Climate Surprises Could Spring from Changes in River Flow,
Ice Sheets and Sea Ice
Dr. Terry Hughes of the University of Maine at Orono in
a telephone interview
delved into many sources of unforeseen climate events. We are
saving his
interesting remarks on how icebergs from West Antarctica may
affect
El Ninos for a later issue of Climate Alert.
River flow
The Mackenzie River in Canada and the many rivers in Siberia:
the Ob, Yenisey, Lena, and Indigirka, drain enormous areas. While
they are frozen in winter, in summer they carry a very heavy discharge.
Under greenhouse warming, the rapid runoff would be extended a
few weeks at each end of the summer season, two to three months
in all, and the pulse-like discharges would probably be much heavier.
A large influx of fresh water would have a big effect on sea ice
which is delicately balanced. Warmer winters bringing more precipitation
could upset that balance, increasing the thickness by a few centimeters
per year. There would be a change in the interglacial balance,
both in thickness and to a much larger extent in area of sea ice.
There is a question of how the mass balance would shift. If the
snow didn't melt during cooler summers, the shift would be in
the negative direction, and the ice would thicken. The sea ice
may become grounded on the shallow Arctic continental shelf of
Asia and North America. Grounded sea ice, called "fast" ice, would
cause the draining rivers to flow over the top of the pack ice
instead of under it. This could extend over an enormous area.
As winter precipitation fell on grounded ice, it would cause the
sea ice to become thicker and thicker until it behaved like an
ice sheet, advancing on to land and eventually generating its
own local climate as Greenland and Antarctica do today. As the
area became bigger, local climate would become regional climate
and finally would become global climate. This is a long-term process,
but once it got started it would be hard to stop. The prospect
of forestalling it would be daunting. Could explosives prevent
sea ice from grounding, nipping the process in the bud?
Changes in Ice Sheet Mass
NASA has been using laser beams and radar to detect any raising
or lowering of the level of ice sheets. In two areas studied,
western Greenland flowing into Baffin Bay and Antarctica north
of 72 degrees South, both show elevation increasing, in the ball
park of 2 - 3 cm, indicating a thickening of the ice sheet. This
is despite the many glaciers ringing Greenland and moving toward
the sea. One of them, Jakobshavn on the western coast is the fastest
moving glacier in the world, advancing at the rate of 8 km a year
and producing icebergs the size of a city block which move into
the North Atlantic shipping lanes and down to Grand Banks near
where the Titanic sank. In other areas of Greenland, the elevation
picture is mixed: in some sectors the ice is thinning at 3 cm
per year, farther north it is thickening at 6 cm per year and
north of that it is thinning at 3 cm per year.
Collapse in West Antarctica
The situation in Antarctica is quite different. The East Antarctic
Ice Sheet, grounded above sea level, has been less researched.
The West Antarctic Ice Sheet is grounded below sea level, and
two-thirds of it has completely collapsed in the Holocene. The
jury is still out on just what is happening to the remaining third,
but there is one definite conclusion: it is not in equilibrium.
Parts of it show net lowering leading to collapse; other parts
do not. Actually, very little has been studied; sampling only
covers a small area, and a great deal of research remains to be
done.
One sample of this area is the Pine Island Glacier which enters
Pine Island Bay on the Amundsen side of the West Antarctic Ice
Sheet. Its underside is melting rapidly because of greenhouse
warming. According to a paper in Geophysical Research Letters
of May 1, 1996, by Stanley Jacobs and Hartmut Hellmer of Lamont
Doherty Earth Observatory and Adrian Jenkins of the British Antarctic
Survey, the underside of this glacier is melting at the rate of
12 meters a year. The entire rise in sea level since the last
glacial maximum, spread over 12,000 years, amounts to 120 meters.
At the rate of the melting measured by Jacobs et al we could have
the equivalent in a decade of the entire sea level rise since
the last glacial maximum, as both melting and rising sea level
cause retreat of the grounding line for ice sheets grounded below
sea level.
The effect of rapid basal melting has been studied by Eric Rignot,
working at the Jet Propulsion Laboratory (JPL) in Pasadena. He
has shown through satellite imagery that the grounding line of
Pine Island Glacier has been retreating at the rate of 5 kilometers
in four years. It can be expected to accelerate as the grounding
line retreats down an increasingly steep slope.
Pine Island Glacier and Thwaite's Glacier enter Pine Island Bay
and together drain one-third of the remaining WAIS. They have
been discharging icebergs steadily, with one 100-kilometer iceberg,
the size of Connecticut, breaking up in the last 10 years. All
around West Antarctica, from the Weddell Sea on both sides of
the Antarctic Peninsula to the Ross Sea, enormous crevasses have
formed and led to massive outbursts in the last decade. It may
be an early warning system for something more dramatic that is
just beginning, not merely a coincidence.
Dr. Brenda Hall (University of Maine) is researching the history
of the West Antarctic Ice Sheet grounded in the Ross Sea between
8,000 and 6,600 years ago. She found that the grounding and calving
front retreated 500 kilometers in the western Ross Sea within
a 1400-year interval - geologically an almost instantaneous period.
And yet this was a time when ice was flowing into the Ross Sea
from both sides. But now, ice is flowing out, into West Antarctica
and into the Ross Sea embayment on the east and the Weddell Sea
embayment on the west. A retreat in the Amundsen Sea augmented
by ice flowing out from both sides could cause the ice sheet to
collapse a lot faster than in Hall's historical 1400-year period.
