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Unlike coal power plants which produce CO2 and intensify the greenhouse effect, every engine produces thermal pollution because of the constraints on engine efficiency. No engine can convert 100% of its heat input into work because doing so would violate the third law of thermodynamics. Thus, the extraneous heat (QL) can raise water and air temperatures. In the case of rivers, this could negatively affect aquatic life (warm water holds less O2).

However, how does raising air temperature affect the weather, and how is that process different than what is traditionally thought of as climate change from CO2? Does the output QL of every engine make the environment hotter? Is the same true for refrigerators and air conditioners which also output heat into the atmosphere?

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  • $\begingroup$ Just want to point out that a heat engine can convert 100% of its heat input to work in a process without violating the second law of thermodynamics. A reversible isothermal expansion process does that. A heat engine cannot convert 100% of its heat input into work in a cycle. That would violate the second law. $\endgroup$
    – Bob D
    Commented Jun 18, 2019 at 1:22
  • $\begingroup$ @BobD, there are no reversible thermodynamic processes in the real world. Reversibility requires an infinitesimal temperature difference for heat transfer. Since the driving force for heat transfer is a temperature difference, reversibility would require an infinite amount of time to accomplish. $\endgroup$
    – David White
    Commented Jun 18, 2019 at 2:11
  • $\begingroup$ @David White I’m well aware of that. Just pointing out that completely converting heat into work does not violate the second law in a process $\endgroup$
    – Bob D
    Commented Jun 18, 2019 at 2:55
  • $\begingroup$ I didn't phrase the question correctly. It would violate the third law which states that absolute zero is unattainable. Nevertheless, because of the laws of thermodynamics, a perfect engine is impossible. You will always have some heat (QL) released in addition to work produced. But my question is about how that heat or thermal pollution is different than CO2 trapping more heat in the atmosphere. $\endgroup$
    – user27343
    Commented Jun 18, 2019 at 3:24

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Well, one could definitely argue about the global, large-scale effects of the heat released by engines and I doubt that it will be significant.

The problem with global warming due to increasing CO2 levels is very different and there is only one way to see it. The earth radiatea out heat energy, primarily in the form of infrared radiation. The molecular structure of CO2 (and some other gases) happen to be such that they absorb this particular type of radiation. When the levels of these gases in the atmosphere increases, more of the heat energy is absorbed and radiated back to the earth instead of escaping into space. This increases the net temperature of the earth.

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  • $\begingroup$ So in other words, the heat radiated by engines is not the type trapped by CO2, and it can escape into space? The heat radiated by engines is not infrared and is not absorbed by CO2? $\endgroup$
    – user27343
    Commented Jun 18, 2019 at 20:32

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