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Understanding ER over EIThe first concept you need to understand to weed through the conflicting information you hear about the world's oil supply is a simple formula, ERoEI. It stands for "Energy Returned over Energy Invested." ERoEI is not so much a mathematical statement as a logical one. If I can invest one unit of energy (could be calories, kilowatts, or joules; it doesn't matter which unit) and get back 100, that's a good trade. On the other hand, if I get back only two units for every one I invest, I begin to wonder whether I want to invest the effort. If I get back only one for each unit I invest, I'm just pushing energy around for no good reason. And if get back less than one unit for each unit I invest, well that's just dumb. In the first days of oil extraction in the U.S., it was not uncommon to get back 50 barrels of oil for every barrel consumed. Now, because all the "gushers" are tapped out, it's more typical to get rates of 2:1 or even less. And when someone tells you how much conventional oil there is remaining in the world, remember that the easy oil is gone or going. What is left will be oil that is in deeper waters, in less hospitable climates, harder to find, and in smaller deposits. All that difficulty translates to more and more unappealing rates of ERoEI. So the oil will still be there, and we will still be able to get at it. But when it takes more energy to get it than we get back, continuing to extract it would be, well, dumb.
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