Volcanoes have played a larger role in changing the climate than previously thought, and their climatic effects may have contributed to past societal and economic change.
Studying the growth rings of more than 9,000 living and dead trees, a study published in the journal Dendrochronologia reconstructed the summer temperatures in North America and Eurasia for the last 2,000 years.
The researchers then compared the occurrence of colder and warmer periods with known volcanic eruptions. Previous observations suggested that the effect of volcanoes on the global climate is punctuated and short, lasting just a few years. The new results show a long-lasting effect of volcanic eruptions on global temperatures, at least before humanity started to change the climate.
Large volcanic eruptions can lower global average temperatures. The main factor is the amount of sulfur emitted during the eruption that reaches the higher layers of Earth’s atmosphere, where sulfur-compounds form a mist of minute particles and droplets that block some sunlight from reaching the surface. The reduced incoming radiation causes temperatures to drop, resulting in shorter growing seasons and reduced harvests. Poor harvests and cold weather can lead to famine, disease, conflict and migration. The study shows how periods characterized by more active volcanism often coincided with times of conflict and economic decline in past civilizations.
Conversely, in periods when fewer volcanic eruptions occur, less sunlight is absorbed and temperatures rise. A period with relatively warmer summers than the long-term average and plentiful harvests helped the Roman Empire to sustain a large population and army, allowing its expansion. A relatively constant warmth during medieval times helped to feed the growing population, most European urban centers date back to the 12th and 14th century.
Ulf Büntgen from Cambridge’s Department of Geography and the study’s lead author notes that “While nothing about the future is certain, we would do well to learn how climate change has affected human civilization in the past.”