ABSTRACT

INTRODUCTION

Observation of the differing terminus behavior of North Cascade glaciers in response to the same climate changes during the last century (Tangborn and others, 1990), has prompted the evaluation of the terminus behavior of 38 glaciers in the North Cascades for the 1890-1990’s period.  The objective is to determine the characteristics that lead to differential terminus behavior among North Cascade glaciers. 

To complete this task requires examination of the response time of glaciers to climate change.  For any glacier there is a lag time (Ts) between a significant climate change and the initial observed terminus response (Paterson, 1994), this is also referred to as the reaction time of the glacier.  It should be noted that Ts cannot be considered a physical property of a glacier and is expected to depend on the mass balance history and physical characteristics of the glacier.  In this paper Ts is simply defined as the time lag from an observed climate shift to the initial observed change from an advancing to a retreating termini or from a retreating to an advancing termini.    

In addition, for each glacier there is a response time to approach a new steady state for a given climate driven mass balance change (Tm), referred to as length of memory by Johannesson and others (1989). Johannesson and others (1989) defined Tm as the time-scale for exponential asymptotic approach to a final steady state (approximately 63% of a full response), resulting from a sudden change in climate to a new constant climate.  The magnitudes of Ts and Tm are crucial to interpreting past and current glacier fluctuations and predicting future changes (Paterson, 1994; Johannesson and others, 1989).

In nature a step change in climate causing an evolution from an initial to a final steady state never happens (Schwitter and Raymond, 1993).   The variability of climate forcing and the continuous changes in the glacier superimpose new disturbances on the response of the glaciers to previous climate.  This leads to the conclusion that Tm cannot be defined directly from observations in nature.  However, Tm cannot be accurately defined from models alone either. 

In this study we recognize that the response of the terminus is influenced by ongoing changes in forcing, limiting the accuracy of determination of Tm from terminus observations.  We use terminus observations to establish limits on the range of Tm, rather than attempting direct calculation of Tm from observations.  We use terminus observations to establish time limits on the range of Tm.  Our estimates of Tm are based on the understanding that, during a period of relatively constant climate, the glacier should advance to about 2/3 of its final adjustment in its terminus positions and the rate of advance/retreat of the terminus should be reduced to approximately 1/3 over a time period of length Tm. The difficulty of examining terminus response to a specific step change is minimized with respect to the post Little Ice Age warming , because the climate change and associated terminus response is large compared to climate changes and resulting terminus changes that have occurred in the last half-century

(Schwitter and Raymond, 1993; Burbank, 1981; Porter, 1986).  

In the North Cascades the disparate terminus behavior of glaciers has been noted by Hubley (1956), Post and others (1971), Tangborn and others (1990) and Pelto (1993). The primary goal of this paper is to examine the observed terminus behavior and glacier geographic characteristics of 38 North Cascade glaciers to determine why the terminus behavior history varies.  The secondary goal is to identify the ranges of Ts and Tm for the response of North Cascade glaciers to climate changes. 

DATA USED

The North Cascade Glacier Climate Project (NCGCP) has been observing terminus behavior on 47 North Cascade glaciers and measuring annual balance on 9 glaciers since 1984 (Pelto, 1993; 1996).   The primary objective of this glacier research has been to identify the magnitude and timing of glacier response to a climate change.  This NCGCP data set is extensive in its breadth, but not in its length of record, extensive USGS aerial photographic collections and the ongoing research by the USGS on South Cascade Glacier have been indispensable to this research in providing long-term records.

Terminus Observations

The magnitude of terminus changes from the Little Ice Age Maximum (LIAM) has been measured on 38 glaciers by utilizing 720 USGS vertical aerial photographs, taken by Austin Post between 1962 and 1979, 150 aerial photographs taken by Richard Hubley at the University of Washington from 1950 to 1955, and 120 photographs taken by J.B. Richardson and William Long of the US Forest Service from 1940 to 1960 (all of these photographs have been donated to NCGCP by William Long, Austin Post, and the USGS).            

