Projection variants and uncertainty intervals: not the same thing
A prediction interval says how uncertain the UN’s main projection is: by its statistical model, the population has a 95 percent probability of lying within the 95 percent interval. A variant, such as high or low fertility, shows what would follow from a different assumption and has no probability attached. The high and low variants are not the range of likely outcomes.
The medium projection
The UN World Population Prospects 2024 project the population of every country with a probabilistic model: many possible future paths of fertility and mortality, each giving a population. The middle of those paths, the median, is the medium projection, the main result. For the world in 2100 it is 10.2 billion.
Prediction intervals: how uncertain
The same paths give the intervals. By the UN’s model, the world population in 2100 has an 80 percent probability of lying between 9.41 billion and 11 billion, and a 95 percent probability of lying between 9.05 billion and 11.4 billion.
The intervals widen the further ahead the projection reaches. They measure uncertainty within the model, not every possible surprise.
Variants: what if
A variant keeps the medium assumptions and changes one of them. The high-fertility variant assumes more births, the low-fertility variant fewer, the zero-migration variant no migration, and so on. Each is a calculated “what if”.
For the world in 2100, the high-fertility variant gives 14.4 billion and the low-fertility variant 6.99 billion. Both lie outside the 95 percent interval. They show what a lasting change in fertility would mean, not how likely it is.
On Anthrotempo
The population scenarios tool draws the intervals as shaded bands, labelled with their level, and the variants as separate lines. A variant is never called a confidence range, and an interval is never called a scenario.
Last updated 26 October 2026