New Ice Stream
In the last decade an enormous ice stream has been discovered
in northwest Greenland which no one previously knew existed. It
flows northeast from the central dome of Greenland and breaks
into outlet glaciers, the main trunk being the Zacharia Ice Stream.
The ice stream is 700 kilometers long, a very long stretch for
an ice stream and an indication that it is draining a large area
and pulling out a large amount of ice. While the present mass
balance is not known, such a large drainage may mean that the
positive mass balance of the ice sheet may be switched. If the
elevation of the ice sheet is being lowered and the ice dumped
into the East Greenland current and into the Greenland and Labrador
Seas, both areas of deep water formation for the Conveyor Belt,
the implications are extreme. This linkage could yield a very
fast response, actually in a matter of days (if the iceberg discharge
is large enough).
Work on the Greenland ice core has shown very rapid climate changes
in short spaces of time occurring back through geologic ages.
The more closely you can sample what is happening, the more distinctly
you can resolve the time that elapsed. These changes recorded
in the ice core are illustrated by changing dust concentrations
and can only show the upper limit of such changes. The changes
couldn't have been slower; they might have happened a lot faster.
As we have seen, satellite technology looking at what is happening
in the west part of Greenland draining into Baffin Bay, show that
the ice elevation is increasing and at the same time sea level
rise is also increasing by the same few centimeters. Somewhere
the ice surface must be lowering to override the ice surface increases.
The likely sites include the West Antarctic Ice Sheet facing the
Amundsen Sea and the Greenland drainage basin of the giant ice
stream.
The well-documented changes happening just within a decade is
a numbing prospect, Hughes says, and we have only hints of exactly
what is going on.
Albedo
The ability of ice to reflect solar heat from the earth's surface
- known as albedo - cools or warms Earth's surface depending on
whether there was an increase or decrease in the amount of Arctic
or Antarctic sea ice. Increased sea-ice cover reflects more sunlight,
and air temperature is reduced; decreased cover leads to more
warmth. Sea ice cover in the polar oceans is an important component
of the Earth's climate system. It has a higher albedo than the
open ocean and thus modifies the energy balance of the polar regions.
It also acts as an insulating blanket, reducing the transfer of
heat from the underlying oceans to the cold polar atmosphere.
Perplexing Data
A decline in sea-ice is commonly predicted under global warming,
but satellite observations have shown no clear trends. According
to a recent Nature article by William K. de la Mare of the Australian
Department of the Environment, comparisons between satellite observations
and ice-edge charts from early ship records suggest that sea-ice
covered a smaller area in the 1970s than during the 1930s, but
the observations are seen as "inconclusive." However, details
from whaling records which de la Mare has uncovered, including
every whale caught since 1931, show that the sea-ice edge moved
south 2.8 degrees latitude from the mid 50s to the early 70s.
This analysis, based on circumpolar whale catch records averaged
over October to April, suggests a decline of about 25 percent
in the area covered by sea ice. Such an abrupt change "could imply
changes in Antarctic deep-water formation and in biological productivity,
both important processes affecting atmospheric CO2 concentrations."
Other data do not conform with these observations. Satellite
observations only began in 1973, and they show sea-ice coverage
has been roughly stable since then, but there is a great deal
of regional variability. (In the Bellingshausen Sea, west of the
Antarctic Peninsula, there has been a reduction in sea-ice in
the past couple of decades.) The fact that the whaling data show
abrupt change from 1950 to 1970 and later analysis shows it to
appear roughly stable is an "interesting challenge for coupled
atmosphere-ocean general circulation models ... and may give insight
into fundamental climate processes," say two British Antarctic
Survey members who also comment that, "the variability may be
natural and not connected to any human-induced changes. But as
yet we do not know."
Complete Warming Not Necessary
Earth's albedo also decreases if greenhouse warming disintegrates
the floating ice shelves that surround the WAIS. It is not commonly
understood that for the floating ice shelves of the WAIS to disintegrate
it does not have to warm so much that the ice melts completely,
Hughes says. All that is necessary is for some melt water to get
into crevasses, which are lines of weakness in the ice shelves.
Surface melting exploits the line of weakness, and the surrounding
area of the ice sheet can disintegrate without having gone through
a longer term melting process.
The snow line, above which precipitation falls as snow, intersects
the shelves at a very low angle. Therefore, if climate warming
raises the snowline slightly, it can change an ice shelf from
having a snow-covered surface to a melting bare-ice surface. This
is what happened to the Wordie Ice Shelf on the Antarctic Peninsula.
Just a little bit of warming during the summer was enough to change
the whole pattern of snow buildup in winter followed by melting
in summer. Sea ice is also full of cracks, and so it can disintegrate
before it melts.
Cloud cover
In the long winter night in the Arctic, there is black body radiation.
But even at night, part of it can be reflected back to Earth by
clouds, and if there is an increase in cloud cover, this too would
tend to increase warming. Removing sea ice increases ocean evaporation
and therefore should increase cloudiness in the Arctic and Antarctic.
Permafrost
The permafrost is divided into zones: most northerly zones it
is extremely thick with little melting; as you move outward the
permafrost is less thick and is marked by lakes, permanent indentations
scoured out by glaciers where water collects and remains permanently.
Farther out the permafrost is thinner and may be subject to quite
a bit of melting in the summer. By and large it is not subject
to sudden change.
The albedo is much lower in high latitudes and therefore allows
more thickening of the sea ice. But it is not clear how it would
be affected.
Temperature rise
Global warming would also get rid of sea ice, with the same implications
as the change in albedo.
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