -18 C). This aligns with the second law of thermodynamics, which states that the total entropy of the system (Earth and its surroundings) must increase.
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Radiative forcing, the result of GHG emissions
Radiative forcing is what happens when the amount of energy that enters the Earth’s atmosphere is different from the amount of energy that leaves it. Energy travels in the form of radiation: solar radiation entering the atmosphere from the sun, and infrared radiation exiting as heat. If more radiation is entering Earth than leaving—as is happening today—then the atmosphere will warm up. This is called radiative forcing because the difference in energy can force changes in the Earth’s climate.
| Content source |
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| MIT edu |
| https://climate.mit.edu/explainers/radiative-forcing |
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Earth's energy balance
The earth-atmosphere energy balance is achieved as the energy received from the Sun balances the energy lost by the Earth back into space. In this way, the Earth maintains a stable average temperature and, therefore, a stable climate. Low entropy energy released from the Sun is emitted as shortwave light and ultraviolet energy. When it reaches the Earth, some is reflected back to space by clouds, some is absorbed by the atmosphere, and some is absorbed at the Earth's surface.
Using 100 units of energy from the sun as a baseline the energy balance is as follows:
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| IN | Incoming energy from the sun | Outgoing energy from the Earth | OUT |
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| 100 | Shortwave radiation from the sun | Shortwave radiation reflected back to space by clouds | 23 |
| Shortwave radiation reflected to space by the earth's surface | 7 | ||
| Longwave radiation from the atmosphere into space | 49 | ||
| Longwave radiation from clouds into space | 9 | ||
| Longwave radiation from the earth's surface into space | 12 |
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| IN | Energy into the atmosphere | Outgoing energy from atmosphere | OUT |
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| 19 | Absorbed shortwave radiation by gases in the atmosphere. | Longwave radiation emitted to space by clouds | 9 |
| 4 | Absorbed shortwave radiation by clouds. | Longwave radiation emitted to space by gases in atmosphere | 49 |
| 104 | Absorbed longwave radiation from earth's surface. | Longwave radiation emitted to earth's surface by gases in atmosphere | 98 |
| 5 | From convective currents (rising air warms the atmosphere) | ||
| 24 | Condensation /Deposition of water vapor (heat is released into the atmosphere by process) |
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| IN | Energy absorbed | Energy released | OUT |
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| 47 | Absorbed shortwave radiation from the sun | Longwave radiation emitted by the surface. | 116 |
| 98 | Absorbed longwave radiation from gases in atmosphere | Removal of heat by convection (rising warm air) | 5 |
| Heat required by the processes of evaporation and sublimation and therefore removed from the surface | 24 |
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The absorption of infrared radiation trying to escape from the Earth back to space is particularly important to the global energy balance. Energy absorption by the atmosphere stores more energy near its surface than it would if there was no atmosphere. The average surface temperature of the moon, which has no atmosphere, is -18°C. By contrast, the average surface temperature of the Earth is 15°C. This heating effect is called the greenhouse effect..
| Content source |
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| NOAA |
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The Hydrologic Cycle
The hydrologic cycle involves the continuous circulation of water in the Earth-Atmosphere system. At its core, the water cycle is the motion of the water from the ground to the atmosphere and back again. Of the many processes involved in the hydrologic cycle, the most important are:
Evaporation
Evaporation is the state change in a substance from a liquid to a gas. In meteorology, the substance we are concerned about the most is water. Energy is required for evaporation to take place. The energy can come from any source: the sun, the atmosphere, the Earth, or objects on the Earth such as humans. Everyone has experienced evaporation personally. When the body heats up due to the air temperature or through exercise, the body sweats, secreting water onto the skin. The purpose is to cause the body to use its heat to evaporate the liquid, thereby removing heat and cooling the body. The same effect can be seen when you step out of a shower or swimming pool. The coolness you feel is the removal of body heat through the evaporation of the water on your skin.
