Thermodynamics of the Climate System

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, Earth has experienced vastly different climates, including “snowball Earth” episodes, during which the planet is believed to have been entirely covered in ice, and hothouse periods, during which prehistoric alligators may have roamed the Arctic. Recent anthropogenic greenhouse gas emissions are the cause of modern, rapid climatic change, which poses a growing hazard to societies and ecosystems.

The climate system comprises the fluid envelopes of Earth: the atmosphere, oceans, and cryosphere. Those constituents, along with the evolving surface properties of the solid lithosphere, are responsible for reflecting some and absorbing most radiation received from the Sun. The climate system is close to an energy balance at all times. The total energy doesn’t significantly fluctuate in time because terrestrial radiation is emitted to space at approximately the same rate at which solar energy is absorbed.

Being in nearly exact energy balance with the universe allows Earth to have a relatively familiar climate tomorrow and a century from now. But over time, small deviations from a strict energy balance can induce massive changes in climate. Such small deviations are due to the diurnal and seasonal cycles, orbital variations— the Milankovitch cycles, for example (see the article by Mark Maslin, Physics TodayMay 2020, page 48)—and internal forcings, such as anthropogenic emissions of carbon dioxide.

Another characteristic of Earth’s climate—indeed, any planetary climate—is that it evolves irreversibly. Imagine watching a 10-second video of a field with a leafy tree on a sunny day. Would you notice if that video had been shown in reverse? Maybe not. Now imagine watching a 10-second clip of the same field and tree during a windy rainstorm. You could probably immediately assess whether the clip was run forward or backward in time. Some obvious tells stand out: Rain should fall toward the ground, and leaves should separate from, not attach to, the tree.

The climate system contains myriad irreversible processes, and on both a calm day and a stormy day 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 continuously produced by irreversible processes. Although physicists often consider ideal, reversible processes, all real physical processes are irreversible and therefore produce entropy.

In accordance with the second law of thermodynamics, irreversibility in the climate system permanently increases the total entropy of the universe. As in the case for total energy, though, the total entropy in the climate system is relatively steady. That’s because the climate is an open system that receives much less entropy from the Sun than it exports to the universe (see box 1). The difference between what is imported and what is exported is produced locally, through friction, mixing, or irreversible phase changes.

Although the climate is approximately steady, it is far from thermodynamic equilibrium, which would be a very cold and boring state with no motion. Instead, the climate system may be thought of as an engine, fueled by the unequal distribution of solar radiation incident upon it. It is those gradients in energy, and the resulting gradients in temperature and pressure they produce, that allow the wind to blow.1

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The article then moves on through the Climate System as a Heat Engine

But how do climate scientists characterize the work performed by the planetary heat engine? Earth cannot push on any external body, and in the framework of a classic heat engine, its work output is identically zero! The oceans and atmosphere do, however, perform work on themselves and each other, and that work generates the familiar winds and ocean currents that scientists observe. For climate scientists, useful work is that used to drive atmospheric and oceanic circulations.

Then drilling down with sections on Irreversible Processes,

The resultant cycle of energy production and dissipation, beautifully described in 1955 by Edward Lorenz,4 implies a balance between work and frictional dissipation in the climate system.

drivers of Global Circulation,

On global scales, the atmospheric circulation is driven by the differential heating associated with the Sun’s angle. It manifests as large overturning cells and jet streams. All planets in orbit around a star are heated most strongly at any given moment at the substellar point, where the planet’s surface is directly perpendicular to the star’s radiation. 

modeling irreversibility,

Indeed, analysis of the entropy budget of climate models has allowed scientists to probe the climate system’s irreversibility far beyond what observations alone would allow. Such studies have shed light on the role played by moist processes in governing how Earth’s planetary heat engine may 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 induce an overturning circulation, with the fact that observed large-scale vortices can be predicted by models that forbid temperature gradients? Tropical cyclones certainly have an important overturning circulation that responds to surface heating and upper-level cooling, but the much larger stratospheric polar vortex does not: It is a 2D phenomenon that is amenable to description using Boltzmann entropy. The most useful interpretation of the second law of thermodynamics is evidently feature-dependent in the climate system.

Read the full article here.

via Watts Up With That?

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July 2, 2022 at 09:00AM

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