++ Wrangell Range
Mass Balance of the Wrangell Range Glaciers, Alaska
Figure 1. The world's longest interior valley glacier, the Nabesna Glacier, is over 75 miles long. It is just one of the Wrangell Range's more than 30 glaciers.
Daily and annual mass balances ( Net, Ablation, and Accumulation) of the Wrangell Range Glaciers are calculated for the 1951-2011 period. Calculations are made with the precipitation-temperature-area-altitude model (PTAA), using daily precipitation and temperature observations collected at the Big Delta and McKinley Park, Alaska weather stations, together with the area-altitude distribution of the approximately thirty glaciers in this range. The average mass balance for 61 years of the south-facing glaciers is - 0.68 mwe (meters water equivalent) and - 0.42 mwe for north-facing. Total thinning of the south-facing glaciers is 47 meters versus 28 meters for north-facing during this period. An unexpected finding is that average annual ablation positive anomalies of the Wrangell glaciers are correlated (r=+0.81) with global temperature positive anomalies averaged for 7000 weather stations in the Northern Hemisphere. In addition, the time-trends of glacier ablation and global temperatures are both increasing exponentially.
The PTAA Model (precipitation-temperature-area-altitude) is applied to all the glaciers (about 30) in the Wrangell Range, Alaska, shown in Figure 1. Input to the model are daily temperature and precipitation observations at McKinley Park (elevation = 631 meters) and Big Delta (470 m), Alaska, located approximately 350 km NW of the Wrangell Range, and the area-altitude distributions of the glaciers. The glaciers are divided into two sets of approximately equal areas based on their north-south orientations. For mass balance analyses, the total areas of north and south facing glaciers are treated as two large glaciers. The area-altitude distributions of both north and south-facing glaciers are shown in Figure 2.
The Wrangell Range has 12 south-facing glaciers with a total area of 2308 square kilometers. The largest is the Kennicott (551 square kilometers), followed by the Regal, Rohn, Frederika, Middle Fork, Nizina , West Fork Glaciers and 5-6 that are unnamed. There are also about 12 named north-facing glaciers that have a total area of 2418 square kilometers. The largest is the Nabesna (253 square kilometers) and numerous smaller glaciers, both named and unnamed. The varied orientations together with the calculated mass balances provide a unique combination for understanding how mass balance, glacier flow and the current weather in this region are linked. A major goal of this project is to explain the physical mechanism(s) that can force the daily and annual mass balance of a glacier by applying weather observations collected at distant stations to the unique area-altitude distribution of each glacier's surface.
The study is part of a larger project which will eventually display the mass balances of 200 glaciers worldwide in near real-time (depending on when weather data will be available on-line after it is collected). Both the current year and historical balances (Net, Accumulation and Ablation) will be shown, primarily in a graphical format. The altitude distribution of area, average snow accumulation, ablation and balance will also be shown for each glacier.
Mass Balance Results
The daily and annual mass balances for the Wrangell Range glaciers are determined with the PTAA model using daily precipitation and temperature observations collected by the National Weather Service and the area-altitude distributions of each glacier's surface. There is a significant difference in the area-altitude distributions of the south facing versus the north facing glaciers (Figure 2) that can be attributed to greater erosion of the bedrock underlying the south facing glaciers. The difference is also reflected in the mass balances due to higher ablation rates for glaciers with a more southern exposure. The annual balances of south- facing versus north-facing glaciers, shown in Figure 3, are approximately 0.3 mwe more negative because of higher ablation rates (Table 1).
Mass balance terminology used in this report deviates slightly from that proposed by the WMG Working Group (Cogley, 2012). Prior to approximately 1995, mass balance is defined by b(z), or balance is dependent on elevation. In recent decades this relationship has been reversed so that elevation is now a function of mass balance, which is both physically and mathematically incorrect. Also, the terms Accumulation Balance and Ablation Balance are preferred over Winter Balance and Summer Balance. For most Alaskan glaciers, snow accumulation at higher elevations occurs throughout the year and can be especially heavy in August and September. For many Himalayan Range glaciers, snow accumulation is greatest during the Monsoon season, from June through September, and at lower elevations ablation often occurs during the winter months (Yang, 2011).
