Norse and Aztecs · 1250
Can climate cause a society’s success or failure?
The seminar question: “Can climate ever be the cause of a society’s success or failure, or only its context?”
Between about 950 and 1250 much of the North Atlantic enjoyed a warm spell scientists call the Medieval Climate Anomaly; from about 1275–1300 it slid into the colder Little Ice Age. Proxies such as Greenland ice cores and lake-mud records show the change was uneven, warming one coast while a neighbour cooled.
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Medieval climate changed unevenly from place to place, which is why scientists call the warm spell an anomaly, meaning a regional departure from the usual pattern, much as one unusually hot summer stands out from the long-run average in a city’s records. Comparing hundreds of records in 2009, Michael Mann and his colleagues found that the years 950–1250 were warm around the North Atlantic while parts of the tropical Pacific were cool. In south Greenland, midge remains in the mud of Scoop Lake point to summers about 1.5 °C warmer between 900 and 1400 than in the colder centuries on either side. Yet lakes near Kangerlussuaq, further north and inland, record a fall of about 4 °C in some eighty years from around 1100. The cold returned abruptly, and Gifford Miller’s team dates its onset to 1275–1300, after a burst of volcanic eruptions whose sulphur dimmed the sun and whose effect spreading sea ice then prolonged. A coast could therefore be warm for one community while its neighbour cooled, and that unevenness is what turns climate into history.
The short answer
- Living at the margin magnifies small changes. A society that farms close to the limit of what its crops and animals can tolerate feels a small shift in climate as a large shift in harvests. Greenland’s Norse depended on enough summer warmth and moisture to grow the hay that fed their cattle through eight winter months, while the Mexica grew maize at about 2,240 metres, where an early autumn frost can kill a crop before it ripens. In both places a change of a degree or two decided whether a year was ordinary or desperate.
- Ocean currents and sea ice shape the routes. The North Atlantic’s climate is carried by the sea as much as by the air, which matters enormously to a people who travel by ship. The cold East Greenland Current, drawn as the grey arrow, carries Arctic ice south along Greenland’s east coast and around Cape Farewell. When the climate cooled, that drift ice spread further and lasted longer each summer, blocking harbours, lengthening voyages and making the link between Greenland and Norway slower and riskier.
- Volcanoes can tip a climate into a new state. Large eruptions inject sulphur high into the atmosphere, where it reflects sunlight and cools the world for several years. Gifford Miller’s team argued in 2012 that a cluster of four large eruptions in about fifty years, including the great eruption of 1257–58 later traced to Samalas in Indonesia, began the Little Ice Age between 1275 and 1300. Spreading sea ice then reflected more sunlight and kept summers cold long after the volcanic haze had cleared.
- Societies build their own vulnerabilities. The same weather harms societies differently, depending on how many people they must feed, how varied their food is and how far their trade reaches. A colony of perhaps 2,000–2,500 people on the edge of the Arctic had little margin for a run of bad years, while a city of perhaps 200,000 drew on an empire but concentrated its people on one lake. Climate exposed these differences more than it created them, and that is why the same cold or drought could be survived in one place and fatal in another.
THE TURNING POINT
Summit, Greenland, 1 July 1993. On this day the GISP2 drill reached bedrock 3,053 metres below the summit of the Greenland ice sheet, completing a record of past climate that can be read almost year by year. This is the hinge of the chapter because it changed what historians could ask. Before the great cores, the climate of the Norse age was inferred largely from the sagas and annals whose story it was meant to explain, so the argument risked going in a circle. After them, climate became independent evidence that could be tested against the documents, and the documents tested against it.
What it changed
The warm centuries open the sea road. The warmth of roughly 950–1250 coincided with the boldest phase of Norse voyaging, when settlers crossed open water to Iceland, Greenland and North America. The next chapter asks what made those crossings possible in the first place, and it finds the answer in a ship and in the farming society that paid for it, with climate as the setting that made the gamble of open-ocean sailing pay off more often than it failed.
The cold tests the farms. The shift into the Little Ice Age after about 1275–1300 fell hardest on the communities furthest out, above all the Greenland settlements that lived on hay, cattle and walrus ivory. Chapter 5 follows that test in detail and weighs climate against markets, isolation and choice as explanations of why the Norse left while the Inuit, newly arrived, stayed and prospered.
A second laboratory in Mexico. The same questions about margins, buffers and invention apply to the Basin of Mexico, where frost and drought struck in the 1450s and floods in 1449 and 1499. Chapter 8 shows how the Mexica answered with dikes, aqueducts and granaries, and how each answer, by letting more people live on the lake, created a new and larger kind of risk.
The full answer, argued
The evidence in this atlas supports climate as a powerful context that becomes decisive only through a society’s structure. The warm centuries plainly widened the window for Norse farming in Greenland, and the cooling after 1275–1300 narrowed it, so a purely human account ignores real physical change. Yet the same cooling did not end Iceland, and the Thule Inuit expanded into Greenland during exactly the centuries when the Norse withdrew, which shows that the cold was survivable with the right tools and food. The proxies also disagree, since Lake 578 shows drying where the summit cores show cold. A careful answer treats climate as a pressure whose effect depends on population, trade, technology and choice, and historians call this the study of vulnerability. The transferable insight is that the same shock can be an inconvenience for one society and a catastrophe for another, so explanation must always ask who was exposed and why.
An interesting fact
The principle behind ice-core thermometers began with beer bottles. On a rainy Saturday at the end of June 1952 the Danish physicist Willi Dansgaard set out an empty beer bottle with a funnel in his garden in Herlev, near Copenhagen, to catch the rain from successive showers, and found that the share of heavy oxygen in the water changed with the temperature of the cloud it fell from. If falling snow kept the same signature, he reasoned, the Greenland ice sheet would be an archive of past temperatures. Two decades later he read that archive beside the history of the Greenland Norse.
This is the study layer of Medieval Warm Period and Little Ice Age, chapter 1 of Climate and Innovation: The Norse and the Aztecs, c. 790–1521; the full index of the atlas is here.
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