Climate Is Not Weather
Weather is what happened on Tuesday. Climate is the distribution weather is drawn from — the averages, and just as importantly the variance. Confusing the two is the oldest error in the field, and it runs in both directions: a cold winter is not evidence against a warming climate, and a warm decade did not by itself end a dynasty.
For history, the variance often matters more than the mean. A region with reliable rainfall and one with the same average rainfall arriving unpredictably are not the same place at all. The second cannot support the same population, because populations live at the level the bad years permit, not the average ones.
The Köppen System
Wladimir Köppen's 1884 classification is still the standard, and its elegance is that it classifies climate by vegetation — the plants integrate temperature and rainfall over the year better than any index a person could design.
| Group | Name | Character |
|---|---|---|
| A | Tropical | Hot, wet; no real winter |
| B | Arid | Evaporation exceeds precipitation |
| C | Temperate | Mild winters — the historic heartland of dense agrarian states |
| D | Continental | Cold winters, warm summers; big seasonal swing |
| E | Polar | No warm season |
Group B is defined by a relation, not a temperature: arid means evaporation exceeds precipitation. That is why a cold desert is a desert. Aridity is a budget, not a thermometer reading.
Climate's Fingerprints on the Record
Climate leaves signatures a historian can read, and the last century of paleoclimatology has made this genuinely quantitative:
- Tree rings (dendrochronology): annual, exactly dated, sensitive to growing-season conditions.
- Ice cores: trapped air gives past atmospheric composition; layers give volcanic ash and dating.
- Pollen cores: which plants grew, and therefore the biome, century by century.
- Speleothems: cave deposits recording rainfall regimes.
The correlations are real and dramatic. The Medieval Climate Anomaly (c. 950–1250) coincides with Norse settlement of Greenland and European agricultural expansion. The Little Ice Age (c. 1300–1850) coincides with famine, the abandonment of Greenland, and the seventeenth-century general crisis. The 536 CE dust veil — a volcanic event that dimmed the sun for over a year — coincides with famine and plague across Eurasia.
And here is the discipline's hard-won rule about all of that: climate is a stressor, not a script. The same Little Ice Age that helped wreck one polity found another that adapted, traded, or stored its way through. Climate changes the difficulty setting; institutions, reserves, and choices determine the outcome. The Norse abandoned Greenland while the Inuit, in the identical climate, did not — which is the single most useful fact in this lesson, because it kills determinism with a controlled experiment nature happened to run for us.
The methodological trap is correlation. Two dates lining up is a hypothesis, not a finding — and paleoclimate series are now precise enough that you can always find a climatic wobble near any event you care about. The demand is the same as ever: show the mechanism. Harvest yields, grain prices, mortality records, migration. A cold decade next to a revolt is a coincidence until you can trace how the cold reached the crowd.
Nature Ran the Experiment
The animation opens on the Little Ice Age arriving over Greenland as a falling temperature curve, and two settlements sitting underneath it — Norse and Inuit — in the same place, in the same century, under the identical cold.
The Norse column builds first: cattle and sheep, hay meadows, turf byres, a European farming package carried north and held onto. As the curve descends, the growing season shortens and the hay yield bar drops. The animation traces the mechanism rather than asserting it — shorter season, less hay, fewer cattle overwintered, less milk, more hunger, and the trade link to Norway thinning at the same time. The settlement empties.
The Inuit column runs beside it under the same curve. Ringed seal, sea ice, kayaks, dogs, a technology aimed at the sea rather than the meadow. Cold lengthens the ice season. The same descending curve that starves the hay meadow extends the hunting platform, and the second bar rises while the first falls.
The final beat holds both columns side by side under one temperature line, and states the thing plainly: the cold did not decide anything. It changed the difficulty setting. What the Norse could not do was stop being Norse farmers — and that was a choice about identity, not a fact about the weather.
Climate is a stressor, not a script. Nature ran the controlled experiment and the answer survived.