Modelling the Response of Mountain Glacier Discharge to Climate Warming

Transcription

1 Modelling the Response of Mountain Glacier Discharge to Climate Warming Regine Hock 1*, Peter Jansson 1, and Ludwig N. Braun 2 1 Department of Physical Geography and Quaternary Geology, Stockholm University, SE Stockholm, Sweden 2 Commission for Glaciology, Bavarian Academy of Sciences, Marstallplatz 8, D Munich, Germany * phone , fax , regine.hock@natgeo.su.se Keywords: Climate change, Glacier discharge, Glacier mass balance, Modelling, Storglaciären, Vernagtferner. 1. Introduction Glaciers are characteristic features of mountain environments but are often not recognized for their strong influence on catchment runoff quantity and distribution. Such modification occurs with glacierization of only a few percent of the total catchment area, and affects adjacent lowlands far beyond the limits of mountain ranges. The main impact occurs because glaciers temporarily store water as snow and ice on many different time scales (Jansson et al. 2003), the release from storage being controlled by both climate and internal drainage mechanisms. Mountain glaciers have generally experienced a worldwide retreat and thinning since the beginning of the 20 th century in response to a ~0.6 C increase in mean global temperature (IPCC 2001). General Circulation Models (GCMs) predict enhanced global warming in the coming decades due to anthropogenically-induced greenhouse warming (IPCC 2001), which will likely accelerate the current glacier decline. Consequently, additional water is expected to be released from glacier storage thus modifying current streamflow regimes further. Any change in storage and release of water by glaciers is important for all aspects of watershed management, including the

2 244 R. Hock et al. operation of hydroelectric facilities and flood forecasting, and hence such changes have direct economic implications in many parts of the world. In this paper, we discuss the specific characteristics of glacier discharge and the expected changes from climate warming. We emphasize a need to maintain and establish long-term monitoring programmes combining glacier mass balance and discharge measurements. In addition, further studies are needed to quantitatively assess the impact of climate change on runoff by fully exploiting the growing availability of modern, sophisticated modelling tools. 2. Characteristics of glacier discharge The distinct modification of streamflow by glaciers is well documented in the literature (e.g. Meier and Tangborn 1961; Stenborg 1970; Fountain and Tangborn 1985; Lang 1986; Braithwaite and Olesen 1988; Chen and Ohmura 1990; Collins and Taylor 1990; Hopkinson and Young 1998). Five specific characteristics can be identified (Table 1): 1. Specific runoff: : Total streamflow is reduced in years of positive glacier net balance, when water is withdrawn from the annual hydrological cycle and put into glacier storage. The opposite occurs in years of negative glacier mass balance since water is released from long-term glacier storage, thereby increasing streamflow. 2. Seasonal variation: In climatic regions subject to distinct accumulation and ablation periods, most annual runoff is concentrated during the summer melt season, while runoff is negligible during winter when most precipitation is stored as snow. Kuhn and Batlogg (1998) found the ratio of maximum monthly runoff to mean monthly runoff to increase linearly with increasing glacierization above a 5% level. In contrast to snow-covered basins subject to spring meltwater peaks, the seasonal glacier-meltwater runoff-peak is delayed due to refreezing and firn saturation at the onset of melt. In addition, melt rates are generally higher in summer due to larger sun altitude angles, higher air temperatures and lower albedo of ice compared to snow. 3. Diurnal variation: Glacier discharge is characterized by large diurnal fluctuations caused by the pronounced diurnal cyclicity in meltwater production. Daily peak discharges increase by up to several hundred percent of daily minimum flows during precipitation-free days. 4. Year-to-year runoff variability: Glaciers tend to dampen annual streamflow variations, generally referred to as the glacier compensation effect (Lang 1986), where ablation variations offset precipitation variations. A moderate fraction of 10 to 40% glacierization reduces year-to-year variability to a minimum. The variability becomes larger at both higher and lower glacierization-levels. 5. Runoff correlation: Finally, there is a tendency for runoff to correlate positively with temperature and negatively with precipitation with increasing glacierization, while glacier-free basins show positive correlations between runoff and precipitation. This indicates the dominance of glacier melt water over precipitation as contributor to runoff in highly glacierized basins.

