In 1783, Benjamin Franklin wrote a paper blaming an extremely cool summer on volcanic dust from the eruption of Iceland’s Laki volcano. Furthermore, Franklin blamed the huge amounts of erupted sulfur dioxide for the death of much of Iceland’s livestock and for the famine that wiped out a quarter of the island’s human population.
What the statesman was referring to is known as a volcanic winter. After a large and particularly explosive type of volcanic eruption, volcanic ash and droplets of sulfuric acid obscure the sun and raise Earth’s ability to reflect solar radiation, resulting in a drop in global temperatures for a period of time.
The 1991 explosion of the stratovolcano Mount Pinatubo in the Philippines cooled global temperatures for 2-3 years by half a degree Celsius.
Though there’s considerable debate among climate scientists regarding the Little Ice Age, many are coming to accept the explanation that volcanic eruptions caused the Little Ice Age. Disagreements continue, though, as to when the period began and what prolonged it.
Researchers from the University of Colorado may have a good start on unraveling the riddles. Gifford Miller and Kurt Refsnider subjected dead vegetation they’d collected in the Canadian Arctic to radiocarbon dating. Most of the dates at which the vegetation died from the encroachment of snow and ice clustered at two periods of time: 1275 and 1450 CE.
Miller and his colleagues pointed to a likely cause of the sudden cooling: a period of at least four major volcanic eruptions starting in 1275 and continuing through the 1800s.
David Schneider, a climate scientist at the National Center for Atmospheric Research in Boulder, CO, expressed a remaining unanswered question in the minds of researchers: “Volcanism explains the abruptness (of the Little Ice Age), but it can’t account for the longevity (of the age). This has always been the problem with the volcanic explanation. . . . Volcanoes can make it cold, but they can’t keep it cold.”
Miller and his team used computer modeling to determine how repeated, short-lived episodes of volcanism could trigger centuries-long cold periods. Their modeling suggested that the persistent cold summers after the eruptions could be explained by a sea ice-ocean feedback originating in the North Atlantic Ocean.
Prolonged cooling from repeated eruptions would have caused Arctic sea ice to expand to the south until it reached warmer temperatures and melted. Because sea ice contains almost no salt, when it melted it created a less-dense freshwater cap over the seawater. The cap acted as an insulator, weakening heat transfer from the tropics to the North Atlantic and, in effect, creating a self-sustaining feedback system that could last long after the effects of the volcanic aerosols subsided, Miller says.
Schneider admits that such a scenario is plausible. “This study is really the first to explain how a short-lived event like a volcanic eruption can trigger cooling that lasts for centuries.”
However, he still has questions. “Before the modern era, there are only a few lines of evidence to figure out when and where volcanic eruptions occurred.” Dating of ash in ice core records suggests an eruption on the island of Vanuatu in 1450. But records of a 1275 eruption are ambiguous.
from Wikipedia, “Volcanic Winter” EARTH, The Science behind the Headlines, May 8, 2012—study published in Geophysical Research Letters