Ablation is approximately 14 % greater on a glacier that flows in a southward direction in the Wrangell Range. Daily ablation is calculated by the PTAA model for both north and south sets. Ablation is greater for south-facing glaciers because solar radiation strikes their surfaces at a slightly less oblique angle than it does for north-facing glaciers. Thus, there is a greater amount of solar energy absorbed and consequently more ablation, greater turnover of mass and increased flow rates of these glaciers, resulting in greater erosion of the underlying bedrock. A comparison of AA distributions for the two sets of glaciers in Figure 2 reflects these differences. There is a greater surface area at lower elevations of the south facing glaciers and more surface area at mid-elevations of north facing glaciers. These unique patterns have developed throughout geologic time and reflect the climate that existed as these glaciers developed. The influence of glacier aspect variations on mass balance has been emphasized in previous studies (Evans and Cox, 2010). Most studies indicate glacier altitudes, especially equilibrium-line altitudes (ELAs), as climate indicators. Variations of glacier aspect can provide useful information on cloudiness and wind patterns at or near the glacier (Evans, 1977).
Figure 2. Area-altitude distribution of the north and south-facing Wrangell Range glaciers. The total area of these glaciers is 4786 km2 and there are 150 altitude intervals spaced at 30.5 m (100 feet), ranging from 427 to 4850 meters in elevation. The difference in area distributions of the two sets is due to unequal erosion rates of the south and north-facing glaciers.
Figure 3. Annual ablation balances of south and north facing glaciers for the 1951-2011 period. The average annual ablation balance for south-facing is -1.46 mwe, for north-facing is -1.28 mwe.
Figure 4. Cumulative balance of the north and south-facing glaciers. South- facing glaciers thinned 19 meters more than north-facing during the 61 year period. Total thinning of the south-facing glaciers 47 meters or 0.8m of ice per year, and the north facing is 28 meters of ice or 0.5 m per year.
The cumulative mass balance of the Hintereisferner in the Austrian Alps for the 1953-2003 period, based on manual measurements, is -26.2 mwe (total thinning of 28 meters or 0.6 m per year) (Fischer, 2010). Therefore, mass loss of the Hintereisferner during the past half-century is nearly identical to the rate of mass loss of the north-facing glaciers in the Wrangell Range
Figure 5. Annual net balances of north and south facing glaciers. The average annual balance for north-facing glacier is -0.42 and for south facing is -0.68 mwe.
Balance Versus Elevation
The Net, Accumulation, and Ablation balances as functions of elevation for the south-facing glaciers is shown in Figure 6a, and for north-facing glaciers in Figure 6b. The difference between these balance versus elevation curves provides an insight into heat balance processes of the two sets of glaciers. The Equilibrium Line Altitude (ELA) is 200 meters higher for north-facing glaciers (2300 versus 2100 meters), which is inconsistent with the expected ELA for these orientations. The annual balances at the termini are -8.0 mwe for south-facing and -6.0 mwe for north-facing, which is consistent for south and north orientations. Except at lower elevations, the north-facing glaciers have more negative balances at each elevation interval. The average annual balance is more negative for south-facing glaciers because of the greater mass loss of these glaciers in the terminus area.
The significance of the balance versus elevation differences for south and north-facing glaciers is the generally higher ablation rates of north-facing glaciers, which indicates that air temperature has greater influence than solar radiation on ablation of these glaciers. Such a finding is especially important for relating global temperatures to glacier ablation.
Figure 6a. Average Net, Accumulation and Ablation balances of south-facing glaciers as a function of elevation, averaged for the 1951-2011 period. The ELA is 2100 meters.
