From Physics Today HT / Leif Svalgaard
Martin S. Singh and Morgan E O’Neill “Thermodynamics of the Climate System”, Physics Today 75, 30-37 (2022) https://doi.org/10.1063/PT.3.5038
Here is the introduction:
Throughout its history, the Earth has experienced vastly different climates, including “snowball Earth” periods in which the planet is believed to be completely covered in ice and ungulate period in which prehistoric crocodiles may have roamed the Arctic. Recent anthropogenic greenhouse gas emissions are the cause of rapid, modern climate change that poses an increasing danger to society and ecosystems.
The climate system includes the liquid crust of the Earth: the atmosphere, oceans, and cryosphere. Those components, along with the evolving surface properties of the solid lithosphere, are responsible for reflecting some and absorbing most of the radiation received from the Sun. The climate system is always near energy balance. The total energy does not fluctuate significantly over time because terrestrial radiation is emitted into space at a rate approximately equal to the rate of solar absorption.
An almost exact balance of energy with the universe allows Earth to have a relatively familiar climate tomorrow and a century from now. But over time, small deviations from the strict energy balance can cause big changes in climate. Such small deviations are due to daily and seasonal cycles, orbital variations – for example Milankovitch cycles (see Mark Maslin’s article, Physics today, May 2020, page 48)—And the constructions inside, such as human-caused carbon dioxide emissions.
Another feature of Earth’s climate – really, any planetary climate – is that it evolves irreversibly. Imagine you are watching a 10-second video of a field with leafy trees on a sunny day. Did you notice that the video is played in reverse? Maybe not. Now, imagine you are watching a 10-second clip of the same field and tree during a thunderstorm. You can instantly judge whether the clip is running forward or backward in time. Some obvious things stand out: The rain will fall on the ground, and the leaves must separate from the tree, not stick to the tree.
The climate system contains a multitude of irreversible processes, and on both calm and stormy days, they produce entropy. Like energy, entropy is a property of any thermodynamic system, and it can be calculated if one knows the state of the system. But unlike energy, entropy is not conserved. Rather, it is produced continuously by irreversible processes. Although physicists generally consider ideal processes to be reversible, all actual physical processes are irreversible and thus produce entropy.
According to the second law of thermodynamics, irreversibility in the climate system permanently increases the total entropy of the universe. However, as in the case of total energy, the total entropy in the climate system is relatively constant. That’s because the climate is an open system, receiving far less entropy from the Sun than it exports to space (see box). first). The difference between what is imported and what is exported is produced domestically, through friction, mixing or irreversible phase changes.
Although the climate is nearly stable, it is still far from thermodynamic equilibrium, which would be a very cold and dull state with no motion. Instead, the climate system can be thought of as an engine, driven by the uneven distribution of solar radiation breakdown across it. Those are gradients in energy, and the resulting gradients in temperature and pressure they create, allowing the wind to blow.first
https://physicstoday.scitation.org/doi/10.1063/PT.3.5038
The article then turns to the Climate System as a Heat Engine
But how do climate scientists describe the work done by the planet’s heat engines? The Earth cannot push on any external objects, and within the framework of a classical heat engine, its working output is identical to zero! However, the ocean and atmosphere do work for themselves and each other, and that work creates the familiar winds and ocean currents that scientists observe. For climate scientists, useful work is used to promote cycles in the atmosphere and oceans.
Then dive into the sections on Irreversible Processes,
The result of the energy production and dissipation cycle, beautifully described in 1955 by Edward Lorenz,4 implies a balance between work and friction dissipation in the climate system.
the dynamics of Global Circulation,
On a global scale, atmospheric circulation is driven by differential heating relative to the angle of the Sun. It manifests as large overturned cells and jet streams. All planets in orbit around a star are most heated at any given time at the subpoint, where the planet’s surface is directly perpendicular to the star’s radiation.
irreversible modeling,
Indeed, analyzing the entropy fund of climate models has allowed scientists to probe the irreversibility of the climate system far beyond what observations alone allow. Such studies shed light on the role of moisture processes in regulating how planet Earth’s heat engine can respond to climate change.
and beyond classical thermodynamics,
How can climate scientists reconcile a conceptual model of a planetary heat engine, which requires a temperature gradient to create an inverted cycle, with the fact that vortices Can the observed large-scale water be predicted by temperature gradient prohibition models? Tropical cyclones certainly have an important circulation reversal that responds to surface warming and upper cooling, but much larger stratospheric cyclones do not: It’s a 2D phenomenon that has may be suitable for description using Boltzmann entropy. The most useful interpretation of the second law of thermodynamics is clearly character-dependent in the climate system.
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