Each year, Earth passes through two meteor showers produced by dusty fragments of Halley's comet. Throughout the 530s and into the 540s, these meteor showers may have been unusually heavy, and continued to top-up Earth's atmosphere with cooling dust – and perhaps more.
There are other surprising clues in the ice. Cores from around the beginning of 536 contain the frozen remains of microorganisms normally found in shallow tropical seas, while samples from 538 also contain fossils of much more ancient marine microorganisms. Abbott thinks there is only one way these microorganisms could have ended up in Greenland ice.
Halley's comet might have shed a few especially large fragments during its journey through the inner solar system in 530. In the following years, perhaps including 536 and 538, these fragments slammed into Earth's oceans. When they did, dust and debris – containing living marine microbes in the water and fossils in the rocks that were struck – were thrown high into the atmosphere and global temperatures plummeted.
Abbot's team may have even found where one of these collisions occurred. Gravity anomalies and metallic spherules in a sediment layer suggest a large object struck Australia's Gulf of Carpentaria sometime in the first millennium AD, she says.