On a global scale, the Smithsonian Institution's Global Volcanism Program reports that baseline volcanic activity remains relatively constant over long periods. The appearance of a surge around 1,200 years ago (the 800s CE) down through the High Middle Ages is primarily caused by two factors:
- Ice Core Preservation: Volcanologists locate ancient eruptions by drilling into polar ice sheets to measure sulfate spikes. Because younger ice layers are less compressed and have suffered less melting or distortion than older layers, scientists can more easily identify smaller, distinct eruptions from 1,200 years ago than from 12,000 years ago.
- Increased Human Record-Keeping: Around 1,200 years ago, literate societies across Europe, Byzantium, the Middle East, and Tang Dynasty China were actively recording atmospheric anomalies. Chroniclers often noted "diminished sunlight" or "deeply red skies", which modern historians now match with ice core data to confirm historical eruptions.
Natural Climate Cycles and Volcanic Clusters
While the overall global rate of volcanism is steady, volcanoes do occasionally experience random statistical clustering.
Between the 6th and 13th centuries, Earth did undergo several massive, climate-altering eruptions. For example, the infamous
volcanic winter of 536 CE—often cited as one of the worst periods to be alive—was triggered by major eruptions in Iceland and the Americas. A few centuries later, during the Medieval period, another cluster of major tropical eruptions occurred, culminating in the massive 1257 eruption of Mount Samalas in Indonesia.
These heavy-hitting events injected massive amounts of sulfur dioxide into the stratosphere, blocking sunlight and noticeably cooling the global climate. Because these specific, large events heavily disrupted human history and left massive chemical signatures in polar ice, they stand out prominently in the geological record.
