Poor misunderstood rats. The Black Death originated in Asia and arrived at European harbors in 1347 via the Silk Road trade routes. This marked the beginning of the second plague pandemic, which lasted until the 19th century in Europe. For a long time, researchers thought there was just one introduction of the plague bacterium Yersinia pestis into Europe, and we blamed the infection’s persistence on the establishment of that pathogen in flea-infested rats over four centuries. But now, researchers studying tree-ring data think that the outbreaks in Europe were driven by climate events in Asia.

That means black rats (Rattus rattus) living in Europe aren’t the main culprits: It was wild central Asian rodents who flourished during warmer, wetter climate periods. The findings, published in Proceedings of the National Academy of Sciences this week, might explain why the plague showed up in large parts of northern Europe where rats were absent.

A team led by Boris Schmid and Nils Stenseth from University of Oslo set out to find rodent reservoirs of recurrent plague emergence by studying climate fluctuations that occurred before regional outbreaks. They examined a dataset of 7,711 historical outbreaks and 15 tree-ring climate records from Europe and Asia. The thickness of rings reflect annual conditions for vegetation growth, making them great proxies for climate fluctuations.

The team found no evidence to support the existence of a climate-sensitive rodent plague reservoir in medieval Europe. Instead, they write, variable climate conditions that affect wildlife outbreaks of plague in Asia nowadays were identified in juniper tree rings from northern Pakistan’s Karakorum Mountains. For example, in Kazakhstan, warmer springs and wetter summers can lead to increases in the density of great gerbils (Rhombomys opimus) and their fleas—and thus plague prevalence. As gerbil populations collapse due to sudden temperature drops, fleas seek out alternative hosts, like humans and maybe camels. At the high altitudes of the Karakorum range, the dominant rodent reservoirs of plague are long-tailed ground squirrels (Spermophilus undulates) and Altai marmots (Marmota baibacina). "We have previously shown that an increase of 1 degree Celsius doubles the prevalence [of plague] in wild rodents in central Asia,” Stenseth tells New Scientist.

Such climate events in Asia (and not those in Europe) consistently preceded plague reintroductions in Europe by 15 years—suggesting that the infection spread from distant rodents during multiple waves through trade. The team found 61 potential maritime introductions in 17 trade harbors, mostly along the Mediterranean and Black Sea coasts. Then they identified 16 years between 1346 and 1837 that faced all new episodes of plague outbreaks. Based on the timing, rats living in Europe weren’t plague reservoirs: The bacterium was continuously reimported. Although rats may have helped maintain outbreaks on ships and harbors.

As for that 15-year delay, the team proposes three stages: The fleas spent one to two years looking for new hosts after rodent populations crash, then the pathogen crosses 4,000 kilometers (2,485 miles) westward over 10 to 12 years. Finally, it takes less than three years for the new pulse of plague to spread through the European mainland.