Deep beneath the barren landscape of Antarctica lies secrets to the climatic history of the world. Frozen in the ice are chemical markers which, when studied by scientists, can help reveal a historical record of key climate events.

Jihong Cole-Dai is a professor in South Dakota State University’s College of Natural Sciences. He has made numerous trips to both the North Pole and South Pole to retrieve ice cores—cylinders of ice that are drilled from the depths of glaciers and ice sheets. When studied, ice cores can reveal evidence of important climatic events like volcanic eruptions, dust storms and wind patterns.

This evidence enables scientists, like Cole-Dai, to reconstruct the climatic history of the world.

“That history is a key to understanding the present and future climatic changes,” Cole-Dai said.

Recently, Cole-Dai and colleagues published a paper in Atmosphere that revealed the 13th century had an unprecedented five major volcanic eruptions.

“The number of very large volcanic eruptions in the 13th century (A.D. 1201–1300.) was higher than in any other century in the last several thousand years,” Cole-Dai said. “The high number makes this century highly important and susceptible to volcanic influence on climate variations in the last millennium.

By comparing ice cores from both Greenland and Antarctica, Cole-Dai and his research team were able to discern the volcanic events were all major eruptions. Because the chemical signals of the eruptions were found in both Greenland ice cores and Antarctic ice cores, the eruptions all likely occurred in the tropics.

“The bipolar signals of these volcanic events indicate that the erupting volcanoes were located in the low latitudes near the equator,” Cole-Dai explained.

Scientists know that major volcanic eruptions shoot large amounts of gases into the air. The sulfate aerosols from the gases alter the properties of the atmosphere. While invisible to the human eye, the volcanic aerosols create a veil in the atmosphere blocking some of the incoming solar radiation.

This upsets the delicate balance between incoming solar energy and outgoing heat, and works to effectively cool the Earth for a few years.

“Volcanic eruptions are one of the main causes of natural climate variations,” Cole-Dai said. “The volcano-climate connection is an important aspect of the global climate system.”

For example, the 1991 eruption of Mount Pinatubo in the Philippines had a two-year cooling effect on the world’s climate. The Earth saw a 10% reduction in sunlight and an average global temperature decrease of 0.7°F. Scientists also believe the eruption played a role in the 1993 “Storm of the Century.”

Large volcanic eruptions, as described above, often have a two-, sometimes three-year effect on the Earth’s climate. But what happens to the climate with repeated major eruptions in a relatively short amount of time? Cole-Dai believes feedback mechanisms in the climate system have a compounding effect that can change the baseline level of the climate—with long-term consequences.

The five eruptions occurring over a span of approximately 55 years in the 13th century may have helped trigger what is known as the “Little Ice Age,” a cold period that lasted from around A.D. 1300 to 1850.

“The cumulative and positive feedback impact on climate may be more severe when eruptions occur on an interval of a decade or two,” Cole-Dai said. “Such frequency and pacing of large volcanic eruptions are not seen in other centuries in the last millennium.”

Scientists have long suspected that volcanic activity may have played some role in the Little Ice Age, but Cole-Dai’s study, which provides unambiguous evidence that the century leading up to the Little Ice Age had five large volcanic eruptions, supports the hypothesis that the Little Ice Age was caused by these volcanic eruptions.

Of the five major eruptions, only one has been positively identified: the Mount Samalas volcano eruption near present-day Lombok Island, Indonesia, in A.D. 1257. Pioneering work by a group of French scientists in 2013 was able to make the identification by studying ice cores, volcanic ash, stratigraphic data, radiocarbon age of materials near the volcano, and information in medieval chronicles.

The Mount Samalas eruption is believed to be potentially the largest and most explosive volcano in the last 7,000 years. It’s impact on the world’s climate was immense. Medieval chronicles from the Northern Hemisphere noted a medieval “year without summer” in A.D. 1258. The chronicles detailed a cold summer with incessant rains, flooding and poor harvests. Mount Samalas, a volcano that erupted on the other side of the globe, was confirmed as the source of the volcanic veil observed at that time.

The source of the other four eruptions is unknown. Cole-Dai notes it will be valuable to determine the locations and identities of those volcanos in future research.