When discussing engines, the ratio of output work to input work expressed as a percentage is called .

Efficiency

Explanation: Efficiency of the engine is defined as percentage of the input work that is converted into output work.

Efficiency = %

Thus if an engine converts all of the input into output, i.e. input work = output work, the efficiency of the engine will be 100%.

But in real scenario, efficiency is always less than 100% as some of the input gets lost in the form of heat, or to overcome frictional forces.

causes

fossil fuels burn

cities become more industrialized

human population grows worldwide