life-cycle assessment
Life-cycle assessment tallies a product's total environmental footprint from cradle to grave — mining, making, transporting, using, and finally disposing of it. It is like counting every calorie a meal really cost to grow and cook, not just what ends up on the plate.
The analysis is split into phases: raw-material extraction, manufacture, transport, use, and end-of-life. Through each you track quantities such as embodied energy (in MJ per kg) and carbon footprint (in kg of CO2 per kg). For example, making aluminum from ore takes about 200 MJ/kg versus only about 30 MJ/kg for steel — a big up-front penalty — yet an aluminum car body is lighter and burns less fuel in use, so for a heavily driven car the use-phase saving can more than pay it back. For cars and appliances the use phase often dominates the whole footprint.
The honest caveat: LCA is data-hungry and very sensitive to where you draw the boundaries. Results shift with your assumptions — the electricity mix, the product's lifetime, transport distances. Recycled aluminum, for instance, needs only about 5 percent of the energy of virgin aluminum, so recycled content alone can swing the answer.
Should a car body be steel or aluminum? Aluminum takes about 200 MJ/kg to make from ore versus about 30 MJ/kg for steel — a big upfront penalty. But an aluminum body is lighter, so over 200,000 km of driving it burns less fuel; for a car driven a lot, the use-phase fuel saving can outweigh the higher making energy. For a car barely driven, steel wins. The answer depends on the whole life, not one stage.
Comparing steel and aluminum only makes sense when you count making, using, and recycling together.
LCA numbers swing with the assumptions — electricity mix, product lifetime, recycled content — so an honest study reports its boundaries and runs a sensitivity check rather than one headline figure.