3 Modelling the response of mountain glacier discharge 245 Table 1: Schematic summary of discharge characteristics in highly glacierized basins and expected changes when balanced glacier mass balances turn negative for prolonged periods due to a warming climate. Responses are shown both for an initial phase and a later stage when glacier sizes and volumes have been significantly reduced by long-term glacier mass loss. N 3. Effects of climate warming Various studies have investigated the effects of climate warming on glacier discharge by analysing historic discharge data from glacierized and non-glacierized basins (Fountain and Tangborn 1985; Chen and Ohmura 1990) or by runoff modelling (Singh and Kumar 1997; Kuhn and Batlogg 1998; Braun et al. 2000). The primary effect of climate warming on runoff is increased streamflow caused by larger glacier melt rates. These are further accelerated by positive feedback mechanisms, such as enlargement of bare ice areas that reduce albedo compared to that of snow-cover. Runoff distribution will also be modified by changes in meltwater routing through the glacier, caused by a reduction in the thickness and extent of firn and snow cover. Since water flow velocities through ice considerably exceed those through snow and firn (Fountain and Walder 1998), a reduced firn body and snow cover will reduce the water retention capacity and will thus speed up average transport of water on and through the glacier. Enhanced diurnal discharge peaks and amplitudes are a direct consequence (Fig. 1). The response of glacier runoff to climate warming is a matter of timescale. Although the response is immediate, some response-variables will change sign at a later stage when enhanced melt rates have caused glacier volume to decrease significantly (Table 1). Assuming unaltered precipitation conditions, specific runoff will initially increase due to release of water from storage when mass balances are negative. However, prolonged long-term mass loss will reduce glacier volume, which in turn will lead to reduced water yields (Braun et al. 2000; Jansson et al. 2003). For basins in Washington State, Fountain and Tangborn (1985) found that, for glacier mass balances of 1 m, a mean annual runoff of 2 m and glacierization of 20%, streamflow is enhanced by about 10% due to glacier mass loss. Singh and Kumar (1997) and Kuhn

4 246 R. Hock et al. Figure 1: Short-term effects of climate warming on glacier discharge including feedback mechanisms. and Batlogg (1998) modelled enhanced specific runoff owing to increased glacier melt for Himalayan and Austrian Alpine basins, respectively. Singh and Kumar s study is based on daily time steps and a GCM-output for a future climate forcing, while Kuhn and Battlogg s study uses monthly time steps and assumes uniform temperature shifts. Both assume unchanged glacier size, thus failing to model potential runoff reduction due to glacier retreat. Braun et al. (2000) modelled the effect of a 2xCO 2 scenario, obtained from a regional climate model, on daily discharge of a 40% glacierized basin in the Austrian Alps using the conceptual HBV3-ETH9 runoff model and employing different assumptions on deglaciation. Current water yields during summer are more than doubled using present-day glacier boundaries, while water yields are drastically reduced when a markedly reduced glacierized area is prescribed. In this case, the runoff regime approaches the characteristics of present-day nivo-pluvial regimes of lower-alpine areas. The long-term effect of runoff reduction, resulting from a decrease in ice-covered area, was also detected by Chen and Ohmura (1990), who analyzed multi-decadal discharge records in the Swiss Alps. Climate warming is expected to prolong the melt season, thus reducing seasonal runoff concentration, i.e. the ratio of maximum monthly to mean monthly discharge. In addition, diurnal discharge fluctuation will, at least in an initial phase, be amplified by enhanced daily melt water production and more efficient water transport through the glacier (Braun et al. 2000; Willis et al. 2002). Figure 2 shows the temporal development of bare ice surface at the end of the melt season and the amplitude of the diurnal variation of discharge of Vernagtferner, Austria, since the beginning of measurements in In the 1970s, when mass balance was positive (Fig. 3) and 10 30% of the glacier area consisted of bare ice at the end of the balance year, typical mean diurnal variation of discharge during the melt season was m 3 s -1 with maximum amplitudes of ~5 m 3 s -1, based on hourly mean values. In the 1990s, bare ice area was as large as 90% as a result of strongly negative mass balances (Fig. 3). Mean diurnal variation of summer discharge was up to 3 m 3 s -1 with maximum amplitudes exceeding 10 m 3 s -1. Preliminary modelling results from Storglaciären (3

