Running the numbers for fusion power, if fusion could match the global energy output of 1995, current Lithium reserves (for lithium fusion, obviously) would last 3000 years. Lithium extracted from sea water would last 60 million years. Deuterium (Hydrogen-2) extracted from sea water, for the complicated Deuterium only fuel cycle, would last 150 billion years. To put that in perspective, over ten times the age of the universe and 30 times the remaining lifespan of the sun. This is a potential non-renewable resource that could theoretically outlast, by an order of magnitude, so-called "renewable" energy sources.
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