Conditions for sustainable human societies

All populations, including humans, are sustained by fluxes of energy and materials from a finite environment. Physical constraints on biological design result in ubiquitous and predictable "allometric scaling laws" (Brown et al. 2004), pervasive in ecological theory. However, unique to the human species is its capacity to harness extra-metabolic energy in the form of renewables and fossil fuels to power the development of more complex societies, from agricultural and industrial to modern technological lifestyles. We use ecological theory to compare variation in densities and individual energy use in human societies (varying in societal complexity) to other land mammals. We show that societal complexity (from hunter-gatherers to modern cities) not only associates to greater energy fluxes (both per capita and at a population scale), but also allows escaping from ecological laws. Moreover, densest cities across the globe flux greater energy than net primary productivity on a per area basis, becoming sinks. This condition poses formidable challenges for establishing a sustainable relationship on a finite planet. In an attempt of evaluating sustainable conditions of such demands, we developed a mathematical model, coupling human population growth, the benefits they obtain from the natural system or "ecosystem services" and technological development. In our model, high population numbers attaining basic standards of living can only be sustained under "clean technology" (technology capable of having a net positive impact on ecosystem services flow compared to their consumption). Otherwise, those numbers signify the establishment of "mad-max" scenarios (sensu Costanza 2000), with societies attaining less than basic standard of living conditions or the acceptance of inequalities. A rapid shift to clean technology to power modern societies, is necessary now more than ever