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| 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.
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| 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.
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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