May 10, 2012
Was Little Ice Age Caused By Increased Volcanism In The Middle Ages?
A large part of the Northern Hemisphere was in the midst of an unusual cold snap for nearly 500 years, from the Middle Ages through the early 19th century, in what scientists now call the “Little Ice Age.”
A new study, published in the journal Geophysical Research Letters, has probed the longstanding mystery of when this event actually began, what caused it and how it was sustained for such a long period.Gifford Miller, a climatologist at the University of Colorado at Boulder and lead author of the study, said there has been a vague consensus by experts on when this period of cooling actually began, with estimates ranging from the 13th century to the 16th century.
EARTH Magazine reports that, to narrow the date of onset, Miller and colleagues used radiocarbon dating on dead vegetation emerging from the ice caps on Baffin Island in the Canadian Arctic to get a clearer understanding on when the cooling may have begun. Their dating methods revealed a number of dates that clustered around two distinct periods of time: 1275 and 1450.
“Everybody tends to think of this as a gradual cooling, so we were quite surprised when we got the dates back,” said Miller, adding that during both cooling events, plants at lower elevations froze at nearly the same time as those at higher elevations, indicating a fairly rapid onset.
The team also tested sediment cores from glacial lakes in Iceland and found changes in the erosion rates in the 13th and the 15th centuries, matching the time periods for what they observed in the Canadian Arctic.
With the newest evidence, Miller and colleagues were able to pinpoint the culprit of the Little Ice Age: a period of active volcanism, starting with an eruption in 1275 and continuing periodically through the 19th century. The team discovered at least four major eruptions occurring during this time period.
Volcanism has, in more recent times, also played a role in global cooling.
In 1991, after the eruption of Mt. Pinatubo in the Philippines, global temperatures dropped about 1 degree Fahrenheit due to volcanic airborne matter that blocked solar radiation. However, the effects of most volcanoes do not last more than a few years, said David Schneider, a climatologist at the National Center for Atmospheric Research (NCAR) in Colorado.
“Volcanism explains the abruptness but it can´t account for the longevity” of the Little Ice Age, Schneider, who was not involved in the study, told EARTH Magazine's Mary Caperton Morton. “This has always been the problem with the volcanic explanation of the Little Ice Age. Volcanoes can make it cold but they can´t keep it cold.”
But a long period of volcanism could keep it cold, suggests Miller. He and colleagues used computer modeling to determine how repeated short-lived volcanic eruptions might trigger a cooling event lasting several hundred years. They found that the persistence of cold summers following eruptions could be explained by a sea ice-ocean feedback originating in the North Atlantic Ocean.
Sustained cooling from repeated eruptions would have caused sea ice to expand southward until it eventually reached warmer waters and melted.
Such was the case in the winter of 1780, when New York Harbor froze for the first time in recorded history, allowing people to walk from Manhattan to Staten Island.
Miller noted that since the salt content of sea ice is practically nil, it creates a less-dense freshwater cap over salty seawater once it melts. This freshwater cap inhibits mixing and weakens heat transport from the tropics to the North Atlantic, creating a self-sustaining feedback system that could have lasted long after the effects of the volcanic aerosols subsided, he added.
Schneider concurred that Miller´s scenario was plausible.
“However, there are still questions,” he said. “As of now, the weakest link in the study is the computer modeling, which depends on mere estimates of the size of the volcanic eruptions. There´s no accurate account of volcanic eruptions during this period.”
“Before the modern era, there are only a few lines of evidence to figure out when and where volcanic eruptions occurred,” noted Schneider. Dating of ash in ice core records suggests a 1450 eruption occurred on the island of Vanuatu, but other records for a 1275 eruption and others throughout the Little Ice Age remain vague at best.
Miller´s dating models indicate that the Little Ice Age reached a maximum in the early 1800s and then began tapering off. As volcanic activity subsided, more sun was able to penetrate through the atmosphere, helping temperatures rebound, and eventually breaking the cooling cycle, the team concluded.