Energy returned versus energy invested (the fraction ER/EI) is important to you whether you know it or not. I prefer the fraction to the acronym because it makes clear there is an absolute threshold limit involved: if you divide your energy return by your energy invested and get a result less than 1, your system is losing energy and will not long remain a system. In practice, since complexity and adaptability have energy costs, some sorts of systems require higher ER/EI than others: a jellyfish might get by on an ER/EI of 1.5, while a tribe of hunter-gatherer humans might require an ER/EI of 5. Your current civilization was built on an ER/EI of 20:1+ and expects and depends on such a surplus continuing. ER/EI is a fundamental limit on life itself, but few humans are even aware of it as a concept because for the last two centuries, fossil energy has been essentially “free” in comparison with its true value, with ER/EI initially well over 100 (though now having fallen to around 20 and dropping). By convention, those few human academics who are aware of the need to numerate and denominate in natural resource terms tend to refer to it as EROI, even though that’s confusing ly similar to the more frequently used financial term “return on investment.”. Mathematicians refer to it as the fraction ER/EI, and ecologists tend to call it EROEI (Energy Returned On Energy Invested) to make it less confusing. On my planet we call it Derr (which roughly sounds like the English “duh”), but whatever it’s called, anywhere in the universe it’s a trade that underlies all of life’s game moves and determines their net result. If you expend more energy than you find, you eventually perish. This is such a general thermodynamic truism that it transcends life itself and applies equally well to a forest fire or hurricane. Like them, life is a thermodynamic process which obeys the rules of reality. Nothing – not even humans – gets a “pass” on this re ality. The Bottom line: A living system expends energy to get energy. There is a difference between Gross and Net. Net energy is what keeps living systems alive, and different sorts of life have different “net energy” needs to exist.
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