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Glaciers like Greenland’s Jakobshavn
Isbrae (shown) are now expected to
add less to sea level rise this century
than worst-case scenarios predict.
Greenland ice
flow stop-and-go
Motion suggests worst-case
sea level predictions unlikely
By Devin Powell
Time-lapse snapshots showing
Greenland’s glaciers racing toward the
sea in recent years have turned up some
good news and some bad news.
As the island’s glaciers disintegrate
over coming decades, they won’t raise
the world’s oceans as much as the most
pessimistic forecasts had shown possible,
researchers report in the May 4 Science.
“We’re certainly looking at signifi-
cant rises in sea level, but some of the
worst-case scenarios that people have
imagined don’t seem likely,” says glaci-
ologist Twila Moon of the University of
Washington in Seattle.
That’s in contrast to the situation in
some parts of Antarctica, where recent
work suggests that researchers’ worst
fears may be realized (Page 5).
Moon’s team used satellite measurements from 2000 to 2011 to clock the
speeds of more than 200 of Greenland’s
outlet glaciers — flowing tongues of frozen water that carry ice away from the
vast ice sheet that blankets most of the
country. Where the glaciers extend offshore, they tend to fall apart and dump
ice into the ocean.
Some of these icy conveyor belts have
already been spotted moving — and thus
melting—faster in recent years. The
giant glacier Jakobshavn Isbrae, for
instance, accelerated from 9. 4 kilometers
per year in 2000 to 12. 6 kilometers per
year in 2003. A 2008 study in Science
estimated how much such acceleration
might contribute to rising sea level. If
every glacier could suddenly increase
its speed tenfold, sea level would rise
about half a meter by 2100, researchers
found. A more realistic doubling of speed
between 2000 and 2010, followed by
leveling off, would contribute a smaller
rise of about nine centimeters.
“We were trying to set some really
firm upper limits on sea level rise using
values that seemed within the realm of
possibility,” says glaciologist Tad Pfeffer
of the University of Colorado Boulder, a
coauthor of the 2008 study.
The new data show that glaciers as a
whole haven’t accelerated that much, or
that uniformly, from winter to winter.
On average, they moved about 30 percent faster at the end of the first decade
of the 21st century than they did at
the beginning.
Sea salinity has shifted since ’50s
Warmer atmosphere may be cause of water cycle changes
Simulations in the new study suggest
evaporation and rainfall got a 4 percent
boost as surface temperatures rose half
a degree Celsius. That boost is a bigger
change than previous studies had sug-
gested, but fits with the idea that a warmer
atmosphere can hold more moisture.
By Devin Powell
More water moved into and out of the
atmosphere in 2000 than in 1950, making
saltier parts of the world’s oceans saltier
and fresher waters less salty, researchers
report in the April 27 Science.
“We see big broad patterns of change,”
says Paul Durack, an oceanographer at
Lawrence Livermore National Laboratory in California.
Ingram, an atmospheric physicist at the
Met Office Hadley Centre in Exeter and
the University of Oxford in England.
Small fluctuations in evaporation and
rainfall tend to get smoothed out over
time, helping scientists to tease out long-term trends.
A warming planet may be to blame.
Measuring such global changes in
Earth’s evaporation and rain cycle has
never been easy. Rain gauges on land or
at sea tend to be sparsely distributed,
and the exact positions of such instruments decades ago isn’t always known.
Ocean salinity provides a fairly stable,
reliable way to measure how much water
goes up and comes down, says William
Durack’s team analyzed 1. 7 million
salinity measurements made by ships
during the second half of the 20th century. Other researchers had already seen
patterns of change in these data, but
Durack and his colleagues sharpened the
picture. A net work of autonomous buoys
deployed in the 21st century helped to
fill in gaps in the record, particularly at
high latitudes where winter storms keep
ships away.
