The classic definition of sustainability, “providing for society’s needs today without compromising society’s ability to meet those needs tomorrow” originated in the  Brundtland Report, 1987 and can be found quoted by the Canadian Lawyers Abroad, Genentech, and elsewhere.

The authors of the Brundtland Report must have been lawyers not engineers. Their definition is good, but abstract. When you ask the canonical engineering question: ‘How do we make it work?’ The answer is:

Sustainable systems harness a process, rather than consuming a resource. Solar panels transform the energy in sunlight. Wind turbines transform the kinetic energy in wind. Geothermal systems use the heat of the earth.

The sun will shine and the winds will blow regardless of the presence or absence of solar panels and wind turbines. And the core of the earth will stay hot for a very long time – on the order of five billion years.