Imagine possible futures of humanity. Apply logic, common sense, the fundamental laws of mass and energy conservation, as well as more advanced thermodynamics to evaluate overall efficiencies of major human energy supply schemes: fossil, nuclear, solar, wind, and biomass. Define and quantify the irreversible linear processes, and sustainable/unsustainable cycles. Emphasize the relationships between ecosystems (ancient and new), their energy storage and throughput, energy production and its side effects.