Oxygen, one half in 5000 years. Available energy can't be recycled. Active life is not sustainable without an adequate supply of available energy. Energy is a basic necessity of an ecosystem. Exergy is the energy available in a given environment, the ability to do work, the cause of all actions.
Energy is the ultimate connection between society and the environment.
Nuclear fuels should be reduced, due to waste. Large scale biomass is inefficient, and in conflict with food production. Hydro power not an option Everything ruled out except direct solar, and efficiency. Total exergy needed is 5 kW per person. 3 kW of solar heat requires 24 square metres of sunshine.