42 Scientific American, February 2019
PRECEDING PAGE: GETTY IMAGES (
hand, water, buildings
)
IN BRIEF
Big glaciers on
Greenland, such as
Jakobshavn, are
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If it does retreat,
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Richard B. Alley is a professor of geosciences
at Pennsylvania State University. He has spent
more than 40 years studying ice sheet s and
has advised the U.S. government on a variety
of climate issues.
Global warming is melting glaciers up in mountain-
ous areas and expanding ocean water, while shrinking
ice at both poles. Averaged over the planet’s oceans for
the past 25 years, sea level has risen just over a tenth of
an inch per year, or about a foot per century. Melting
the rest of the globe’s mountain glaciers would raise the
sea a little more than another foot. But the enormous
ice sheets on land in the Arctic and Antarctic hold more
than 200 feet of sea-level rise; a small change to them
can create big changes to our coasts. Ice cliffs many
miles long and thousands of feet high could steadily
break off and disappear, raising seas significantly.
Well-reasoned projections for additional sea-level
rise this century have remained modest—maybe two
feet for moderate warming and less than four feet even
with strong warming. Scientists have solid evidence
that long-term, sustained heating will add a lot to that
over ensuing centuries. But the world might be entering
an era of even more rapid ice melt if the front edges of
the ice sheets retreat.
To learn whether this could happen, we look for
clues from changes underway today, aided by insights
gained about Earth’s past and from the physics of ice.
Many of the clues have come from dramatic changes
that started about two decades ago on Jakobshavn Gla-
cier, an important piece of the Greenland Ice Sheet. Gla-
ciers spread under their own weight toward the sea,
where the front edges melt or fall off, to be replaced by
ice flowing from behind. When the loss is faster than the
flow from behind, the leading edge retreats backward,
shrinking the ice sheet on land and raising sea level.
During the 1980s Jakobshavn was among the fastest-
moving glaciers known, racing toward Baffin Bay, even
though it was being held back by an ice shelf—an exten-
sion of the ice floating on top of the sea. In the 1990s
ocean warming of about 1.8 degrees Fahrenheit (one de-
gree Celsius) dismantled the ice shelf, and the glacier on
land behind it responded by more than doubling its
speed toward the shore. Today Jakobshavn is retreating
and thinning extensively and is one of the largest single
contributors to global sea-level rise. Geologic records in
rocks there show that comparable events have occurred
in the past. Our current observations reveal similar ac-
tions transforming other Greenland glaciers.
If Thwaites, far larger, unzips the way Jakobshavn
did, it and adjacent ice could crumble, perhaps in as lit-
tle as a few decades, raising sea level 11 feet. So are we
risking catastrophic sea-level rise in the near future? Or
is the risk overhyped? How will we know how Thwaites
will behave? Data are coming in right now.
WAFFLES ON THE COAST
CALCULATING THWAITES’S THREAT is complex. To make
sense of it, let’s begin with breakfast. If you pour batter
on a waffle iron, your mound will spread across the
iron’s crosshatched grid. Physically, the weight of the
batter pushes the mound outward against the friction
on the grid below it. This spreading slows as cooking
stiffens the batter—or if you hold the batter back with
your spatula.
Glacial ice sheets are like big waffles, up to two miles
thick and a continent wide. Snow falls on top and is
squeezed into ice under the weight of subsequent snow-
falls. These huge ice mounds are strong—I have landed
on them in heavy ski-equipped military transport
planes—but they still spread. Their temperature is often
within a few degrees of the melting point, making the
ice soft enough to slowly flow from the high, central re-
gion toward the edges, where it more readily melts and
breaks off. Thicker or steeper mounds such as those on
Greenland and Antarctica spread faster.
Left to itself, an ice sheet grows until it is thick and
steep enough for the spreading, melting and breaking
to balance the ongoing, additional snowfall. The mound
can stay at one size for a long time. But that is not the
case on our warming planet. The moisture in the snow
that falls on Greenland and Antarctica each year, which
almost entirely comes from the sea, is equal to a layer of
water evaporated from all oceans, just over a quarter of
an inch deep. The ice sheets are now returning about
15 percent more than this amount to the oceans, by
meltwater runoff or icebergs that “calve” off, raising sea
level a little. If melting remains greater than snowfall
for long enough, an ice sheet can disappear. But that
could take almost 100,000 years at recent rates. If
warming rises, however, the melting quickens. That is
the case we are facing globally.
LACIERS ARE MELTING. SEAS ARE
rising. We already know ocean
water will move inland along the
Eastern Seaboard, the Gulf of
Mexico and coastlines around the
world. What scientists are urgent-
ly trying to figure out is whether
the inundation will be much worse than anticipated—
many feet instead of a few. The big question is: Are
we entering an era of even faster ice melt? If so, how
much and how fast? The answer depends greatly on
how the gigantic Thwaites Glacier in West Antarctica
responds to hu man decisions. It will determine wheth-
er the stingrays cruising seaside streets are sports cars
or stealthy creatures with long, ominous tails.
© 2019 Scientific American
