|
A stash of 80-year-old photo plates in a Danish
basement has proved that Greenland's ice was melting even faster then
that it is now.
In the thirties, Greenland's ice was melting rapidly, then there was a
cooling period in the middle part of the twentieth century, and now it
is melting again, accelerating in the 2000s.
Images of ice shelves from the pre-satellite era are extremely rare, so
it's often difficult to assess the scale and speed of Arctic ice melting
today.
Researchers at the National Survey and Cadastre of Denmark had been
storing the glass plates since explorer Knud Rasmussen's expedition to
the southeast coast of Greenland in the early 1930s.
|
|
 |
|
In this week's online edition of Nature Geoscience, Ohio State
University researchers and colleagues in Denmark describe how they
analyzed ice loss in the region by comparing the images on the plates to
aerial photographs and satellite images taken from World War II to
today.
Taken together, the imagery shows that glaciers in the region were
melting even faster in the 1930s than they are today, said Jason Box,
associate professor of geography and researcher at the Byrd Polar
Research Center at Ohio State.
A brief cooling period starting in the mid-20th century allowed new ice
to form, and then the melting began to accelerate again in the 2000s.
‘Because of this study, we now have a detailed historical analogue for
more recent glacier loss,’ Box said. ‘And we've confirmed that glaciers
are very sensitive indicators of climate.’
|
|
Images recorded in 1933 (left) and 2010 from a
glacier northeast of Tasiilaq: During the 77 year period the glacier has
retreated 1.7km |
|
Pre-satellite observations of Greenland glaciers are rare - but some are
available.
Anders Anker Bjørk, doctoral fellow at the Natural History Museum of
Denmark and lead author of the study, is trying to compile all such
imagery. He found a clue in the archives of The Arctic Institute in
Copenhagen in 2011.
‘We found flight journals for some old planes, and in them was a
reference to National Survey and Cadastre of Denmark,’ Bjørk said.
As it happens, researchers at the National Survey had already contacted
Bjørk about a find of their own.
‘They were cleaning up in the basement and had found some old glass
plates with glaciers on them. The reason the plates were forgotten was
that they were recorded for mapping, and once the map was produced they
didn't have much value.’
Those plates turned out to be documentation of Rasmussen's 7th Thule
Expedition to Greenland. They contained aerial photographs of land, sea
and glaciers in the southeast region of the country, along with travel
photos of Rasmussen's team.
The researchers digitized all the old images and used software to look
for differences in the shape of the southeast Greenland coastline where
the ice meets the Atlantic Ocean.
|
|
A stash of 80-year-old photo plates in a Danish
basement has proved that Greenland's ice was melting even faster then
that it is now |
|
Then they calculated the distance the ice front moved in each time
period.
Over the 80 years, two events stand out: glacial retreats from 1933-1934
and 2000-2010. In the 1930s, fewer glaciers were melting than are today,
and most of those that were melting were land-terminating glaciers,
meaning that they did not contact the sea.
Those that were melting retreated an average of 20 meters per year - the
fastest retreating at 374 meters per year.
Fifty-five percent of the glaciers in the study had similar or higher
retreat rates during the 1930s than they do today.
Still, more glaciers in southeast Greenland are retreating today, and
the average ice loss is 50 meters per year.
That's because a few glaciers with very fast melting rates - including
one retreating at 887 meters per year - boost the overall average.
But to Box, the most interesting part of the study is what happened
between the two melting events.
From 1943-1972, southeast Greenland cooled - probably due to sulfur
pollution, which reflects sunlight away from the earth.
|
|
Knud Rasmussen, the Danish polar explorer whose
photo archive contained the shock news about Arctic ice melt |
|
Sulfur dioxide is a poisonous gas produced by volcanoes and industrial
processes. It has been tied to serious health problems and death, and is
also the main ingredient in acid rain. Its presence in the atmosphere
peaked just after the Clean Air Act was established in 1963. As it was
removed from the atmosphere, the earlier warming resumed.
The important point is not that deadly pollution caused the climate to
cool, but rather that the brief cooling allowed researchers to see how
Greenland ice responded to the changing climate.
The glaciers responded to the cooling more rapidly than researchers had
seen in earlier studies. Sixty percent of the glaciers advanced during
that time, while 12 percent were stationary. And now that the warming
has resumed, the glacial retreat is dominated by marine-terminating
outlet glaciers, the melting of which contributes to sea level rise.
‘From these images, we see that the mid-century cooling stabilized the
glaciers,’ Box said. ‘That suggests that if we want to stabilize today's
accelerating ice loss, we need to see a little cooling of our own.’
|
|
The Heinkel hydroplane after returning from a
surveying mission during Knud Rasmussen expedition to Greenland in the
1930s |
|
Southeast Greenland is a good place to study the effects of climate
change, he explained, because the region is closely tied to air and
water circulation patterns in the North Atlantic.
‘By far, more storms pass through this region - transporting heat into
the Arctic - than anywhere else in the Northern Hemisphere. Climate
change brings changes in snowfall and air temperature that compete for
influence on a glacier's net behavior,’ he said.
|