Schwitter and Raymond (1993) noted the ease of identification and utility of the well-preserved Little Ice Age moraines for reconstructing former glacier profiles in the North Cascades and elsewhere.  The distance from the typically well preserved, fresh LIA moraines and trimlines to the current glacier front has been measured in each case using a laser ranging device (+1m).  Additionally on each of the 38 glaciers field measurements from the same LIA moraines to the current glacier front was completed on at least two occasions between 1984 and 1998.  The goal being to verify both the LIAM and the actual terminus position changes from 1984-1998.    In cases where the field measurements and photographic measurements differed by more than 20 m the glacier is not used in this analysis.  This was the case on two glaciers. 

Terminus change from 1850-1950 is the distance from the aforementioned LIAM and the position of the glacier terminus as in 1950 noted by Hubley (1956).   Terminus change from 1950-1979 is the change between the position noted by Hubley (1956) and the USGS aerial observations in 1979.  Neither, 1950 or 1979 perfectly match the timing of climate changes noted in the following section; however, they are closest to the climate shifts, beginning in 1944 and 1976 respectively, for which adequate aerial photographic observations were made.  Terminus change from 1979 to the present is based on comparison of the USGS aerial observations and repeated field measurements of the North Cascade Glacier Climate Project from 1984-1998 (Pelto, 1993; 1996).

To determine Ts a group of 21 North Cascade was observed to switch from retreat to advance shortly after 1944, and from advance to retreat shortly after 1976.  Observations of the initial post-1944 glacier advances were made by Long (1955 and 1956) and Hubley (1956).  Observations of the onset of retreat after 1976 are from Heikinnen (1984), NCGCP field observations (Pelto, 1993) and USGS aerial photographs from 1979. 

Glacier Characteristics

On seventeen of the aforementioned 38 glaciers more detailed observations are used to calculate theoretical estimates of Tm for comparison with observations of terminus behavior.  Each of these 17 glaciers had at least five terminus observations during the 1850 to 1998 period (Table 1)(Hubley, 1956; Long, 1955; Meier and Post, 1962; Pelto, 1993; Post and others, 1971).  To be able to calculate Tm terminus velocity u(t), mass balance near the terminus b(t), ice thickness (h) and glacier length (l) have to be determined.  Nine of the 17 glaciers have had annual balance measurements over a span of more than 10 years (Table 1)(Pelto, 1996; Krimmel, 1996).  Krimmel (1999) has observed b(t) and u(t) on South Cascade Glacier.  On the other 16 glaciers b(t) and u(t) have been directly observed for at  least two hydrologic years using stakes drilled into the glacier terminus area by NCGCP (Pelto, 1996; Krimmel, 1999). Measurement of u(t) relied on at least three points, in the lower 25% of the glacier’s length, which is within 200 m of the terminus in each case.    The longer term annual balance measured on nine of these glaciers indicates that the period of record for terminus mass balance measurement (1994-1996) was close to the 1984-1997 mean.

In 1998, Krimmel (1999) observed the length of the South Cascade Glacier.  Glacier length on the other 16 glaciers length has been taken from the most recent USGS maps compiled from aerial photographs between 1982 and 1984, and adjusted for the observed retreat up 1998.

Glacier thickness (h) has been measured on three North Cascade glaciers South Cascade Glacier (Krimmel, 1970), Easton Glacier (Harper, 1993) and Lewis Glacier.   The first two are both comparatively large glaciers for the area.  Each had a maximum mean profile depth of 60-80 m.   These measurements along with those of Driedger and Kennard (1986) on Cascade volcanoes, which yielded a mean thickness of 50 m on Mt. Rainier, and 35 m on Mt. Hood, illustrate that these glaciers are comparatively thin.  On Lewis Glacier the thickness was measured in a moulin that reached the base of the glacier in 1985, 1986 and 1987.  The thickness of each of the other 14 glaciers where direct measurement has not been made is assumed to be 50 m on thin concave or slope glaciers and 100 m on thicker convex or valley glaciers.  In both cases the resulting h is a maximum value, thus the Tm would be a maximum value too (Post and others, 1971).

Climate Data

A good single climate indicator for North Cascade glaciers behavior is the Pacific Northwest Index (PNI), developed by Ebbesmeyer and others (1991).  Figure 1 is a graph of the PNI during this century.  The index is based on March 15 snowpack depth at Paradise on Mt. Rainier, WA, mean annual temperature at Olga, WA., and total annual precipitation at Cedar Lake, WA.  Positive values reflect negative glacier mass balances. 

