For example, sulfur dioxide gas ejected during a volcanic eruption can react with water vapor in the air to form acid rain, which then leaves chemical fingerprints in polar ice.
Furthermore, bristlecone pine trees high in the Sierra Nevada mountains experienced stunted growth and frost damage in 1761, Pang said.
The researcher also looked through old Chinese weather chronicles from the early 1760s. Those records revealed that large parts of China experienced an unusually bitter winter and heavy snowfall in 1761 and 1762.
Rivers and wells across central China froze, ships could not sail, and innumerable trees, birds, and livestock died due to the cold, the chronicles state.
Finding a Culprit
A good candidate for the cause of the 1761 events is the Makian volcano on the Indonesian island of Halmahera, Pang thinks. Records show that this volcano experienced a series of eruptions beginning in September of 1760 and lasting until spring of the following year.
Makian's equatorial position could explain why evidence of its eruption was found at both poles.
But it's also possible the culprit volcano went unrecorded, Pang added.
Richard Keen is a climatologist at the University of Colorado in Boulder who was not involved in the study.
"[Pang] is absolutely correct in saying that volcanoes can darken a lunar eclipse," Keen said. But for the 1761 event, he noted, historical accounts about the dimness of the moon varied by geographical location.
The differing accounts could be due to the patchy distribution of dust and sulfur particles that occurs shortly after a volcanic eruption, said Fred Espenak, an eclipse expert at NASA's Goddard Space Flight Center in Maryland.
"But if this dust is up there long enough, it tends to uniformly distribute itself over weeks or months," said Espenak, who also did not participate in the research.
Espenak witnessed a modern dimming of the moon during a total lunar eclipse in 1992. That event was also probably due to a volcano, he said.
"It was here in Maryland, but the volcano was Mount Pinatubo in the Philippines the year before."
Michael Baillie, a paleoecologist at Queen's University Belfast in the U.K., said Pang's claim is "very interesting and indeed plausible," but he questioned the scientific usefulness of using lunar eclipses to pinpoint historic volcanic eruptions.
Pang is "almost intimating that we should be able to look at very black eclipses and assess that volcanoes have gone off," Baillie said.
"But it's always going to be a patchy thing. If someone didn't see an eclipse, is that because it was obscured [by volcanic smog] or because it was cloudy that night?"
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