Can climate cause a society’s success or failure?

The short answer

• Living at the margin magnifies small changes. Hay linked Greenland's short summer to the survival of cattle through eight winter months. Too little warmth or moisture could leave the animals without enough feed. Mexica maize faced a different limit at about 2,240 metres: an early autumn frost could kill the crop before it ripened. Near such limits a shift of a degree or two could turn an ordinary harvest into a desperate year.
• Ocean currents and sea ice shape the routes. Ships connected Greenland to Norway, but the sea also carried an obstacle. The cold East Greenland Current, shown by the grey arrow, moves Arctic ice down the east coast and around Cape Farewell. As cooling made drift ice more extensive and longer-lasting in summer, harbors could be blocked and voyages stretched out. The connection became slower and riskier just as the settlements depended on it.
• Volcanoes can tip a climate into a new state. Sulphur from a large eruption can reach high into the atmosphere, reflect sunlight and cool the world for several years. Gifford Miller's team argued in 2012 that four large eruptions over roughly fifty years helped initiate the Little Ice Age in 1275–1300; the sequence included the 1257–58 eruption later traced to Samalas in Indonesia. Expanding sea ice could then reflect more sunlight, keeping summers cold after the volcanic haze had cleared.
• Societies build their own vulnerabilities. A settlement of perhaps 2,000–2,500 people in Norse Greenland had a small margin against repeated bad years. Tenochtitlan, perhaps 200,000 strong, could draw on an empire for food, but placed a large population on one lake. Population, food diversity and trade determined what resources each society had when cold or drought struck. A shock that one community survived could be fatal to another.

What it changed

The warm centuries open the sea road. Norse voyages to Iceland, Greenland and North America coincided with the relatively warm centuries of roughly 950–1250. Climate helped set the conditions for open-ocean crossings; ships and the farming society that supported them made the voyages possible. Chapter 2 takes up that combination.

The cold tests the farms. Greenland's outlying farms depended on hay and cattle, while walrus ivory supported trade. Cooling after about 1275–1300 placed those arrangements under pressure. Chapter 5 weighs that pressure with markets, isolation and choice in asking why the Norse left while newly arrived Inuit stayed and prospered.

A second laboratory in Mexico. The Basin of Mexico faced floods in 1449 and 1499 and frost and drought in the 1450s. Chapter 8 examines dikes, aqueducts and granaries as Mexica responses. By supporting a larger population on the lake, those measures also meant that a later failure could harm more people.