In the summer of A.D. 536, a mysterious cloud appeared over the Mediterranean basin. “The sun gave forth its light without brightness,” wrote the Byzantine historian Procopius, “and it seemed exceedingly like the sun in eclipse, for the beams it shed were not clear.” In the wake of the cloud's appearance, local climate cooled for more than a decade. Crops failed, and there was widespread famine. From 541 to 542, a pandemic known as the Plague of Justinian swept through the Eastern Roman Empire.
Scientists had long suspected that the cause of all this misery might be a volcanic eruption, probably from Ilopango in El Salvador, which filled Earth's atmosphere with ash. But now researchers say there were two eruptions—one in 535 or 536 in the northern hemisphere and another in 539 or 540 in the tropics—that kept temperatures in the north cool until 550.
The revelation comes from a new analysis that combines ice cores collected in Antarctica and Greenland with data from tree rings. It shows that the sixth-century tragedy is just one chapter in a long history of volcanic interference. According to the data, nearly all extreme summer cooling events in the northern hemisphere in the past 2,500 years can be traced to volcanoes.
When a volcano erupts, it spews sulfur particles called aerosols into the air, where they can persist for two to three years. These aerosols block out some of the sun’s incoming radiation, causing cooling. How much light gets blocked and how long the effect lasts depends on the location of the volcano and the magnitude of the eruption, as well as other variables in Earth's natural climate-control system.
Trees record the climate impacts of an eruption in the size of their rings—when a climate-related event occurs, the rings may appear wider or thinner than average, depending on whether the region is typically wet or dry and the normal length of the growing season. Meanwhile, the sulfur particles eventually fall to Earth and get incorporated into polar and glacial ice, providing a record of the eruptions.
Combining the two types of records, though, has proven difficult in the past. So Michael Sigl of the Desert Research Institute and his colleagues used more ice cores than any previous study. They also employed a method to enhance the resolution in the data obtained from the cores: melting the core from one end and continuously analyzing the meltwater. The team then used a sophisticated algorithm to match up their ice core data with existing tree ring datasets.
The researchers detected 238 eruptions from the past 2,500 years, they report today in Nature. About half were in the mid- to high-latitudes in the northern hemisphere, while 81 were in the tropics. (Because of the rotation of the Earth, material from tropical volcanoes ends up in both Greenland and Antarctica, while material from northern volcanoes tends to stay in the north.) The exact sources of most of the eruptions are as yet unknown, but the team was able to match their effects on climate to the tree ring records.
The analysis not only reinforces evidence that volcanoes can have long-lasting global effects, but it also fleshes out historical accounts, including what happened in the sixth-century Roman Empire. The first eruption, in late 535 or early 536, injected large amounts of sulfate and ash into the atmosphere. According to historical accounts, the atmosphere had dimmed by March 536, and it stayed that way for another 18 months.
Tree rings, and people of the time, recorded cold temperatures in North America, Asia and Europe, where summer temperatures dropped by 2.9 to 4.5 degrees Fahrenheit below the average of the previous 30 years. Then, in 539 or 540, another volcano erupted. It spewed 10 percent more aerosols into the atmosphere than the huge eruption of Tambora in Indonesia in 1815, which caused the infamous “year without a summer”. More misery ensued, including the famines and pandemics. The same eruptions may have even contributed to a decline in the Maya empire, the authors say.
“We were amazed at the close correspondence and the consistency of the climate response to volcanic sulfate forcing during the entire 2,500-year period,” says coauthor Joe McConnell of the Desert Research Institute. “This clearly shows the marked impact that volcanic eruptions have on our climate and, in some cases, on human health, economics and so history.”