Figure 6b. Average Net, Accumulation, and Ablation balances of north-facing glaciers as a function of elevation, averaged for the 1951-2011 period. The ELA is 2300 meters.
The model is calibrated for each glacier by calculating the daily balance using 15 coefficients that convert precipitation and temperature observations to snow accumulation and snow and ice ablation for each elevation interval. One iteration performs this operation for each day of the full period of record and for each elevation interval, from the terminus to the head of the glacier. Both the annual balance and calibration error are calculated for each iteration. In Figure 7a, the first 16 iterations use pre-set coefficients; the remaining are determined automatically by the simplex optimizing procedure. In Figure 7b, the annual balance and calibration error is shown for each iteration.
Figure 7a. Mean annual balance of south-facing glaciers for the 1951-2011 period for each iteration of the simplex. Each point represents 61 years of model runs, calculating daily and annual balances. The first 16 iteration balances are calculated from pre-set coefficients, and the remainder are calculated by the simplex optimizing process. The final mean annual balance after 250 iterations is -0.60 mwe.
Figure 7b. Mean annual balance of south-facing glaciers for the 1951-2011 period versus calibration error. Each point represents 61 years of model-runs calculating daily and annual balances. The minimum calibration error is 42 % when the balance is - 0.68 mwe.
Real-Time Mass Balances
The glaciers will be monitored daily when temperature and precipitation observations for the appropriate weather stations used in the PTAA model are routinely available. The ability to simultaneously compare the daily balances of all the glaciers in the project (200 eventually) will provide the information needed to assess glacier-climate interactions. Figure 8 shows the daily Net, Ablation, and Accumulation balances for the full 2011 balance year. The accumulation balance is +0.62 mwe, the ablation balance -1.0 mwe and the annual balance -0.35 mwe on September 30, 2011.
Figure 8. Daily balances (Net, Accumulation and Ablation) throughout the 2011 balance year. The accumulation balance is +0.62 mwe, the ablation balance -1.0 mwe and the annual balance -0.35 mwe on September 30, 2011. In future years balances will be monitored daily when it is feasible.
Predicting Wrangell Glacier Ablation From Global Temperature Anomalies
The difference in ablation rates between the two sets of glaciers provides the means to analyze the effect of global temperatures on the mass balances of these glaciers. Average annual ablation of south-facing glaciers in the Wrangell Range is approximately 0.2 mwe greater than north-facing glaciers (-1.46 mwe versus -1.28 mwe) because the south sloping surfaces receive greater solar radiation and the greater surface area at lower elevations of south facing glaciers that has higher ablation (Figures 3 and 4). However, there is an increasingly greater daily variation in the north-south differences likely caused by variations in global temperature anomalies. Figure 9 demonstrates that until about 1990, north-facing glacier ablation is frequently greater than south-facing. After 1990, ablation of south-facing glaciers is dominant.
Ablation of north-facing glaciers is less reliant on solar radiation than south- facing, and more dependent on air temperature. Therefore, temperature anomaly variations influence north-facing glaciers to a greater degree than does radiation.
Figure 9. Cumulative difference in ablation anomalies of south facing minus north facing glaciers. Before 1990, daily ablation rates on north and south facing glaciers are nearly equal. After 1990, south-facing glacier ablation is predominately greater than north-facing.