Transpiration
Transpiration is the evaporation of water from plants through stomata. Stomata are small openings on the underside of leaves connected to vascular plant tissues. In most plants, transpiration is a passive process primarily controlled by the atmosphere's humidity and the soil's moisture content. Of the transpired water passing through a plant, only 1% is used in the plant's growth process. The remaining 99% is passed into the atmosphere.
Condensation
Condensation is the process whereby water vapour is changed into a liquid state. In the atmosphere, condensation may appear as clouds or dew. This is also the process whereby water appears on the side of an uninsulated cold drink can or bottle. Condensation is not a matter of one particular temperature but of a difference between the air temperature and the dewpoint temperature. The dew point is the temperature at which dew can form - it is the point at which air becomes saturated and can not hold any more water vapour. Any additional cooling causes water vapour to condense. Foggy conditions often occur when air temperature and dew point are equal. Condensation is the opposite of evaporation. Since water vapour has a higher energy level than liquid water, excess energy in the form of heat energy is released when condensation occurs.
Precipitation
Precipitation results when tiny condensation particles grow too large through collision and coalescence for the rising air to support and thus fall to the Earth. Precipitation can be in the form of rain, hail, snow, or sleet. Precipitation is the primary way we receive fresh water on Earth. On average, the world gets about 980 mm yearly over the oceans and land masses.
Runoff
Runoff occurs when there is excessive precipitation and the ground is saturated (cannot absorb any more water). Rivers and lakes are the result of runoff. Some runoff evaporates into the atmosphere, but most water in rivers and lakes returns to the oceans. If runoff water flows into a lake with no outlet for water to flow out, evaporation is the only means for water to return to the atmosphere. As water evaporates, impurities or salts are left behind. As a result, the lake becomes salty, as in the case of the Great Salt Lake in Utah or the Dead Sea in Israel. Evaporation of this runoff into the atmosphere begins the hydrologic cycle over again. Some of the water percolates into the soil and ground water, only to be drawn into plants again for transpiration to occur..
| Content source |
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| NOAA |
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Photosynthesis
Life fundamentally depends on free energy, which is primarily derived from the sun on earth. Through photosynthesis - from the Greek words 'photo' (light) and 'synthesis' (putting together) - sunlight is captured and transformed into chemical energy. Plants and numerous bacteria carry out this crucial process. A pivotal evolutionary breakthrough was the advent of oxygenic photosynthesis. In this process, sunlight drives the splitting of abundant water molecules into molecular oxygen (O₂), which we rely on for respiration. Intricate biochemical reactions utilise the resulting protons and electrons to reduce carbon dioxide (CO₂) into carbohydrates. The ubiquity of water as an electron source enabled oxygenic organisms to proliferate and diversify rapidly. Although the oxygen produced was initially toxic to many species, those adapting to the increasingly oxygen-rich atmosphere gained a significant advantage: extracting additional energy by oxidising organic matter. All life on Earth comprises one or more cells, and each cell depends on chemical energy, primarily carbohydrates. These molecules, produced predominantly via photosynthesis, store solar energy and convert it into chemical bonds.
When organisms metabolise food, the stored energy is released, driving cellular processes such as respiration or muscular contraction. This continuous flow of energy, initiated by photosynthesis, sustains ecosystems worldwide, transferring energy from one organism to another. Thus, photosynthesis indirectly or directly supports nearly all life on Earth.
Moreover, photosynthesis replenishes atmospheric oxygen, making it indispensable for life as we know it. Humans, for example, depend heavily on photosynthetic organisms for sustenance and breathable air.
Plants typically convert sunlight into chemical energy with an efficiency of 3–6%, dissipating unused energy primarily as heat. Photosynthetic efficiency varies depending on light frequency, intensity, temperature, and atmospheric CO₂ levels, ranging from as low as 0.1% to as high as 8%. By contrast, solar panels achieve approximately 6–20% conversion efficiencies in mass-produced models, with experimental devices exceeding 40%.
| Do you want to know more? |
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| https://en.wikipedia.org/wiki/Photosynthesis |
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