A new method of analyzing ice cores has allowed scientists to more precisely determine when volcanic eruptions occurred on Earth over the last 2,500 years.
A multidisciplinary team of scientists melted down giant ice cores drilled from sites in Greenland and Antarctica and painstakingly analyzed sulfate deposits in the ice as they went. They were able to assign more accurate dates to 238 eruptions, including two particularly dramatic ones in the 6th century that triggered lower temperatures and possibly contributed to plagues, famines and other woes at the dawn of the Middle Ages.
The work, published this week in the journal Nature, reinforces scientists' understanding of how volcanoes influence variations in climate and confirms that they have a significant cooling effect, some experts said.
The study also helps resolve a long-standing discrepancy about the timing of eruptions as measured by ice cores (which record them via sulfate deposits) and tree rings (which bear signatures of the subsequent cooling), they said.
"Before this work, the tree rings and the ice core records diverged. In the new dating, they line right up," said hydrologist and study coauthor Joe McConnell of the Desert Research Institute in Reno. "We can look at the tree and say, 'There's the cooling that's associated with this volcanic event.'"
What set the new study apart from ice core analyses performed in the past, McConnell said, was the team's ability to make higher-resolution measurements of the sulfates deposited by eruptions and the decision to analyze a larger-than-usual number of cores. The team also calibrated its data by studying signatures of a "cosmic ray event" in space, dated to around the year 775, in both the ice cores and the tree rings.
Geologists had been waiting for this degree of precision in timing eruptions for two decades, said volcanologists and climate scientists David Pieri and Florian Schwandner from NASA's Jet Propulsion Laboratories in La Cañada-Flintridge, who were not involved in the new study.
"This is a really tough thing to do," Pieri said. "They bit off a big chunk."
The team drew connections between the ice core evidence and historical texts describing diminished sunlight over long periods of time. The analysis revealed that there had been a massive eruption in North America in 535 or 536, followed by another in the tropics in 539 or 540 -- corresponding to temperature dips in the tree ring records starting in 536.
Of that year, the study noted, the historian Procopius had written, "For the sun gave forth its light without brightness, like the moon, during this whole year, and it seemed exceedingly like the sun in eclipse."
The authors wrote that the cooling, which extended for more than a decade, "provides a notable environmental context to widespread famine and the great Justinian Plague of 541-543 CE that was responsible for decimating populations in the Mediterranean and potentially China."
"A lot of quite dramatic accounts in ancient records of weakened or discolored sunlight from ancient Babylon to China now match the dates of big eruptions," study coauthor Francis Ludlow, a postdoctoral fellow at the Yale Climate & Energy Institute, said in a statement.
McConnell said that by getting the timing of the volcanic events right, the new research could help scientists predict future climate conditions with greater certainty.
Some outside experts, including the JPL scientists, agreed. But climate researchers Michael Mann of Penn State University and Scott Rutherford of Roger Williams University expressed concerns about the tree ring data the authors used. Noting that the measurements were collected from just five sites, they argued that there was not sufficient spatial coverage to infer broad, hemisphere-wide cooling.
McConnell said the team used these particular data because they provided a continuous look at the 2,000-year period being studied.