Two major shifts in the earth's temperature patterns (Figure 10) appear to be related to these glacier ablation anomalies. The shifts are revealed by summing the difference between the maximum and minimum daily temperature anomalies:
CTR = ∑ (dTx - dTn), where CTR = cumulative daily temperature range, dTx and dTn are the maximum and minimum temperature deviations from the 1961-1990 period, averaged for 7000 global weather stations by the Hadley Climatic Center (Caesar, et al, 2006). (Daily anomalies of both temperature and glacier ablation are based on departures from the 1961-90 reference period). The first shift on about 1977-78 occurred when minimum temperature anomalies are positive more frequently than maximum anomalies, which translates to a decrease in cloud-cover (an above normal temperature range usually means less cloud-cover). The second shift occurred on about 1999-2000 when maximum temperature anomalies are more frequently positive, and have remained dominant since then. An unusual feature of cumulative anomalies during this period is the annual cyclic perturbations occurring in mid-January, caused by winter warming episodes lasting several weeks (Tangborn, 2003). Similar temperature patterns found in the Austrian Alps, based on cumulative anomalies from the climatologic mean (1961-90), are shown to be linked to mass and area changes of glaciers in that region (Abermann et al, 2009, Abermann et al, 2011)
Figure 10. The cumulative difference between global maximum and minimum temperature anomalies produced from the Hadley Climate Center data set (HadGHCND). Major shifts in worldwide temperature patterns occurred in 1976-77 and 2000-2001. After 2000 there are small cyclical perturbations of temperature increases occurring in mid-January and lasting for several weeks.
There is a similarity in the curves generated by cumulative glacier ablation differences (Figure 9) and those produced by the cumulative global temperature range (Figure10) that suggests both variables are controlled by the same physical mechanism. Annual averages of positive anomalies of both ablation and maximum temperature plotted as line graphs suggest similar non-linear trends (Figure 11).
Figure 11. Annual averages of positive anomalies of south-facing glacier ablation and global maximum temperature anomalies. The exponential trend lines indicate both are increasing at similar rates and have been accelerating since the mid-1980s.
The correlation between annual ablation for south-facing glaciers and maximum global temperature anomalies (Figure 12) equates ablation partly caused by solar radiation with maximum temperature anomalies. The correlation between annual ablation of north-facing glaciers and minimum global temperature anomalies (Figure 13) equates ablation due primarily to warmer air with minimum temperature anomalies.
The similarity of the annual ablation and temperature trends shown in Figure 11 and the significant correlations (r=+0.80) in Figures 12 and 13 suggest that a common heat source is controlling both glacier ablation and temperatures at weather stations throughout the Northern Hemisphere. These results could also be explained by a relatively abrupt increase in solar radiation. However, there is no evidence that the sun's irradiance has changed more than +/- 0.5 % in recent decades - not enough to significantly affect the earth's mean temperature (Wild, et al, 2005).
The earth is currently taking in more energy than it radiates to space causing its surface temperature to increase approximately 0.015 C each year (Hansen et al., 2005, 2011). As the concentration of carbon dioxide increases, the atmosphere will become more unstable and respond to the overload of CO2 in ways that are not easily understood or predictable. One response might be pulses of abrupt heating, such as is indicated after the year 2000 in Figure 10. Another sign of instability is that glaciers are now losing mass at rate greater than the global temperature increase predicts, which suggests that factors other than simple atmospheric heating are responsible for the current high rate of glacier ablation.
The steady rise in the concentration of carbon dioxide in the atmosphere will eventually reach a critical threshold, if it has not already, so that heating variations due to trapped radiation override temperature variations produced by atmospheric circulation. Figure 11 suggests such a threshold was reached about 1990 and that the earth's climate will be undergoing a major shift within the next 2-3 decades.
Figure 12. Annual ablation anomalies of south-facing glaciers versus global maximum temperature anomalies (the same annual averages as Figure 11). The daily temperature anomalies, provided by the Hadley Climate Center, are averages of observations at 7000 weather stations in the Northern Hemisphere, referenced to the 1961-90 period.
Figure 13. Annual ablation anomalies of north-facing glaciers versus global minimum temperature anomalies
The annual mass balances of glaciers that are south-oriented are more negative than those oriented to the north, but the balances at individual elevation intervals are more negative for north-oriented glaciers. The cause of this disparity is the difference in the area-altitude distributions of south and north-facing glaciers. Ablation of the Wrangell Range glaciers is predicted from global temperatures because uniformly distributed atmospheric heating is controlling both glacier ablation and worldwide surface temperatures.
June 12, 2012
Beedle, Matthew , www.GlacierChange.org
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