Greenland Climate: Now vs. Then, Part II. Record Greenland Melt
Area?
World Climate Report, November 5, 2007
Recently the press was more ablaze than California with NASA
proclamations that the surface area of Greenland had melted in 2007 at a
record-high rate. This is true, if the record only extends back only 20 years
or so—which is the case of the NASA dataset. If you could peer back a bit
further into the past, say back into the 1950s, it is quite likely that the
melt area in Greenland then was about the same as it is now, effectively
rendering the 2007 melt area hardly newsworthy. Just another NASA
climate-change exaggeration?
The NASA press release announcing all of this is rather
cryptically headlined ÒNASA Finds Greenland Snow Melting Hit Record High in
High PlacesÓ and then goes on to try to explain:
A new NASA-supported study reports that 2007
marked an overall rise in the melting trend over the entire Greenland ice sheet
and, remarkably, melting in high-altitude areas was greater than ever at 150
percent more than average. In fact, the amount of snow that has melted this
year over Greenland could cover the surface size of the U.S. more than twice.
A Òrecord high in high placesÓ? High altitude melting Ògreater
than everÓ? Amount of snow melt this year Òcould cover the surface size of the
U.S. more than twiceÓ? At what depth?
The NASA press release also included the following graphic (Figure
1).
Figure 1. The trend of the melting index in Greenland between
1988 and 2007 in relation to how many surface sizes of the U.S. match the
melting index observed each year. (source: NASA press release)
This has to be one of the more bizarre graphs ever put out by
NASA. It shows the annual Òmelt area indexÓ of Greenland in relation to the
size of the United States for each year from 1988 to 2007. The value for this
year is a bit more than two times the size of the continental U.S. Now
considering that the total area of Greenland is just more than one-quarter the
area of the lower 48, you may wonder how an area of more than twice the size
continental U.S. melted this year in Greenland. Good question.
The answer lies in what exactly the Òmelt area indexÓ represents.
Contrary to what it might seem—the area of Greenland that experience some
surface snowmelt—it is in fact, the sum of the area of Greenland the
experienced surface snowmelt across all days of the year that melting occurred.
Thus if an area of Greenland equal to 1/365th the area of the U.S. experienced
melting every day of the year, this would produce a Òmelt area indexÓ for that
location equal to the size of the entire contiguous United States. This
certainly makes for a wacky comparison, although it certainly sounds
impressive!
Rather than using a picture of the continental U.S. as a metric in
their graph, even though it would have been less sensational, NASA should have
simply plotted out the time history of the Òmelt area indexÓ for Greenland and
left it at that (we do this service for you in our Figure 2). That would have
shown a general rise since NASA Greenland snow melt records began in 1988 up
through 2007, but it would also have shown that all of the rise took place from
1988 to 1997. Since then, the Òmelt area indexÓ has shown year-to-year
variation, but no real overall change.
Figure
2. The
same NASA data showing the Òmelt area indexÓ of Greenland without using the
area of the U.S. as a metric.
Then why all the news this year when Figure 2 shows that 6 of the
past 10 years have had a larger Òmelt area indexÓ than 2007? Well, if instead of
calculating the Òmelt area indexÓ for all of Greenland, if you limited your
calculation only those regions that lie at elevations above 2,000 meters (in
NASA parlance Òhigh placesÓ), voila! you find that the value for 2007 is the
highest on record since 1988 (or, as NASA described it, Ògreater than everÓ).
Figure 3. The Òmelt area indexÓ from altitudes on the Greenland
ice sheet that are above 2,000 meters (source: Tedesco, 2007).
But, as we have discussed in earlier installments of our series on
GreenlandÕs climate history, Greenland was at least as warm as it is now, back
in the early-to-mid 20th century and the coldest period of the past 100 years
was probably the mid-to-late1980s. So, it should come as little surprise that
the melt area of Greenland has been increasing as temperatures have warmed
since the 1980s.
