6. Energy Sources: Renewable vs. Non-Renewable

Energy choices are at the heart of environmental science — the type of energy we use determines our carbon footprint and pollution levels.

Non-renewable energy sources:
These are finite resources that took millions of years to form and cannot be replenished on human timescales.

• Coal: most carbon-intensive fossil fuel; burning produces CO₂, SO₂, NOₓ, and particulates. Still used heavily for electricity generation in many countries.
• Oil (petroleum): primarily used for transportation fuels (gasoline, diesel, jet fuel) and plastics. Drilling and spills cause environmental damage.
• Natural gas (methane): cleanest-burning fossil fuel — produces less CO₂ per unit energy than coal or oil, but methane leaks during extraction and transport are potent warming agents.
• Nuclear energy: splits (fission) uranium atoms to produce heat. Zero direct CO₂ emissions during operation. Issues: radioactive waste storage (lasting thousands of years), high construction costs, rare but severe accident risk (Chernobyl 1986, Fukushima 2011).

Renewable energy sources:
These are replenished naturally and do not run out on human timescales.

Energy efficiency and conservation:
The cleanest energy is the energy you don't use. Energy efficiency improvements (better insulation, LED lighting, efficient vehicles) reduce demand, cutting both costs and emissions. The rebound effect means some savings are spent on more consumption, partially offsetting efficiency gains.

EROI — Energy Return on Investment:
The ratio of energy obtained to energy invested in obtaining it. Coal and conventional oil have historically had high EROI (30:1 or more). As fossil fuel reserves decline, EROI falls. Solar and wind EROI has improved dramatically as technology advances.