What does a carbon footprint not tell you?

A footprint is an estimate, not an exact meter reading.

• Factors change. Grid emission factors, fuel factors and GWP values are updated over time, so the same activity can give a different number in different years.
• Methods differ. Two calculators can give different answers for the same family because they include different items or use different factors. Compare like with like.
• Trees are not a fixed eraser. Trees take carbon dioxide out of the air and store it as they grow, which is a removal. Using less electricity or fuel is an avoided emission. These are different things. How much carbon a tree stores varies widely with species, age, soil, rainfall and whether it survives, so one tree is not a fixed number of kilograms per year. Planting and caring for trees is valuable, but it is not a precise way to cancel a stated footprint.
• It is not a moral score. Many emissions, such as those from the grid or public buses, depend on choices made by governments and companies, not only by individuals.