In economic thermodynamics, an entropy watershed is a hypothetical point in time when, supposedly, nonrenewable fossil fuels or the established energy source run out, disorder increases to a maximum, and a society collapses. [1] More specifically, an entropy watershed, according to American economist Jeremy Rifkin, is: [2]

“[a point in time in which] the particular matter-energy base that a society is using becomes depleted, as a result of natural forces at work or as a result of people consuming resources faster than nature can reproduce them.”

The term has little substantial justification and was conceived, using a great deal of backwards logic, by American economist Jeremy Rifkin in 1980. [3] The term is based on the "material entropy" hypothesis, which argues that available fossil fuels represent low-entropy states of matter and that society, in accordance with Romanian-born American mathematician and economist Nicholas Georgescu-Roegen's fourth law of thermodynamics, will tend to use up all the fuel leaving a state of high-entropy waste and disorder.

In the correct light, "entropy" as defined by German physicist Rudolf Clausius, is the measure of the irreversible work energy that the molecules of a system due on each other in a heat engine cycle. [5] When this model is translated into the human sphere, the "matter-energy base", Rifkin refers to is the substrate on which human molecules react. [6] In this sense, the matter-energy base only effects the "activation energy" barrier to human chemical reactions, thus having an effect on the speed of human progress and evolution. In a second sense, the attraction of human molecules to different regions of the matter-energy base, can be studied using the science of surface thermodynamics, in which interactions mediate to the effect that the free energy of the system lowers.

In short, Rifkin uses the phrase entropy watershed to divide one energy era from another, such as the wood-energy era to coal-energy era transition or the oil energy era to the nuclear-energy era transition, etc. Phrased another way, an entropy watershed is a negatively determined transition in which a scarce resource becomes unavailable and a society is forced to switch to a different form of energy use and thus a new economic structure with corresponding social forms and values. [4] Rifkin argues that, supposedly, the "industrialized nations, and the United States in particular, are coming up against an entropy watershed" and that "we stand today at the edge of a historic entropy watershed ... as we transition from the age of nonrenewable resources to the solar age." [1]

References
1. Rifkin, Jeremy. (1989). Entropy: Into the Greenhouse World (revised edition), (ch. 4: "Nonrenewable Energy and the Approaching Entropy Watershed", pgs. 117-36). New York: Bantam.
2. ibid, Postscript (pgs. 293-94).
3. Rifkin, Jeremy. (1980). Entropy: A New World View. Viking Press.
4. Niethammer, Lutz. (1993). Posthistoire: Has History Come to an End?, (pg. 246). Springer.
5. Clausius, R. (1865). The Mechanical Theory of Heat – with its Applications to the Steam Engine and to Physical Properties of Bodies. London: John van Voorst, 1 Paternoster Row. MDCCCLXVII.
6. (a) Thims, Libb. (2007). Human Chemistry (Volume One), (preview). Morrisville, NC: LuLu.
(b) Thims, Libb. (2007). Human Chemistry (Volume Two), (preview). Morrisville, NC: LuLu.