Greenland ice sheet:
Recognizing global warming, regional and local climate, and a bit of physics:
Why warmer temperatures can increase arctic winter snowfall
The July 15, 2006 newsletter of the El Dorado Business Alliance reported
under a headline of Global Warming: Update, first referring to the film
"An Inconvenient Truth" and Al Gore, referring to projections of
consequences of global warming. Then it countered by saying that
"according to a new study by the National Center for Policy Analysis
this is nonsense". Its ultimate quote from the study says “The
average summer air temp-eratures at the summit of the Greenland Ice
Sheet have decreased at the rate of 4 degrees per decade since
measurements began in 1987.”
Actually the data point referenced is from a paper in scientific
literature titled "Global Warming and the Greenland Ice Sheet", by
Peter Chylek, Jason E. Box, and Glen Lesins and published in 2004. The
National Center for Policy Analysis issued a report interpreting the
original research, and their interpretation is not part of the
scientific literature. Original scientific research typically undergoes
a peer review process to qualify it for publication and in some cases
for presentation at conferences and meetings.
The original
paper primarily raises the issue of why certain Greenland observations
are correlated strongly with the North Atlantic Oscillation but not
with global temperatures, which it acknowledges were rising during this
period. The North Atlantic Oscillation is a large scale (but not
global) cause of climate variation, involving winds and ocean currents.
The paper acknowledge the global
trend toward higher air temperatures; its graph of the temperature
anomaly is
reproduced here. For comparison, to its right is a version of the
same data updated to 2005 by the NASA
Goddard Institute for Space Studies, with an additional graphic showing
regional variation from nominal temperatures worldwide in year 2005.
See http://data.giss.nasa.gov/gistemp/2005/
or click on the NASA graphics above to see the data set presentation on the NASA Goddard web site. This has a
very good description, additional graphics depicting warming in the
past 50 years, and references to relevant publications in scientific
literature.
Even the second graph is current only to 2003. In fact, 2002 was a
record-setting year for ice melt on Greenland, and 2005 surpassed the
2002 record (see graphics below, from the University of Colorado's
Cooperative Institute for Research in Environmental Sciences [CIRES]).
The authors of this paper correctly point out that the Greenland
temperature changes are within the range of variation known from the
past, through ice core samples that span several glacial and
interglacial periods, and from historic records gathered over more than
a century.
Extent of Greenland ice melt, 2005 compared with 1992
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Summary of the melt extent and total
melt for the entire Greenland ice Sheet and for the north-western part
(Thule) and the western part (Jakobshavn region) [ PDF ]
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Excerpt from another reference whose co-authors include two of the co-authors of the 2004 paper, Global Warming and the Greenland Ice Sheet: Greenland warming of 1920-1930 and 1995-2005, by Petr Chylek, M.K. Dubey, and G. Lesins, publish May 9, 2006 (see http://meteo.lcd.lu/globalwarming/Chylek/greenland_warming.html)
[15] i) The years 1995 to 2005 have been characterized by generally increasing
temperatures at the Greenland coastal stations. The year 2003 was extremely warm
on the southeastern coast of Greenland. The average annual temperature and the
average summer temperature for 2003 at Ammassalik was a record high since 1895.
The years 2004 and 2005 were closer to normal being well below temperatures
reached in 1930s and 1940s (Figure 2).
Although the annual average temperatures and the average summer temperatures at
Godthab Nuuk, representing the southwestern coast, were also increasing during
the 1995-2005 period, they stayed generally below the values typical for the
1920-1940 period.
A personal note by myself as a past physics major on effects of
temperature on precipitation (snow) in cold areas, such as Greenland
and other arctic and antarctic zones:
Warming can increase
snowfall in localities and regions with cold climates. The reason is
its effect on the saturation partial pressure of water vapor in the
air, and therefore on relative humidity. The basic relation is
illustrated in this graph from Section 14 of a tutorial on remote
sensing on the NASA Goddard web site... (See rst.gsfc.nasa.gov/Sect14/Sect14_1d.html for this section of the tutorial.)
