The landscape of human history is scattered with the remains of societies that, at the peak of their prosperity, presumably seemed to their members no less resistant to decline than industrial society appears to most of us today. If this presumption is reasonable—if a general tendency to base expectations about the future of one’s society on present appearances is indeed a recurring theme in human experience across cultures and time—then we also know that present appearances may prove to be a rather unreliable guide to the future prospects for contemporary ways of life. Thanks to the work of historians and archaeologists, today we have access to detailed records of the life cycles of numerous past societies, and to diverse views on the processes by which they grew in size, influence and complexity before peaking and declining. While each societal story obviously differs significantly in its detail from others, and while different perspectives in relation to any one story emphasise particular factors, energetic considerations represent a recurring, foundational theme in both describing and making sense of the rise and fall of human societies. While they don’t determine a society’s prospects, principal energy sources and the technologies by which they’re harnessed are fundamentally important enabling and constraining factors in shaping a society’s past history and future prospects. Energetic considerations set the available budget for what a society can do, and bound the policy options for how it does what it does.
Industrial society is fossil-fueled. Around eighty percent of global total primary energy supply comes from coal, oil and natural gas. Just under six percent is from nuclear fission of uranium-based fuels. While there’s abundant uncertainty relating to resource sizes and economic reserves for each of these, there’s very little debate regarding their ultimately non-renewable status: the principal primary energy sources with which industrial society has arisen and that it continues to rely on can be treated as finite. Pick a long enough time horizon—and as Tom Murphy at Do the Math demonstrates, we don’t need to look out too far if anything like historical growth in energy use is assumed—and all futures for industrial society based on continued reliance on fossil fuels and uranium run up against their absolute physical limits. Long before such theoretical limits are reached, we’ll be contending with economic limits in the form of diminishing returns on effort, and ecological limits associated with Earth’s capacity to deal with the consequences of all of that fossil-fueled activity. Whichever way you care to look at it, we can safely say that long term futures for human societies will depend not on accumulated energy wealth from the past, but on present energy income. In this respect at the very least, the future for renewable energy looks rather bright indeed!