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 can deliver the same service using less energy. When that energy comes from fossil fuels, burning less fuel generally means less CO2 released, so CO2 emissions are reduced. The overall impact can depend on how demand responds to lower energy costs (a rebound effect); if demand rises enough, the total emissions may be less reduced or even unchanged, but the direct effect of improving efficiency is to lower CO2 emissions per unit of service. Other pollutants like NOx or SO2 typically decrease as well because less fuel is burned, and the idea that efficiency would cause more emissions isn’t the usual outcome.