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Frostbit Eire. Major snowfalls in Ireland (such as the one seen from space in December 2010) are rare, but a new study suggests that most of the cold spells that occurred there during medieval times were triggered by volcanic eruptions el

NASA Goddard/MODIS Rapid Response Team, Jeff Schmaltz

Of Monks, Volcanoes, and Very Cold Weather

Today's state-of-the-art computer simulations are getting a bit of assistance from medieval monks. Chronicles written by religious men in Ireland hundreds of years ago have helped scientists draw a link between frigid periods of the country's history and volcanic eruptions.

While typical indicators of climate like tree rings are sometimes vague, "these historical records are the ultimate form of climate proxy," says Michael McCormick, a historian at Harvard University who wasn't involved in the new study. "They tell us what happened and where."

Scientists have long known that volcanoes, especially those that spew prodigious quantities of light-scattering droplets of sulfur dioxide high into the atmosphere, can temporarily chill the climate. Those tiny aerosols reflect sunlight back into space, decreasing temperatures at ground level as much as 1°C or more for brief periods and by smaller amounts for intervals lasting up to a year or more. Eventually, natural processes, including chemical reactions and precipitation, cleanse those aerosols from the air, and temperatures return to normal. Ice sheets worldwide collect substances called sulfates as they accumulate year by year, enabling researchers to pin down the dates—and sometimes to infer the relative strengths—of ancient eruptions.

But ancient writings also help. For the new analysis, says Francis Ludlow, a historical climatologist at Harvard University, and colleagues delved into a compilation of writings called the Irish Annals. That anthology includes more than 40,000 entries chronicling many aspects of life in medieval Ireland, including unusual climatic events such as droughts and major snowfalls—which are relatively rare due to the island nation's mild climate. He and his team focused on the years between 431 and 1649 C.E. During the first part of that period, entries in the Annals were largely written by monks and scribes in monasteries throughout the country. After the 13th century, however, information also came from family historians of the nation's aristocratic families. In the last few decades of the period, as British colonialism in effect dismantled the network of chroniclers, the team garnered data from personal letters and military historians.

The team scoured the Annals and other sources for reliable reports of particularly cold climate, including major snowfalls, periods of prolonged frost or ice on lakes and rivers, and general observations of abnormally cold weather. During the 1200-plus-year period of interest, the team identified 70 such cold spells. It's clear that the scribes were diligent reporters of such episodes, most probably because of the negative impacts that cold weather had on livestock, crops, and society in general, Ludlow says.

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.

The 10 eruptions that don't appear to have triggered a cold spell may have been small, brief events that didn't produce much climate-cooling sulfate, Ludlow says. Or, they may have occurred at a season when their peak influence didn't affect winter weather. Likewise, the 33 cold spells that didn't seem to result from volcanic eruptions may simply have been normal bouts of unusually cool weather, part of the natural variation often seen from one year to the next.

The new findings will help researchers tweak their climate models to more accurately account for the influence of volcanic eruptions, says Andrei Kurbatov, a volcanologist at the University of Maine, Orono. The research may be particularly useful because the interval that the team studied lies well before industrial emissions started boosting atmospheric concentrations of planet-warming carbon dioxide. "Researchers have long been seeking the range of natural [climate] sensitivity to volcanic eruptions," he notes. "Nobody knows what it is." It's ironic, he adds, that the scribblings of medieval monks could help scientists adjust today's state-of-the-art climate simulations.