 TERMINUS OBSERVATIONS

Since the maximum advance of the Little Ice Age (LIA) there have been three climate changes in the North Cascades sufficient to substantially alter glacier terminus behavior.   During the LIA mean annual temperatures were 1.0-1.5^oC cooler than at present (Burbank, 1981: Porter, 1986).   The lower temperatures in the North Cascades led to a snowline lowering of 100 to 150 m during the LIA (Porter, 1986; Burbank, 1981).  Depending on the glacier, the maximum advance occurred in the 16th, 18th, or 19th century (Miller, 1971; Long, 1956).   North Cascade glaciers maintained advanced terminal positions from 1650-1890, emplacing one or several Little Ice Age terminal moraines. 

Retreat from the LIAM was modest prior to a still stand in the 1880’s (Burbank, 1981, Long, 1956).  Miller (1971) mapped the age of terminal moraines in front of Chickamin and South Cascade Glacier and found in each case that a late-19^th century moraine was emplaced within 100 m of the LIAM.  On Mt. Rainier, just south of the study area, mapping of terminus changes by (Burbank, 1981) indicate that rapid and continuous retreat of Mt. Rainier glaciers from their LIAM began after the 1880-1885 still stand.   Long (1953) noted that retreat on Lyman Glacier and Easton Glacier became substantial only after 1890.  Long (1955) noted that the Lyman Glacier has been retreating steadily since the 1890’s.  Climate warming and retreat began about 1850, but because of the modest retreat and subsequent stillstand or advance of North Cascade glaciers around 1880, 1890 is used as the approximate time for the climate change that initiated a continuous substantial retreat from the LIA advanced positions in the North Cascades. 

This first substantial climate change was a progressive temperature rise from the 1880’s to the 1940’s.  The warming led to ubiquitous rapid retreat of North Cascade Range alpine glaciers from 1890 to 1944 (Rusk, 1924; Burbank, 1981; Long, 1955; Hubley, 1956). Each North Cascade glacier retreated significantly from its LIAM.   It must be emphasized that the entire retreat noted from the LIAM to 1944 did not occur in the 1890-1944 interval, observations do not exist on most glaciers to distinguish the exact timing of the initial post LIAM retreat, though retreat was minor before 1890 on glacier where observations exist.  Average retreat of glaciers on Mt. Baker was 1440 m from LIAM to 1950 (Pelto, 1993).  Average retreat of the 38 North Cascade glaciers in this study was 1215 m. 

The PNI from 1895-1923 is rising, but has a comparatively low mean (-0.34), that is still capable of generating retreat on North Cascade glaciers, that are all still in advanced positions from the LIA.  The PNI average during the 1924 –1944 was the high at 0.44.  This warm dry period has been noted around the world and in the North Cascades as a period of rapid glacier retreat (Burbank, 1981; Long, 1955; Hubley, 1956).  The second substantial change in climate began in 1944 when conditions became cooler and precipitation increased (Hubley 1956; Tangborn, 1980).  The climate change in 1944 is evident in the PNI record (Figure 1). The mean PNI from 1945-1976 was -0.37. This drop in the PNI average of 0.76 from the previous interval marks the climate change that initiated the advance of some North Cascade glaciers and the more positive mass balance for that period (Tangborn, 1980; Hubley, 1956). 

Hubley (1956) and Long (1956) noted that North Cascade glaciers began to advance in the early 1950s, after 30 years of rapid retreat.    This change was reflected in the mass balance of North Cascade glaciers.  A Runoff-Precipitation model constructed for South Cascade Glacier (Tangborn, 1980) yielded a mean annual balance of –1.15 m/a from 1924-1944, compared to –0.15 m/a from 1945-1976. 

Approximately half the North Cascade glaciers advanced during the 1950-1979 period (Hubley, 1956; Meier and Post, 1962).  Advances of Mount Baker glaciers ranged from 120 m to 750 m,  ana verage of  480 m, and ended in 1978 (Heikkinnen, 1984; Harper, 1993; Pelto, 1993).  All 11 Glacier Peak glaciers that advanced during the 1950-1979 period emplaced an identifiable maximum advance terminal moraine, the mean advance was 295 m.  Of the 47 glaciers that NCGCP has observed during the 1984-1998 period, 15 advanced during the 1950-1978 period. 

