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Scotland formally joined England with the Acts of Union (above), following an economic depression that may have been exacerbated by volcano-induced climate cooling.

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How a volcanic eruption helped create modern Scotland

Over seven terrible years in the 1690s, crops failed, farming villages emptied, and severe famine killed up to 15% of the entire population of Scotland. The so-called Scottish ills (named after the biblical plagues) ushered in an era of crippling economic conditions. Soon after, the formerly independent nation joined Great Britain. Now, researchers suggest volcanic eruptions thousands of kilometers away may have helped spark this political transformation.

Scientists have long known that volcanoes can alter Earth’s climate. During large eruptions, light-scattering droplets of sulfuric acid reach the stratosphere and spread around the globe, reflecting some of the Sun’s radiation back into space and cooling the planet. Such cold spells can last from several months to several years—and they can help trigger droughts and crop failures.

The clues to such events are often locked away in the rings of tree, whose growth slows with wild shifts in temperature and precipitation. But until recently, researchers didn’t have any tree-ring records from northern Scotland, where the worst effects of the famine took place. That all changed 2 years ago, when scientists stitched together a complete record of the local climate from 1200 to 2010. They used data from still-living trees and logs that had fallen into lakes, where they were preserved for centuries.

Rosanne D’Arrigo, a paleoclimatologist at Columbia University’s Lamont Doherty Earth Observatory in Palisades, New York, eagerly delved into the chronicle with her colleagues. Their analysis reveals that the second-coldest decade of the past 800 years stretched from 1695 to 1704. Summertime temperatures during this period were about 1.56°C lower than summertime averages from 1961 to 1990, the team will report in a forthcoming issue of the Journal of Volcanology and Geothermal Research.

All of this coincides with two major volcanic eruptions in the tropics: one in 1693 and an even larger one in 1695. The one-two punch likely sent Scotland into a deep chill that triggered massive crop failures and famines for several years, the team speculates.

Tropical volcanoes weren’t Scotland’s only bane. Relatively unsophisticated agricultural techniques, a government policy that encouraged grain exports (which left few reserves when crops failed), and an unsuccessful attempt to set up a Scottish colony in Panama starting in 1698 put the nation in even more dire circumstances. These problems, as well as the economic depression that followed, motivated the Scottish Parliament to end its independence and join Great Britain in 1707, the researchers propose.

D’Arrigo and her colleagues “put forth a credible argument,” says Francis Ludlow, a climate historian at Trinity College Dublin. The most speculative part of their hypothesis, he suggests, is a linkage between eruptions that happened in the mid-1690s and political events that occurred more than 1 decade later. Nevertheless, he adds, the topic deserves further study.