How does the electricity generation mix influence national CO2 emissions?

• A. Higher shares of fossil fuels lower emissions when efficiency improves.
• B. Emissions rise if the grid uses more renewables?
• C. Higher shares of low- or zero-carbon sources lower emissions; heavy fossil-fuel dependence raises emissions even with efficiency improvements.
• D. Emissions are unaffected by electricity generation mix.

Answer: (C)

The main idea here is that the carbon intensity of electricity—the amount of CO2 released per unit of electricity generated—depends on the mix of fuels used. When the grid shifts toward low- or zero-carbon sources like wind, solar, hydro, and nuclear, each kilowatt-hour produced carries far less or no CO2, so national CO2 emissions from electricity fall. Conversely, if the generation mix remains heavily reliant on fossil fuels, especially coal, the carbon intensity stays high, and even if efficiency improvements reduce emissions per kilowatt-hour, total emissions can stay the same or rise if electricity demand grows.

So, increasing shares of clean energy lowers emissions, while heavy fossil-fuel dependence keeps them high; the notion that more renewables would raise emissions or that emissions are unaffected by the mix doesn't align with how carbon intensity works in electricity generation.