Then, the researchers scrutinized the Greenland ice core for evidence of volcanic eruptions, as denoted by ice layers particularly rich in sulfates. During the same 12-century interval, they identified 48 distinct eruptions—many of which likely occurred at high latitudes in the Northern Hemisphere— including Iceland, which lies nearby and upwind of Ireland—but some that may have occurred in the tropics. One of the eruptions, probably the largest of the period, occurred in 1600 in what is now Peru, Ludlow says.
Thirty-eight of the 48 eruptions occurred within 5 years of 37 of the episodes of cold weather, the researchers report online this week in Environmental Research Letters. Five years may seem a long time between volcanic cause and climatic effect, Ludlow says. But the inaccuracy of the ice core's sulfate record (plus-or-minus about 2 years), as well as any time lag between the active phase of an eruption and the time when sulfates actually were deposited, make it difficult to assess any volcano/climate link across a tighter time window.