About 8 billion people now live on Earth. Each person uses water, energy, food and resources, and produces waste. How much people change Earth's systems depends on two things: how many people there are, and how much each person uses on average.
Key idea
Total use = number of people × use per person. A change in either one changes the total, and changes in both multiply together.
Why it works
Picture the total as the area of a rectangle: its width is the number of people and its height is the use per person. Double the width and the area doubles; double the height and it doubles too; do both and it is four times as big. Shrink one while the other grows and the area can stay the same. That is why scientists look at both how many people there are and how resources are used.
Worked example: Total water use
A town of 20,000 people uses about 300 liters of water per person each day. How much does the town use in a day?
- Total = people × use per person.
- $20{,}000 \times 300 = 6{,}000{,}000$ liters a day.
Answer: 6,000,000 L a day
Worked example: Two changes at once
A city's population grows by 20% while each person's energy use falls by 25%. Does the total go up or down, and by how much?
- People are multiplied by 1.20; use per person by 0.75.
- Total is multiplied by $1.20 \times 0.75 = 0.90$.
- So the total falls by 10%.
Answer: Down by 10%
Common mistake
Blaming impact on the number of people alone, or on how much each person uses alone. The total depends on both, multiplied together. Check with numbers: if the population doubles while use per person halves, the total stays the same.
Engineers and scientists also work on how impacts happen: using water more efficiently, cleaner energy, recycling metals. Those change the use per person, the second factor.