5 Modelling the response of mountain glacier discharge 247 km 2 ), northern Sweden, on the effects of a temperature rise on hourly discharge are shown in Figure 4. Melt is modelled with a grid-based energy balance model, based on meteorological data collected on the glacier (Hock 1998). Water is routed through the glacier by a linear reservoir approach. After calibration, the observed temperature record is raised by 2ºC. Results show increased runoff, more pronounced daily amplitudes and a prolonged runoff period. Vernagtferner Figure 2: Bare ice area fraction of Vernagtferner (Oetztal Alps, Austria) at the end of the ablation season, and mean (May-October, solid line) and maximum diurnal amplitude of discharge (m 3 s -1 ) at gauging station Vernagtbach (basin area 11.4 km 2 ), for each year between 1974 and 2000, based on hourly means. Dashed lines denote linear trends. Figure 3: Mass balance (b N ) of Storglaciären, northern Sweden, ( ) and Vernagtferner ( ). Running 5-year moving averages are shown as solid lines. Cumulative net balance for both glaciers with a common point at 0 m w.eq. at 1964 (the start of the Vernagtferner mass balance program).

6 248 R. Hock et al. The impact of climate warming on year-to-year runoff variability will depend on initial glacierization, i.e. whether or not glacierization is above or below the ~10 to 40% yielding minimum variability. When glacierization falls below this threshold, runoff variability tends to increase. In the long term, runoff will correlate positively with precipitation instead of air temperature because the percentage of temperaturedependent melt-derived glacier runoff decreases with decreasing glacierization. However, initially, correlation with air temperature may increase since meltwater yield is increased due to rising temperature. Another issue of concern is how climate change will affect glacier runoff in different climatic regions. Hagg and Braun (2003) made a comparison between the Alps and the continental mountain ranges of the Tien Shan, Central Asia, and showed that the effects of a doubled CO 2 climate on glacier runoff are similar in both areas. In an initial phase, runoff is greatly enhanced due to higher temperature, but with decreasing glacier area, water yield will gradually diminish. Once glaciers have disappeared, summer runoff is greatly reduced. Since melt water from glaciers is the only source for water during dry seasons in the continental areas of Central Asia, loss of glaciers will affect lowland areas even more drastically in Asia than in, e.g. Central Europe. Figure 4: Measured and simulated hourly discharge (Q) from Storglaciären, Sweden, 4 August to 6 September Measured air temperature is linearly shifted by 2ºC in the scenario simulation, leaving all other variables and parameters unchanged. Discharge shows the pronounced melt-induced diurnal cyclicity typical of glacier discharge regimes. 4. Research gaps and strategies for future research 4.1 Measurements While previous research has focussed on the impact of climate change on glacier mass balance (e.g. Braithwaite and Zhang 1999) and glacier size variations (e.g. Oerlemans 1994), far less attention has been paid to the effects on glacier discharge. Glaciers have generally been recognized as sensitive indicators of climate change and contributors to sea level change (Meier 1984). More than 280 glaciers have been subject to annual mass balance measurements at one time or another since 1946 (Dyurgerov 2002). Long-term measurements of glacier discharge have often been

