The 1808 mystery eruption refers to one or more volcanic eruptions that have not been identified. These eruptions caused a large increase in sulfur particles in the stratosphere, which led to a period of global cooling similar to the Year Without a Summer in 1816.

Background

Until the 1990s, scientists who study climate believed that changes in weather during the early 1810s were part of the Little Ice Age, a natural climate pattern. However, a 1991 study of ice cores from Antarctica and Greenland found a sudden increase in sulfate levels in early 1809, about half the level seen after the 1815 eruption of Mount Tambora. This created a challenge for volcanologists, as there were no recorded eruptions large enough to explain the sulfate spike during that time. Further research and data from bristlecone pine tree rings suggested the eruption likely occurred in 1808 instead of early 1809.

At first, scientists thought the sulfate increase and global cooling were caused by a single eruption classified as VEI-6. However, new research indicates that the rise in sulfate levels and cooling were probably caused by a series of eruptions, including some smaller ones.

Location and date

Studies of the chemical makeup of volcanic ash found in sulfate layers of ice cores show that several eruptions likely happened around 1809. These eruptions may have come from places like Antarctica, Alaska, and Indonesia. This suggests that the large increase in sulfate levels in 1809 might have been caused by several eruptions happening close together, rather than one very large volcanic event.

Analysis of sulfur isotope patterns shows a complicated change over time, pointing to multiple eruptions that created the 1809 sulfate layer and caused global cooling. This matches the presence of different types of volcanic ash during this time.

It was expected that eruptions of this size would have been noticed at the time, which made the mystery more confusing. Records from around the world were checked, but nothing clear was found until 2014. Then, PhD student Alvaro Guevara-Murua and Caroline Williams from the University of Bristol discovered a report by Colombian scientist Francisco José de Caldas that described events matching the eruption.

Caldas was the director of the Astronomical Observatory of Bogotá from 1805 to 1810. In 1809, he wrote about a "transparent cloud that blocked the sun's brightness" seen in Bogotá. He first noticed this cloud on December 11, 1808, and it was visible across Colombia. This cloud might have been a "dry fog," a type of sulfuric acid (H₂SO₄) aerosol. He also noted unusually cold weather with frosts.

In Peru, physician Hipólito Unanue made similar observations in Lima. These reports led scientists to think the eruption happened within seven days of December 4, 1808. Caldas' and Unanue's accounts suggest a layer of tiny particles in the atmosphere stretched at least 2,600 kilometers (1,600 miles) across both the northern and southern hemispheres. The most likely source for this would be a tropical volcano, probably in the southern hemisphere but not farther than 20 degrees south latitude.

The area between the southwestern Pacific Ocean, near Indonesia and Tonga, is in the tropics west of Colombia and Peru. This region includes places like Rabaul, which had eruptions with a Volcanic Explosivity Index (VEI) of 6, and the Hunga Tonga–Hunga Haʻapai area, which had a VEI 5–6 eruption in 2022. At the time of the 1809 event, there were no European settlements in this region, and most records of volcanic activity there date back only to the mid-19th century. Some oral histories from local people mention eruptions, but these cannot be dated with certainty.

Known significant eruptions in 1808

In 1808, large volcanic eruptions occurred in Urzelina, Azores, from May 1 to May 4, and at Taal Volcano, Philippines, in March. These events did not take place during the time when visual observations were recorded.

The Chilean Putana volcano also experienced a significant eruption around 1810, though the exact date is uncertain and may be up to 10 years earlier or later. However, it is located 22 degrees south of the equator, which places it slightly outside the preferred latitude range for the study.