When big volcanoes like Mount St. Helens or Mount Pinatubo really blow their tops, the skies darken and temperatures drop. But since such massive eruptions – luckily for us – are fairly uncommon, scientists have few examples to help them piece together the details of how much it cools, and how far and long the chill extends.
A University of Washington glaciologist is the second author of a study published July 8 in Nature that shows that 15 of the 16 coldest summers recorded between 500 B.C. and A.D. 1,000 came after big volcanic eruptions, with the coldest summers occurring shortly after the largest volcanic events.
The study relies on more than 20 ice cores from Greenland and Antarctica that were analyzed for traces of volcanic sulfate at Nevada’s Desert Research Institute, which led the study. The cores provide a yearly record of atmospheric sulfate levels since ancient Roman times.
The researchers compared this volcanic history with land-based temperatures and human written records for nearly 300 eruptions detected in the ice cores.
“We used a new method for producing the timescale,” said co-author Mai Winstrup, a postdoctoral researcher in the UW’s Department of Earth and Space Sciences. “Previously this has been done by hand, but we used a statistical algorithm instead. Together with the state-of-the-art ice-core chemistry measurements, this resulted in a more accurate dating of the ice cores.”
Winstrup’s doctoral research in Denmark developed a statistical method based on principles from machine speech recognition to automate the detection of layers in the deeper section of other ice cores, past 60,000 years ago, where the layers are thin and harder to distinguish by eye.
Using this method in the new study showed that major eruptions led to cooling that lasted for as long as a decade.
“With the newly developed timescale, we could see that the previous ice core timescales were dated wrongly by 5 to 10 years throughout the first millennium,” Winstrup said. “This was causing problems when trying to compare the volcanic eruptions with their impact on the climate seen in the tree ring records.”
The researchers compared the volcanic chemical signature in the ice with temperature clues in the tree rings. Aligning those records precisely allowed them to identify the relationship between the amount of volcanic sulfate in the atmosphere and the temperature where the trees were growing.
Other co-authors on the paper translated and interpreted ancient and medieval documentary records from China, the Middle East and Europe that described unusual atmospheric observations as early as 254 B.C. These phenomena included diminished sunlight, discoloration of the solar disk, the presence of solar coronae and deeply red twilight skies.
Throughout human history, cooler climates from major eruptions have triggered crop failures and famines, contributing to pandemics and social unrest in agricultural communities.
The authors note that their findings may resolve a long-standing debate regarding the causes of one of the most severe climate crises in recent human history. The trouble began in 536, when a “mystery cloud” or dust veil observed in the Mediterranean appeared and persisted for 18 months. Authors linked this harbinger with a large eruption in the northern high latitudes.
A mere four years later, a second volcano erupted somewhere in the tropics and intensified the cooling effect. In the aftermath, unusually cold summers were observed throughout the Northern Hemisphere for nearly 15 years.
Crop failures and famines were common during this period — likely contributing to the outbreak of the Justinian plague that spread throughout the Eastern Roman Empire starting in the year 541. That plague ultimately killed millions of people across Eurasia. Researchers write that the link they established “more firmly implicates volcanic eruptions as catalysts in the major sixth-century pandemics, famines and socioeconomic disruptions in Eurasia and Mesoamerica.”
The ice cores used in the study included the West Antarctic core that UW glaciologist Eric Steig and colleagues helped to drill from 2006 to 2011. A different study using that core recently unraveled the timing of past global temperature seesaws in the Northern and Southern hemispheres, in a study that included Winstrup and several other UW co-authors.
Source: University of Washington