Guide · 1 min
Normal depends on when you were born
Modelled demonstration series · not observed data
Your sense of a "normal" summer was set by the summers you lived through. Ecologists call this a shifting baseline: each generation measures change against the world it first knew.
Same reading, different verdict
The thirty years from 1990 run 1990 – 2019. The Arctic sea-ice series reads −8.60σ against 1951–1980 and −4.30σ against 1991–2020. The number of stations past their own modelled record — 20 — does not move with the baseline (20 against 1991–2020): a record is a fact about the series, not the yardstick.
A birth-year baseline
Not Normal can measure every station against the thirty years starting in the year you were born, instead of a fixed window. Someone born in 1955 and someone born in 1990 can look at the same reading and get different verdicts — both correct for their own yardstick.
Why it matters
A baseline that moves with you makes change feel smaller than it is. Comparing your own window with an older fixed one is the quickest honest way to see how far "normal" has travelled.
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Move the baseline on the field →