What is the effect of increasing energy efficiency on CO2 emissions in a fossil-based system?

• A. Lower CO2 emissions by reducing energy demand, assuming a fossil-based system.
• B. Increase NOx emissions due to higher energy use.
• C. Have no effect on emissions.
• D. Increase SO2 emissions due to cleaner fuels.

Answer: (C)

Increasing energy efficiency means you need less energy to do the same work. In a system that runs on fossil fuels, CO2 emissions come from burning those fuels, so using less energy directly lowers the amount of fossil fuel burned and therefore reduces CO2 emissions. The impact can be affected by the rebound effect—if lower costs lead to more overall energy use, the reduction might be smaller—but the typical outcome is a decrease in emissions as energy demand falls.