Inquiring minds might want to know what the melt area was like
during the warm period 50 to 60 years ago. Obviously, we didnÕt have satellites
orbiting around taking measurements from space back then, but there was a good
deal of climate research taking place on the ground across Greenland. In 1961,
much of this work was summarized in an article by R.W. Gerdel in ÒA
Climatological Study of the Greenland Ice SheetÓ included in the Proceedings of
the Symposium on the Physical Geography of Greenland of the XIX International
Geographical Congress. Included among GerdelÕs discussions of temperature,
precipitation, winds, fog, radiation, etc., is a section on ÒThe Occurrence of
Melting on the High Ice Sheet.Ó
Gerdel reports that there is evidence of summer melting occurring
at least as high as 1700 meters above sea level on the interior of the ice
sheet east of Thule at latitude 76¼N. Air temperature measurements from the
Thule airbase (along the northwestern Greenland coast) coupled with those taken
from elevations on the ice sheet indicate that the temperature lapse rate
(decline in temperature with height) was found to be 0.6¼C/100m. Gerdel used
this lapse rate to calculate the elevation on the ice sheet where the air
temperature would reach the freezing point, extrapolated from the temperatures
taken from coastal stations around Greenland (there were very few temperature
measurements from locations on the interior ice sheet itself). From these data,
Gerdel produced the following Table, which indicates the calculated average
number of days during the summer (June, July, August) that the maximum air
temperature on the ice sheet was at or above freezing for various latitudes and
elevations (based upon observed sea level temperatures during the period
1946-1956).
Figure 4. Table containing the average number of days during
the summer in which the maximum daily temperature was calculated to be at or
above freezing for various elevations and latitudes across Greenland (source:
Gerdel, 1961).
Gerdel described his technique and findings this way:
Assuming that the above relation [the lapse
rate at Thule] was applicable to other sections of the Ice Sheet. The extent of
melting in other areas was estimated from meteorological records reported from
adjacent coastal stations. The number of days having maximum temperatures above
freezing for latitudes of 61N, 67N and 7[6]N are shown in Table 5. The data
indicate that, south of 67N latitude, considerable melting may be expected at
elevations as high as 2700m.
Compare GerdelÕs calculations (based upon mid-20th century data)
to the recently reported record melting announcements by NASA. While the latest
NASA press release proclaiming the 2007 record melt area only reports
anomalies, their press release from back in the spring of 2007 , touting the
big melt in 2006 included a graphic illustrating the actual number of melt
days.
As our Figure 1 illustrates, the Òmelt area indexÓ of 2006 was
very close to the 2007 value, a comparison of GerdelÕs number to the 2006 data
will provide a fair assessment of how current conditions compare to mid-20th
century ones.
Figure 5 shows the number of days with melting observed across
Greenland in 2006 as reported by NASA (blue colors) along with the number of
days of melting for the locations as calculated by Gerdel in 1961 (black
numeric values). Notice that in every case, Gerdel calculated a greater number
of days with melting than occurred in 2006 including in the Òhigh placesÓ on
the ice sheet in the north, south central, and southern portions of Greenland.
In other words, conditions were as favorable, if not more favorable, for snow
melt to occur over the Greenland ice sheet (including NASAÕs Òhigh placesÓ) in
the mid-20th century compared with the present day.
Figure 5. Total number of days with melting in 2006 (colors)
and total number of melt days expected per year calculated from temperature
data collected in the mid-20th century (black numbers). (source: 2006 data from
NASA press release and Gerdel, 1961).
So, all these announcements from NASA seem to be just another case
of a record value being only as significant as the record length. The area of
ice melt in Greenland in recent years (2007 included) is no doubt the highest
in the past 20-30 years (since the end of the cold period there in the
mid-1980s), but is probably very typical of the melting which took place during
the warm period during the years around 1950 (or the decades prior).
WeÕll let it up to each of you to determine how much you want to
get worked up about all of this.
References:
Gerdel, R.W., 1961. A climatological study of the Greenland ice
sheet. Physical Geography of Greenland, 84-106
Tedesco, M., 2007. A new record for 2007 for melting in Greenland.
Eos, Transactions of the American Geophysical Union, 88(39), 383.