by Mary Caperton Morton Tuesday, May 8, 2012
In the winter of 1780, New York Harbor froze, allowing people to walk from Manhattan to Staten Island for the first time in recorded history. The deep freeze didn’t just occur in New York: For close to 500 years, beginning around the end of the Middle Ages and lasting into the early 19th century, unusually cold conditions blanketed much of the Northern Hemisphere.
Climate scientists now call this period of cooling the Little Ice Age. When exactly this period began, what triggered it and how it was sustained for so long are matters of much debate. But a new study, published in Geophysical Research Letters, may answer all three puzzles.
“Estimates regarding the onset of the Little Ice Age range from the 13th century to the 16th century,” says Gifford Miller, a climate scientist at the University of Colorado at Boulder and lead author of the new study. “More exact dates have been totally ambiguous and there is little consensus.”
To narrow the date of onset, Miller and colleagues radiocarbon dated dead vegetation emerging from ice caps on Baffin Island in the Canadian Arctic to determine when the plants died due to the initial encroachment of snow and ice. Most of the dates clustered at two 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,” Miller says. During both cold snaps, plants at lower elevations froze at approximately the same time as those at higher elevations, indicating a swift onset. Testing of sediment cores from glacial lakes in Iceland also indicated a change in erosion rates in the late 13th century and again in the 15th century.
Armed with more exact dates, Miller and colleagues pinpointed a likely culprit: a period of active volcanism, starting with an eruption in 1275 and continuing on and off through the early 1800s, with at least four major volcanic eruptions occurring during this time period.
Volcanism is often implicated in periods of abrupt cooling. After the 1991 eruption of Mount Pinatubo in the Philippines, for example, global temperatures dropped by half a degree Celsius due to airborne particulate matter blocking solar radiation. But the effects don’t usually last more than a few years, says David Schneider, a climate scientist at the National Center for Atmospheric Research in Boulder, Colo., who was not involved in the new study.
“Volcanism explains the abruptness but it can’t account for the longevity” of the Little Ice Age, Schneider says. “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.”
To account for the longevity issue, Miller and colleagues used computer modeling to determine how repeated short-lived episodes of volcanism might trigger a cooling period lasting several centuries. They found that the persistence of cold summers following the eruptions could be explained by a sea ice-ocean feedback originating in the North Atlantic Ocean.
Sustained cooling from repeated eruptions would have caused Arctic sea ice to expand southward until it eventually reached warmer waters and melted. Sea ice contains almost no salt, and when it melts it creates a less-dense freshwater cap over the salty seawater. This 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, Miller says.
Such a scenario is plausible, Schneider says. “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, there are still questions, he says. 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, Schneider says. 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,” he says. Ice cores and other geologic records can help: Dating of ash in ice core records suggests an eruption in 1450 occurred on what is now the island of Vanuatu. But records of an eruption in 1275 and throughout the rest of the Little Ice Age are ambiguous.
Miller’s models showed that the Little Ice Age reached a maximum at the beginning of the 19th century and then began tapering off, Miller says, due to a combination of “a bit more sun, a bit less volcanism and some natural variability that broke the cycle.”
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