The final climate change was a step change in 1977 to a drier-warmer climate in the Pacific Northwest (Ebbesmeyer and others, 1991).  The mean PNI from 1977-1998 was 0.53, even higher than the 1924-1944 period, indicating a warmer drier period that reestablished the ubiquitous retreat of North Cascade glaciers (Pelto, 1993).  This change impacted glacier mass balance, alpine streamflow, and alpine snowpack (Ebbesmeyer and others, 1991).   The impact on North Cascade glacier mass balance is evident from the USGS long-term record of South Cascade Glacier (1958-1998), where mean annual balance was –0.15 m/a from 1958-1976, in contrast to –1.00 m/a from 1977-1998 (Krimmel, 1999).

The retreat and negative mass balances of the 1977-1998 period have been noted by Harper (1993), Krimmel (1994 and 1999), and Pelto (1993 and 1996).  By 1984, all the Mount Baker glaciers, which were advancing in 1975, were again retreating (Pelto, 1993).  In 1997-98, NCGCP measured the retreat of eight Mount Baker glaciers from their recent maximum position (late 1970’s-early 1980’s).  The average retreat had been -197 m.   All of the Mt. Baker termini are still in advance of their 1940 position as observed in photographs from J.B. Richardson, and observations by Austin Post (pers. comm.).  However, the glacier region between the current terminus and 1940 terminus is nearly stagnant on each of the glaciers.  Between 1979 and 1984, 35 of the 47 North Cascade glaciers observed annually by NCGCP had begun retreating.  By 1992 all 47 glaciers termini observed by NCGCP were retreating (Pelto, 1993), and two had disappeared, Lewis Glacier and Milk Lake Glacier. 

INITIAL TERMINUS RESPONSE

                  The time between the onset of a mass balance change and the onset of a significant change in terminus behavior is called the initial terminus response time or reaction time (Ts).  As indicate previously Ts is a descriptive quantity that quantifies the time lag between climate forcing and terminus response for a particular climate event, rather than a physical property of a glacier Ts in this study is based solely on the first observed terminus change from retreat to advance after 1944, and from advance to retreat after 1976.  Ts has been identified from the response of North Cascade glaciers to the relative cooler and wetter weather beginning in 1944 (Hubley, 1956; Long 1955 and 1956; Tangborn, 1980), and to the subsequent warmer and drier conditions beginning in 1977 (Ebbesmeyer and others, 1991; Pelto, 1988; Pelto, 1993; Krimmel, 1994; Harper, 1993).   

Focusing on 21 North Cascade glaciers that responded to these two climate shifTs, all having an area under 10 km², the initial terminus response invariably is less than 16 years (Pelto, 1993; Hubley, 1956; Harper, 1993).   Table 2 is a list of 21 North Cascade glaciers where Ts has been noted for both advance and retreat for the post-1944 period by Hubley (1956), or Long (1955 and 1956).  In each case the glaciers were observed to be in retreat during the 1940’s, and subsequently each advanced within 16 years of the climate change.  Table 2 also notes the response of the same glaciers from a period of advance by each glacier in the early 1970’s to retreat by 1988, 12 years after the climate change (Pelto, 1988 and 1993; Harper, 1993).  The observed Ts is not significantly different on individual glaciers for initiation of advance, versus initiation of retreat. 

Many North Cascade glaciers did not respond to these two climatic changes.  This may be the result of a longer Ts, or as more likely is either due to an ongoing retreat caused by continuing negative mass balances, or that climate change was insufficient to substantially alter mass balance on these glaciers.

ANALYSIS AND CLASSIFICATION OF THE RESPONSE OF THE GLACIERS

                The 38 North Cascade glaciers, where the terminus history has been determined for the 1890-1998 period exhibit three distinct patterns (Table 3): 1) Retreat from 1890 to 1950 then a period of advance from 1950-1976, followed by retreat since 1976.  2) Rapid retreat from 1890 to approximately 1950, slow retreat or equilibrium from 1950-1976 and moderate to rapid retreat since 1976. 3) Continuous retreat from the 1890 to the present.  Distinction of a glacier’s Type is based solely   on iTs terminus behavior in this analysis.