A forest regrows after a fire. Wind keeps blowing. Sunlight hits the ground every morning whether anyone harvests it or not. These are renewable resources: materials and energy sources that nature replenishes on a human timescale. The key phrase is "human timescale." A tree takes decades to mature. Groundwater can recharge in years or centuries depending on the aquifer. Get the harvest rate wrong and a renewable resource behaves like a finite one.

Renewable resources fall into a few broad groups. Solar, wind, and geothermal energy arrive continuously. Biomass, fresh water, and fertile soil renew only if we manage them carefully. Fish stocks, timber, and wildlife populations sit somewhere in between, capable of recovery but vulnerable to overexploitation.

What determines renewability

• Replenishment rate compared with consumption rate
• Whether extraction damages the underlying system
• Time horizon considered

Iceland runs almost entirely on geothermal and hydroelectric power. The Aral Sea, once fed by renewing river flows, shrank to a fraction of its size after Soviet irrigation diverted those rivers. Same category of resource, opposite outcomes. Renewability is not a fixed property. It is a relationship between a resource and how people use it.