"For any human culture to be considered sustainable it must have the capacity (proven only with historical hindsight) to reproduce itself down the generations while providing human material needs without cataclysmic and long-term breakdown. If it is energetically impossible for high energy society to be anything more than a pulse in the long run of human history, then it cannot, by this definition, be sustainable, no matter how much we shuffle the technological deckchairs. ...we need to get over our naive and simplistic notions of sustainability as a likely reality for ourselves or even for our grandchildren and instead accept that our task is use of our familiarity with continuous change to adapt to energy descent." (italics in the original)
David Holmgren, Permaculture: Principles and Pathways Beyond Sustainability, 2002.