Soot likely played a role
in 1800s glacier retreat
A NASA-led team of scientists has uncovered strong evidence that soot
from a rapidly industrializing Europe caused the abrupt retreat of mountain
glaciers in the European Alps that began in the 1860s, a period often
thought of as the end of the Little Ice Age.
The research last month in the Proceedings of the National Academy of
Sciences, may help resolve a longstanding scientific debate.
In the decades following the 1850s, Europe underwent an economic
and atmospheric transformation spurred by industrialization. The use of coal
to heat homes and power transportation and industry in Western Europe
began in earnest, spewing huge quantities of black carbon and other dark
particles into the atmosphere.
Black carbon is the strongest sunlight-absorbing atmospheric particle.
When these particles settle on the snow blanketing glaciers, they darken
the snow surface, speeding its melting and exposing the underlying glacier
ice to sunlight and warmer spring and summer air earlier in the year. This
diminishing of the snow cover earlier in each year causes the glacier ice to
melt faster and retreat.
The Little Ice Age, loosely defined as a cooler period between the 14th
and 19th centuries, was marked by an expansion of mountain glaciers
and a drop in temperatures in Europe of nearly 1.8 degrees Fahrenheit (1
degree Celsius). But glacier records show that between 1860 and 1930,
while temperatures continued to drop, large valley glaciers in the Alps
abruptly retreated by an average of nearly 0.6 mile (1 kilometer) to lengths
not seen in the previous few hundred years. Glaciologists and climatologists
have struggled to reconcile this apparent conflict between climate and
glacier records.
“Something was missing from the equation,” said Thomas Painter, a snow
and ice scientist at NASA’s Jet Propulsion Laboratory in Pasadena, Calif.,
who led the study. “Before now, most glaciologists believed the end of the
Little Ice Age came in the mid-1800s when these glaciers retreated, and
that the retreat was due to a natural climatic shift, distinct from the carbon
dioxide-induced warming that came later in the 20th century. This result
suggests that human influence on glaciers extends back to well before the
industrial temperature increases.”
To help the scientists understand what was driving the glacier retreat,
Painter and his colleagues turned to history. The researchers studied data
from ice cores drilled from high up on several European mountain glaciers
to determine how much black carbon was in the atmosphere and snow
when the Alps glaciers began to retreat. Using the levels of carbon particles
trapped in the ice core layers, and taking into consideration modern
observations of how pollutants are distributed in the Alps, they were able
to estimate how much black carbon was deposited on glacial surfaces at
lower elevations, where levels of black carbon tend to be highest.
The team then ran computer models of glacier behavior, starting with
recorded weather conditions and adding the impact of the lower-elevation
pollution. When this impact was included, the simulated glacier mass loss
and timing finally were consistent with the historic record of glacial retreat,
despite the cooling temperatures at that time.
“This study uncovers likely human