Nearly 40 years
of satellite data from Greenland shows that glaciers on the island have shrunk
so much that even if global warming were to stop today, the ice sheet would
continue shrinking.
The finding, published in the journal Nature
Communications Earth and Environment, means that Greenland’s
glaciers have passed a tipping point of sorts, where the snowfall that
replenishes the ice sheet each year cannot keep up with the ice that is flowing
into the ocean from glaciers.
“We’ve been looking at these remote sensing
observations to study how ice discharge and accumulation have varied,” said
Michalea King, lead author of the study and a researcher at The Ohio State
University’s Byrd Polar and Climate Research Center. “And what we’ve found is
that the ice that’s discharging into the ocean is far surpassing the snow
that’s accumulating on the surface of the ice sheet.”
King and other researchers analyzed monthly satellite
data from more than 200 large glaciers draining into the ocean around
Greenland. Their observations show how much ice breaks off into icebergs or
melts from the glaciers into the ocean. They also show the amount of snowfall
each year — the way these glaciers get replenished.
The researchers found that, throughout the 1980s and
90s, snow gained through accumulation and ice melted or calved from glaciers
were mostly in balance, keeping the ice sheet intact. Through those decades,
the researchers found, the ice sheets generally lost about 450 gigatons (about
450 billion tons) of ice each year from flowing outlet glaciers, which was
replaced with snowfall.
“We are measuring the pulse of the ice sheet — how
much ice glaciers drain at the edges of the ice sheet — which increases in the
summer. And what we see is that it was relatively steady until a big increase
in ice discharging to the ocean during a short five- to six-year period,” King
said.
The researchers’ analysis found that the baseline of
that pulse — the amount of ice being lost each year — started increasing
steadily around 2000, so that the glaciers were losing about 500 gigatons each
year. Snowfall did not increase at the same time, and over the last decade, the
rate of ice loss from glaciers has stayed about the same — meaning the ice
sheet has been losing ice more rapidly than it’s being replenished.
“Glaciers have been sensitive to seasonal melt for as
long as we’ve been able to observe it, with spikes in ice discharge in the
summer,” she said. “But starting in 2000, you start superimposing that seasonal
melt on a higher baseline — so you’re going to get even more losses.”
Before 2000, the ice sheet would have about the same
chance to gain or lose mass each year. In the current climate, the ice sheet
will gain mass in only one out of every 100 years.
King said that large glaciers across Greenland have
retreated about 3 kilometers on average since 1985 — “that’s a lot of
distance,” she said. The glaciers have shrunk back enough that many of them are
sitting in deeper water, meaning more ice is in contact with water. Warm ocean
water melts glacier ice, and also makes it difficult for the glaciers to grow
back to their previous positions.
That means that even if humans were somehow
miraculously able to stop climate change in its tracks, ice lost from glaciers
draining ice to the ocean would likely still exceed ice gained from snow
accumulation, and the ice sheet would continue to shrink for some time.
“Glacier retreat has knocked the dynamics of the whole
ice sheet into a constant state of loss,” said Ian Howat, a co-author on the
paper, professor of earth sciences and distinguished university scholar at Ohio
State. “Even if the climate were to stay the same or even get a little colder,
the ice sheet would still be losing mass.”
Shrinking glaciers in Greenland are a problem for the
entire planet. The ice that melts or breaks off from Greenland’s ice sheets
ends up in the Atlantic Ocean — and, eventually, all of the world’s oceans. Ice
from Greenland is a leading contributor to sea level rise — last year, enough
ice melted or broke off from the Greenland ice sheet to cause the oceans to
rise by 2.2 millimeters in just two months.
The new findings are bleak, but King said there are
silver linings.
“It’s always a positive thing to learn more about
glacier environments, because we can only improve our predictions for how
rapidly things will change in the future,” she said. “And that can only help us
with adaptation and mitigation strategies. The more we know, the better we can
prepare.”
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