Image caption Plants trapped under Iceland's icecaps store a record of ancient temperatures

The Little Ice Age was caused by the cooling effect of massive volcanic eruptions, and sustained by changes in Arctic ice cover, scientists conclude.

An international research team studied ancient plants from Iceland and Canada, and sediments carried by glaciers.

They say a series of eruptions just before 1300 lowered Arctic temperatures enough for ice sheets to expand.

Writing in Geophysical Research Letters, they say this would have kept the Earth cool for centuries.

The exact definition of the Little Ice Age is disputed. While many studies suggest temperatures fell globally in the 1500s, others suggest the Arctic and sub-Arctic began cooling several centuries previously.

The global dip in temperatures was less than 1C, but parts of Europe cooled more, particularly in winter, with the River Thames in London iced thickly enough to be traversable on foot.

What caused it has been uncertain. The new study, led by Gifford Miller at the University of Colorado at Boulder, US, links back to a series of four explosive volcanic eruptions between about 1250 and 1300 in the tropics, which would have blasted huge clouds of sulphate particles into the upper atmosphere.

These tiny aerosol particles are known to cool the globe by reflecting solar energy back into space.

Image caption The Little Ice Age saw an increase in cold winters in parts of Europe, but a small global change

"This is the first time that anyone has clearly identified the specific onset of the cold times marking the start of the Little Ice Age," said Dr Miller.

"We have also provided an understandable climate feedback system that explains how this cold period could be sustained for a long period of time."

The scientists studied several sites in north-eastern Canada and in Iceland where small icecaps have expanded and contracted over the centuries.

When the ice spreads, plants underneath are killed and "entombed" in the ice. Carbon-dating can determine how long ago this happened.

So the plants provide a record of the icecaps' sizes at various times - and therefore, indirectly, of the local temperature.

An additional site at Hvitarvatn in Iceland yielded records of how much sediment was carried by a glacier in different decades, indicating changes in its thickness.

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Putting these records together showed that cooling began fairly abruptly at some point between 1250 and 1300. Temperatures fell another notch between 1430 and 1455.

The first of these periods saw four large volcanic eruptions beginning in 1256, probably from the tropics sources, although the exact locations have not been determined.

The later period incorporated the major Kuwae eruption in Vanuatu.

Aerosols from volcanic eruptions usually cool the climate for just a few years.

When the researchers plugged in the sequence of eruptions into a computer model of climate, they found that the short but intense burst of cooling was enough to initiate growth of summer ice sheets around the Arctic Ocean, as well as glaciers.

The extra ice in turn reflected more solar radiation back into space, and weakened the Atlantic ocean circulation commonly known as the Gulf Stream.

"It's easy to calculate how much colder you could get with volcanoes; but that has no permanence, the skies soon clear," Dr Miller told BBC News.

"And it was climate modelling that showed how sea ice exports into the North Atlantic set up this self-sustaining feedback process, and that's how a perturbation of decades can result in a climate shift of centuries."

Analysis of the later phase of the Little Ice Age also suggests that changes in the Sun's output, particularly in the ultraviolet part of the spectrum, would also have contributed cooling.

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