How long will a mineral or fuel last? Answering that needs two careful definitions, because ore deposits and fossil fuels are counted in different ways depending on what is known and what is profitable.
- Resources: all of a material that exists in a form that could someday be extracted, including deposits not yet found or too poor or costly to use today.
- Reserves: the part of the identified resources that can be produced at a profit now, with today's prices and technology.
Why it works
Because reserves depend on economics, they change. Higher prices or better technology turn lower-grade resources into reserves; new discoveries add more; lower prices shrink them. A common yardstick is the reserves-to-production ratio, R/P = reserves ÷ yearly production: how many years today's reserves would last at today's rate. It is a snapshot, not a countdown. For many materials, reserves have grown about as fast as they were used for decades. On the other hand, if use grows by a fraction $r$ each year, the reserves run down faster than R/P suggests: adding up the growing yearly use gives $n = \dfrac{\ln(1 + r\cdot R/P)}{\ln(1 + r)}$ years. Economists and geologists debate how these effects will balance for each resource, and the answers differ from material to material.
Key idea
Reserves are the part of a resource that can be produced at a profit today. R/P shows how long reserves would last at today's rate, but reserves change with price, technology and discovery, and use can grow or shrink.
Worked examples
Worked example: R/P
A country has reserves of 800 million tonnes and produces 20 million tonnes a year. What is R/P?
- $800 \div 20 = 40$ years.
- That does not mean the material runs out in 40 years: reserves will change as prices, technology and discoveries change.
Answer: 40 years.
Worked example: With growing use
Reserves equal 50 years of today's production. If use grows 2% a year and no reserves are added, how long do they last?
- $n = \dfrac{\ln(1 + 0.02 \times 50)}{\ln 1.02} = \dfrac{\ln 2}{\ln 1.02} = \dfrac{0.693}{0.0198} \approx 35$ years.
- Growth shortens the time; discoveries, recycling and substitutes lengthen it.
Answer: About 35 years.
Common mistake
Reading "reserves ÷ yearly use = 40 years" as the date a resource runs out is a common misreading. Reserves grow and shrink with prices, technology and new discoveries, and use can rise or fall. Check the definition: reserves are only what is known and profitable today.