New research reveals just how bad an idea it was to colonize Greenland and Iceland more than a millennium ago: average temperatures in Iceland plummeted nearly 6°Celsius in the century that followed the island’s Norse settlement in about A.D. 870, a climate record gleaned from mollusk shells shows. 

The record is the most precise year-by-year chronology yet of temperatures experienced by the northern Norse colonies, says William Patterson, an isotope geochemist at the University of Saskatchewan in Saskatoon, Canada, who led the new work. The study will appear online in the Proceedings of the National Academy of Sciences.

“We’re aware from written documents of the kinds of things that people faced in the North Atlantic over the last 1,000 years,” he says. “This is a way to quantify the experiences they had.”

For instance, Icelandic sagas mention several famines that took place in the first century after settlement, at the time temperatures were dropping. But Astrid Ogilvie, an Arctic historian at the University of Colorado at Boulder, says it’s a stretch to blame those famines — in which, as one saga describes it, “the old and helpless were killed and thrown over cliffs” — totally on climate. 

The mollusk temperature record is “all tremendously interesting,” she says, “but there is a caveat — we can’t be 100 percent sure that climate was involved” in the famine.

The study will, however, help historians better understand exactly what was going on in the Norse settlements over the years, Ogilvie says. 

Patterson’s team made detailed measurements of oxygen isotopes contained within 26 mollusk shells taken from sediment cores drilled off the northwestern coast of Iceland. The ratio of oxygen-16 to oxygen-18 in the shells varies depending on water temperature, so the amounts of the two isotopes can be used as a proxy to gauge how hot or cold things were.

The shells show a large amount of variation both within years and from year to year. For instance, the researchers say, winter temperature variability increased between 990 and 1120, a time when written records suggest that crops occasionally failed. By 1250, things heated up again and summer temperatures reached 10°C, possibly the highest in three centuries. Within decades, though, temperatures began to plunge again. 

While Iceland remained settled through the modern day, Norse settlements in Greenland were abandoned by the early 15th century. Many researchers believe that climate changes played at least a minor role.