During the late Middle Ages, many of the glaciers in Arctic Canada and Iceland
experienced abrupt increases in their size, due to substantially cooler summer
months. This advance of glaciers has been termed the Little Ice Age (LIA), and
the expansion of the glaciers has only been undone in the last decade or two.
There are several hypotheses as to the cause of the glacial expansion during the
LIA, but until now, no definitive answers have emerged. However, the retreat of
these glaciers over the last few decades has allowed scientists to study the
plants that were killed by the advance, determine the dates of the expansion of
the glaciers, and connect the timing with global events to try to assign a cause
to the LIA.
First, though, it helps to explain a little astronomy. As the Earth orbits the
sun, there are 3 separate rotations occurring. The fastest is the cycle of day
and night, caused by the Earth moving about its own axis of rotation. The next
longest is the Earth’s rotation around the sun, which causes the seasons.
Imagine a plane that cuts through the center of the Earth and the Sun, and has
on it the ellipse that the Earth follows around the sun. This is called the
plane of the ecliptic. The axis of rotation of the Earth (i.e., the one it
rotates around to cause day and night) is tilted with respect to this plane -
that is to say, it is not perpendicular to the plane. This is what causes the
seasons, as described in the video below.
In addition to these rotations that occur on human time scales, there is a much
longer process called the precession of the
equinoxes,
which occurs on nearly a 26,000 year cycle. As the Earth precesses, the
insolation (the amount of
sunlight received on the surface of the Earth) decreases, and therefore, the
summer temperatures in the Northern Hemisphere have been slowly declining over
the last 8000 years. However, this decrease in insolation is insufficient to
explain several cold snaps that have occurred on the time scale of centuries
instead of millennia.
The present work was done by a group of researchers with representatives from
five different institutions from Colorado, California, Iceland and Scotland.
They traveled to Arctic Canada and Iceland to collect moss samples for use in
radiocarbon dating. These moss samples had all been recently exposed by
retreating glaciers, and by comparing the carbon content of dead samples with
living samples, the authors were able to accurately determine the date of the
advance of the glaciers. This is because organisms that are alive have a
constant amount of one particular isotope of carbon (14C; carbon usually has a
mass of 12 atomic mass units (amu), the superscript 14 indicates that the mass
of this carbon is 14 amu). Once an organism dies, no new 14C is taken in, and
the 14C begins to decay naturally with a known rate. By comparing the amount
of 14C in living organisms and organisms that died due to glaciers, the
authors could place a date on when their samples died. The results of the
radiocarbon dating show that there were two abrupt periods of glacial expansion,
between 1275 and 1300 AD, and again around 1450 AD. Very little vegetation had
dates of death after 1450 AD, indicating that the wide expanse of these glaciers
stuck around from that time until the early 20th century, according to the authors.
In addition to examining the dates of expansion of the glaciers, the authors
conducted computer simulations to determine the cause of the Little Ice Age. It
is relatively well
known
(but don’t read the comments on the linked article, ugh) that volcanic eruptions
can cause short term effects on global temperatures. Indeed, there were four
explosive volcanic events around the time of the first glacial expansion
(1275-1300 AD) and an additional explosion in 1452 AD, corresponding to the
second expansion. However, what was unknown was how the relatively short term
effects of volcanic explosions can cause many centuries of depressed Arctic
temperatures, allowing the glaciers to remain until the 20th century. The
computer simulations showed that there was a positive feedback mechanism between
the glacier expansion caused by the volcanic explosions and summertime air
temperatures, due to the reflective effect of the ice. Glacial and sea ice have
a very strong cooling effect on the air temperature because they tend to reflect
sunlight back into space. Because the air is cooler above glaciers and sea ice,
more sea ice is formed and more sunlight is reflected in a loop that the authors
assert caused the maintenance of the glacial sheets until the modern era. The
authors conclude that the combination of volcanic explosions and
lower-than-modern solar insolation triggered the glacial expansion and a
positive feedback loop between the glaciers and the air temperatures maintained
the glacial extent until the 20th century.
DOI: 10.1029/2011GL050168