Carnot cycle model, thermodynamic system, view of the Earth.

In science, the Earth is the third planet rotating about the sun, intermediate between the orbits of Venus and Mars. [1] The solar flux of energy received by the earth at its surface is 1370 watts per meters squared.

Thermodynamics
The Earth, according to the science of environmental physics, is defined as a thermodynamic entity: [2]

“At the macroscopic level the Earth is a thermodynamic entity. Life is sustained on it because of the temperature gradient maintained by the net energy balance between the energy that comes from the Sun and the energy that leaves the Earth. At the microscopic level, i.e. the level of the individual person or animal, living organisms are also thermodynamic structures exhibiting energy exchanges between themselves and the environment.”

A surface section of earth is defined as an earth-bound thermodynamic system.

Solar chemistry
In human chemistry, the earth is defined as an “earth molecule”, i.e. a large 92-element molecule. [3] In this view, animate atomic structures, such as "human molecules", are attached to the surface of the earth; in same manner as "bacteria molecules" are attached to the surface of humans; in the same manner that lipopolysaccharides, i.e. large endotoxin-type molecules, are attached to the outer surface of gram-negative bacteria.

Thermodynamic system models

See main: System, Thermodynamic system, Biological system, Social system, Economic system, etc.

There are a number of various ways to study the earth from the perspective of thermodynamic analysis, some more intuitive, others less so, depending upon where the boundary is drawn and what time periods or cycles (heat inputs, heat outputs) are studied. A few of these diagrammatic attempts are shown below:

Thermodynamic view of the Earth (c. 1992) by Canadian biophysicist Marek Roland-Mieszkowski. [4]

Radiation gradient between the Sun and the Hawking temperature of outer space (2005) by American ecologist Eric Schneider. [5]

Thermodynamic (temperature gradient) view of the earth (2007) by American chemical engineer Libb Thims. [3]

References
1. Daintith, John. (2005). Dictionary of Science. Oxford University Press.
2. Mason, Nigel, Hughes, Peter, and McMullan, Randall. (2001). Introduction to Environmental Physics (ch. 2: The Human Environment, pgs. 17-55). Taylor and Francis.
3. (a) Thims, Libb. (2008). The Human Molecule (ch. 8: Modern Views, section: “Planetary-sized molecules”, pgs. 57-59) (preview). Morrisville, NC: LuLu.
(b) Thims, Libb. (2007). Human Chemistry (Volume One) (pgs. 20-21, 44, 53) (preview), (Google books). Morrisville, NC: LuLu.
4. Roland-Mieszkowski, Marek. (c.1992). “Life on Earth - Flow of Energy and Entropy.” (PDF), Digital Recordings.
5. Schneider, Eric D. and Sagan, Dorion. (2005). Into the Cool - Energy Flow, Thermodynamics, and Life (pg. 164). Chicago: The University of Chicago Press.