Rock Doc

Ireland enjoys a mild and stable climate. But even in Ireland there are years that stand out as unusual. Recently a team of researchers led by Harvard's Francis Ludlow announced results of a study of Ireland's climate based on the Irish Annals, a body of writings containing more than 40,000 entries.

The Annals record events from 431 A.D. to 1649 A.D. During the Medieval period they were written by monks. From the 1200s onwards some entries were written by historians of the wealthy and aristocratic families of the period. Finally, toward the end of the Annals, entries were contributed by military historians as British colonialism took hold.

As recently reported on the website ScienceNOW, Ludlow's team looked through the Irish Annals for information about weather. Of particular interest were notes about cold times, such as years with ice on certain lakes or winters of heavy snowfall. In the Annals the researchers identified a total of 70 times of unusual cold. Ludlow thinks those who wrote the Annals likely recorded all the significantly cold seasons they experienced because cold has a profound effect on agricultural productivity.

Cold spells can arise for a number of reasons, including the simple natural variation that's always at work in weather and climate. But one specific cause of cold years is volcanic eruptions. Volcanoes eject ash and lava when they erupt, but they also sometimes spew tiny drops of sulfur dioxide into the atmosphere. Those droplets drift downwind and reflect sunlight back into space, cooling the ground below. For short periods of time volcanic eruptions can drop temperatures by a couple degrees Fahrenheit - quite enough to make a real difference.

As it happens, we have a pretty good natural record of volcanic eruptions near Ireland. It's based on variations in Greenland's glacial ice. Scientists have drilled into the icecap and pulled up layered cores of ice. Each pair of layers denotes summer and winter snowfall in Greenland, so researchers can simply count back in time to assign dates to deeper and deeper layers.

The glacial ice is marked by sulfates when volcanic eruptions occurred. During the time spanned by the Irish Annals, scientists found 48 layers denoting significant volcanic eruptions. Many of the eruptions occurred in the Northern Hemisphere at high latitudes, but we have other evidence to think that one large eruption occurred in the year 1600 in Peru.

How does the Greenland record match up with the Irish Annals? In short, there isn't a perfect fit but a number of the sulfate rich ice layers come from around the times the Annals report cold spells. That's good enough for me, as I wouldn't expect the two records to speak only with one voice. Climate is complex and volcanic eruptions are only one factor in what determines temperature. Certainly some cold spells occur simply due to natural cycles in temperature variations, not because of specific volcanic events.

But it's nifty from my point of view to think that the records of Medieval monks could now be fed into computer simulations of climate.

Dr. E. Kirsten Peters, a native of the rural Northwest, was trained as a geologist at Princeton and Harvard. This column is a service of the College of Agricultural, Human and Natural Resource Sciences at Washington State University.