I’m not a big guy. It doesn’t take a tremendous shove to send me crashing to the floor. But what does it take to knock the Earth’s climate off balance? In the case of the Little Ice Age, a recent 400-year cold snap, a new study suggests a few well-timed volcanic eruptions might have done the trick.

Major glacial periods are controlled by the Earth's orbit. Summer temperatures in the Northern Hemisphere have been on a slow decline for the last 8,000 years as the orbital precession cycle pushed summer closer to aphelion, the point in Earth’s orbit where it’s farthest from the Sun. That’s just the first few steps on the road that would eventually lead us down from the peak of our current interglacial and into the next glacial period. But there have been bumps along this road, the most familiar of them being the "Little Ice Age."

The effects of the Little Ice Age were especially clear around the North Atlantic, where many glaciers advanced for a time, leaving small moraine deposits as markers. It would be a mistake to characterize the Little Ice Age as a simple, synchronized global cooling, however. The extent and timing of cooling varied from place to place, but generally occurred between the 1400s and 1800s, when North America and Europe experienced notably colder winters.

Discussions of the Little Ice Age often focus on the Maunder Minimum—a period of low solar activity centered around 1650-1700 AD. While it coincides with the Little Ice Age, researchers have long puzzled over whether the drop in the Sun's output was large enough to cause a significant chill.

In order to dig a little deeper, researchers examined glacial records to identify a more precise start date for the Little Ice Age. They also looked to glaciers around the North Atlantic for a record of summer temperatures. (If snowfall over the glacier is reasonably constant, the primary control of a glacier’s size is the amount of melt that occurs during the summer months.)

First, they examined a small ice cap in northern Canada. Unlike alpine glaciers, there is little topography there to drive ice flow. Because this sort of glacier is frozen to the underlying ground, it does not erode or disturb the sediment beneath it. (Contrary to what you might expect, thick glaciers actually act as large blankets, trapping geothermal heat at the base where it melts some of the ice.)

Mosses and other plants grow right up to the edge of the ice in summer, marking its position like a parent tracks the height of a child on the kitchen door frame. By dating the organic matter in sediment around the glacier, the researchers were able to precisely constrain the time when it started growing at the onset of the Little Ice Age. They found a spike in "kill-dates," recording the growth of the glacier, between 1275 and 1300 AD, and another spike between 1430 and 1455 AD.

They compared this to another record from an ice cap in Iceland named Langjökull. Instead of examining moss casualties, that record is based on the sediment at the bottom of Hvítárvatn—a lake filled by meltwater from Langjökull. Seasonal variation in discharge from the glacier results in visible annual layers (known as varves) in these sediments. Long-term changes in varve thickness depend on how much erosion is driven by the glacier, making it a proxy for the size of the glacier.

Confirming the age of those annual layers is made even easier in Iceland, where a number of ash layers from known eruptions provide absolute anchor dates. Just like the Canadian ice cap, the varve record showed that Langjökull began its expansion in the late 1200s, with another sharp rise in the 1400s. (Incidentally, these records both show that present temperatures around the North Atlantic exceed those experienced during the "Medieval Warm Period" that preceded the Little Ice Age.)

With this information in hand, the researchers could begin to evaluate potential causes of the cooling. It clearly started long before the Maunder Minimum, but the onset correlates nicely with large volcanic eruptions. In 1453, the eruption of Kuwae in the South Pacific ejected about six times as much gas, ash, and rock as the 1991 Pinatubo eruption. An unknown eruption around 1259, possibly in Central America, was even larger. It’s difficult to tell, but it may have been the largest eruption of the last thousand years. As if that weren’t enough, several (much smaller) eruptions followed over the next several decades.

Could these eruptions have pushed the climate into a lasting cool period? To answer that question, the group simulated the eruptions using a climate model. They found that the eruptions triggered several feedbacks that did indeed create a persistent cool period like the Little Ice Age. The initial cooling caused an expansion of Arctic sea ice, which increased the export of sea ice into the North Atlantic. (We know from proxy records of sea ice around Iceland that this occurred, which helps validate the model's behavior.) Sea ice started to show up in the 1200s, spiked again in the mid 1400s, and remained through to the end of the Little Ice Age.

As that sea ice melted, it added lots of freshwater to the North Atlantic, weakening the pump that drives Atlantic ocean circulation. The result? Decreased northward heat transport that lowered temperatures around the North Atlantic and kept sea ice from returning to its pre-eruption state.

The researchers propose that, while the Maunder Minimum may have contributed to the Little Ice Age, it did not cause it. Several large volcanic eruptions nudged the climate into a cooler state, where feedbacks kept it for several centuries.

Geophysical Research Letters, 2012. DOI: 10.1029/2011GL050168 (About DOIs).