The 'Little Ice Age' was a prolonged cooling of the climate from roughly 1650 till roughly 1850 (estimates vary). It is well documented in Europe, but does not appear to be global in extent; evidence from glaciers in North and South America and New Zealand suggests that these areas were effected to some extent, but the literate cultures of East Asia have no record of such a chilling. It was clearly not a true Ice Age, which involve glaciers covering large parts of the temperate continents for tens of thousands of years, but was too long for most short term climate effects; for example cooling caused by major volcanic eruptions seldom lasts more than a decade.
Skaters on Venice Lagoon in 1708. Gabriele Bella.
In a paper published in the journal Geophysical Research Letters on 31 January 2012, a team lead by Gifford Miller of the Institute of Arctic and Alpine Research and the Department of Geological Sciences at the University of Colorado at Boulder, in which they describe a new study of the onset of the Little Ice Age.
They first examined frozen plants recently exposed by retreating glaciers on Baffin Island in the Canadian Arctic. These were carbon dated, to find out the time at which they had died. The largest cluster of plant deaths occurred between 1275 and 1300, considerably before the traditional date for the onset of the Little Ice Age. There was a second cluster of deaths at around 1450, later, but closer to the onset of the European cold spell.
The team then compared this to ice cores from the Langjökull Ice Cap in central Iceland. Ice cores are made up of annual layers of ice, similar to the rings in trees. In cold years the layers are thicker and in warmer years the layers are thinner, giving a good proxy for temperature. The Langjökull Ice Cap cores produced a cluster of thick years in the late thirteenth century, and another in the mid-fourteenth century, matching the Baffin Island plant samples.
Landsat image of the Langjökull Ice Cap. From the United States Geological Survey.
Based upon these findings Miller et al. came up with a scenario by which a major volcanic eruption (from an unknown volcano) in the late thirteenth century caused a drop in temperature over the North Atlantic for long enough for the Arctic Sea Ice to expand. Before this ice had time to melt another major eruption occurred, extending the cold spell and preventing the ice from melting. If this happened for long enough it would have altered the flow of currents in the North Atlantic, causing the cold spell experienced in Europe.
Volcanoes have a complex relationship with climate. They spew out CO₂ and H₂O, which are greenhouse gasses, and therefore can have a warming effect on the climate, but they also produce large volumes of sulphurous aerosols, which act as coolants in the atmosphere. As a rule of thumb major eruptions have a localized cooling effect on the atmosphere, but prolonged eruptive episodes can cause global warming.
The scenario of a freeze in the North Atlantic predating the European Little Ice Age is an interesting one, and ties in with some other evidence for climate change in the North Atlantic, for example the abandonment of the Norse Settlement in Western Greenland in the mid-fourteenth century. However without direct evidence of volcanic eruptions, preferably from named volcanoes, the idea does remain highly speculative. The logical next step in this research would be, therefore, to seek out volcanoes that underwent major eruptions at the appropriate times.
See also Did the first land plants cause an Ordovician glaciation? Snowfall in Namibia and Volcanoes on Sciency Thoughts YouTube.