19th century chronicles offer clues to mystery volcano

Reports of hazy, colorful skies may offer the first known historical evidence of an enigmatic volcanic eruption that occurred more than 200 years ago. The newly uncovered records help researchers narrow down the time and place of the explosion that kicked off the coldest decade of the last 500 years, the 1810s.

Writings from a Colombian astronomer and a Peruvian physician describe a silvery sun, brilliant twilights and dimmed stars starting in December 1808. These are telltale meteorological aftermaths of a mighty eruption that spewed climate-altering particles into the atmosphere.

Because of sulfur deposits in polar ice cores, which act as a physical record of volcanic activity, scientists had suspected that such an eruption occurred in 1808 or 1809. But they knew little else. “This eruption had been speculated for a long time, and until now there were no reports of any kind,” says environmental chemist Jihong Cole-Dai of South Dakota State University in Brookings.

The newfound reports peg the geological fireworks to somewhere in the tropics in late November or early December of 1808. The study, by researchers at the University of Bristol in England, appears September 16 in Climate of the Past.

Volcanoes belch sulfurous particles high into the atmosphere, which can scatter sunlight and cool the Earth, says climatologist Alan Robock of Rutgers University in New Brunswick, N.J. Particles from large eruptions also cause visible hazes over the sky as they fan out across the planet, subduing starlight and distorting the color of the sun and sky.

The 1815 eruption of Mount Tambora in Indonesia, one of the largest of the last millennium, helped trigger a “year without a summer,” in which frosts killed crops and sparked famine across Europe and North America. But scientists have long thought that Tambora couldn’t have been the only eruption around the decade. Cooling had begun well before the blast.

Working with earth scientists, Bristol historian Caroline Williams searched an archive of Spanish colonial documents housed in Seville, Spain, for reference to any eruption. “I searched for every possible term I could think of,” she says. After about a year and a half, she came across an observation by Francisco José de Caldas, the director of an astronomical observatory in Bogotá, Colombia. The record, published in a scientific periodical in February 1809, described sunsets with red, green and blue hues during the previous December.

Williams sent the passage on to her collaborators. “I was on the edge of my seat,” says coauthor Erica Hendy, a biogeochemist. The description perfectly matched the aftereffects of eruptions. Then Williams found a second, similar observation in a footnote of an 1815 book on the climate of Lima, Peru. Written by physician José Hipólito Unanue, the note described prolonged twilights in mid-December 1808.

Based on the dates of the observations and knowledge of how quickly particles disperse in the atmosphere, the researchers estimate that the volcano erupted in late November or early December of 1808. Using the locations of the observations, with Bogotá above the equator and Lima below, the researchers suspect a tropical eruption.

The exact location of the volcano and why so few records can be found of its eruption remain a mystery, says Williams, who will continue to look for clues in the historical records. But, Hendy says, the new data will help tweak computer simulations of past climate to understand how this eruption spurred the cold decade that followed. “There’s some fun modeling to do now,” she says.