Reminder for U.S. readers: Temperatures graphed here are in degrees Celsius.
0º C = 32ºF -17.8º C =
0º F -30º C = -22º F
-40º C = -40º F
The vapor pressures referenced in that equation refer to the partial
pressure of water vapor among the mix of all gases in air. When the
actual pressure reaches the saturation pressure precipitation occurs in
forms such as rain, snow, and dew. The temperature at which this occurs
is the dew point.
A variant of the relative humidity equation which is somewhat more
intuitively clear is identical except that it refers to the ratio of
actual vapor density to saturation vapor density. The key point
is that both forms of the equation represent a measure of how much
moisture the air is capable of carrying as water vapor.
Very cold areas that remain covered with snow and ice, never melting,
typically are deserts with very little precipitation (new snow) because
the air is so cold that it can carry nearly no moisture as water vapor.
In the simplest model of weather water evaporates over oceans, is
diffused into an air mass by convection and is transported to a
continental area by movement of the air mass. Over land it usually
experiences cooling as it is forced to higher elevations by terrain,
especially mountains (orographic lift) or other atmospheric phenomena
such as convection. As it cools its saturation pressure drops, and when
the saturation pressure drops below actual pressure of water vapor
precipitation starts, usually as rain or snow. When the air mass chills
to extremely low temperatures, visible on the graph above at about -25
degrees Celsius, the air has effectively lost its ability to transport
more than trace amounts of water vapor. By then virtually its entire
water content will have been dropped as snow. As the air mass continues
to move it is "bone dry".
Suppose such an area warms. Let's say an air mass that formerly tended
to be about -20 degrees Celsius in past times now tends to be about -10
degrees Celsius. As the graph above shows, the air mass can now
transport much more water vapor. If this air is lifted by topography,
as occurs over Greenland, the warmer air can produce increased winter
snowfall. Greenland's topography is not just mountains, it
includes an ice sheet up to 3,375 meters (11,070 feet) thick.
This situation would be consistent with reports based on remote sensing
from satellites. A NASA press release on March 8, 2006, reported this
based on satellite observations by ICESat: (quoted from http://icesat.gsfc.nasa.gov/)
Image left:
The Greenland ice sheet gained more ice from snowfall at high altitudes
than it lost from melting ice along its coast. Credit: NASA/SVS
In Greenland, the survey saw large ice losses along the southeastern
coast and a large increase in ice thickness at higher elevations in the
interior due to relatively high rates of snowfall. This study suggests
there was a slight gain in the total mass of frozen water in the ice
sheet over the decade studied, contrary to previous assessments.
This situation may have changed in just the past few years, according
to lead author Jay Zwally of NASA's Goddard Space Flight Center,
Greenbelt, Md. Last month NASA scientists at the Jet Propulsion
Laboratory, Pasadena, Calif., reported a speed up of ice flow into the
sea from several Greenland glaciers. That study included observations
through 2005; Zwally's survey concluded with 2002 data.
When the scientists added up the overall gains and losses of ice from
the Greenland and Antarctic ice sheets, there was a net loss of ice to
the sea. The amount of water added to the oceans (20 billion tons) is
equivalent to the total amount of freshwater used in homes, businesses
and farming in New York, New Jersey and Virginia each year.
This also refers to accelerating glaciers. Warmer temperatures
accelerate melting at the toe of the glacier. Meltwater percolates or
flows to the bottom of the glacier through crevasses and related
structures, and the increase of liquid water lubricates the portion of
the glacier which is literally holding it back. The glacier then moves
faster than before. In Greenland and many other arctic areas glaciers
are now physically carrying ice to the sea faster than before.
The brief note above from NASA touches on the balance of two competing
factors that can both be part of global warming: Increased rate
of accumulation of snow and ice at high elevations and increased rate
of carrying it to the sea due to warming at the glaciers' toes.