7 Modelling the response of mountain glacier discharge 249 promoted by hydroelectric power schemes, but have rarely been assessed from a broader water resources aspect. Only very few glaciers are subject to simultaneous mass balance and discharge monitoring programmes. This may partly be due to practical difficulties associated with measuring stream discharge in the glacier environment, which is characterized by considerable turbulence, high sediment load and frequently changing channel geometry. Glaciers subject to such programmes are, e.g. Vernagtferner, Austria (Escher-Vetter and Reinwarth 1994), and South Cascade Glacier, USA (Krimmel 2001), where streamflow monitoring has been combined with annual mass balance measurements since 1974 and 1958, respectively. Storglaciären, Sweden, has the longest, detailed mass balance record in the world (Fig. 3, Holmlund and Jansson 1999), reaching back to 1945/46. However, proglacial discharge records are only available for a few melt seasons and further downstream (21% glacierization) since Obviously, more combined long-term measurements of mass balance and discharge are needed to detect and analyze the effects of climate change on runoff and as a valuable database for modelling experiments. A monitoring network should emphasize geographical distribution in order to encompass different climate settings. For example, Storglaciären experienced positive mass balances for almost a decade due to enhanced winter accumulation, while Vernagtferner showed strongly negative mass balances during the same period (Fig. 3). Consequently, glaciers in different regions respond, at least initially, differently to climate warming, which in turn will trigger different hydrological responses. 4.2 Modelling Although the characteristics of glacier discharge and their importance for catchment runoff have long been recognized, surprisingly few studies have attempted to quantitatively assess the effects of climate change on glacier discharge using numerical models (e.g. Braun et al. 2000). This dearth of studies is surprising considering the volume of investigations that focus on mass balance changes, although it is primarily their hydrological consequences that are immediate and entail direct economic impacts. To capture the full range of influences of climate warming on glacier discharge, better tools than those previously adopted need to be developed. Modelling of glacier discharge involves two principal steps: (1) modelling of glacier mass balance, i.e. snow accumulation and glacier melt, and (2) discharge routing of melt and rain water through the glacier, i.e. transformation of water inputs into a discharge hydrograph. A large number of energy balance and temperatureindex melt-models have been developed to compute melt. The current trend of these models is towards distributed modelling (summaries in Hock 1998; 2003). Temperature-index models are widely used, promoted by ease of application and low data input requirements. However it remains unclear how model parameters will change under a different climate, a limitation that needs further research. Hence, more physically based energy balance models provide a more reliable tool for climate impact studies, although application is restricted to areas with a sufficiently detailed database, including data on air temperature, humidity, wind speed and radiation. A

8 250 R. Hock et al. critical parameter is to determine whether precipitation falls as rain or snow, e.g. a strong summer snowfall can effectively shut off glacier discharge for an extended period of the melt season. Currently, many conceptual runoff models do not include explicit routing routines for water transport through the glacier, taking into account the different hydraulic properties of snow, firn and ice with respect to throughflow velocities. Such routines are necessary to accurately capture the effects of amplified diurnal discharge cycles, resulting from accelerated runoff generation in response to a reduced extent of firn and snow cover under a warming climate. It is obvious that the impact of climate warming on glacier discharge is complex and differs depending on the time-scale considered, in particular whether or not the glacier varies in size. Hence, modelling strategies need to be adjusted to the purpose of the study. When assessing short-term effects, such as amplified diurnal discharge fluctuations, changes in glacier size can be neglected. However, a high temporal resolution, e.g. hourly time steps, is necessary to capture peak flows, especially when considering the enormous diurnal discharge amplitudes typical of glacier regimes. This effect is important for watershed management and flood forecasting in areas close to glacier headwaters, particularly when peak melt rates and intense rain showers coincide. Modelling long-term effects, such as a decline in streamflow contribution caused by a decrease in glacier size, inevitably requires consideration of changes in glacier area. Thus, it is necessary to couple melt models to flow models or other tools capable of adjusting glacier sizes to climate change. A coarser time resolution, such as monthly or annual time steps, is sufficient to capture area changes. The long-term decrease in specific runoff to be expected from continuous glacier retreat is of major concern, especially in arid to semi-arid areas that are fed by rivers originating from remote glacierized mountain areas (e.g. Zhenniang and Xiaogang 1992). Accurate modelling of this effect is essential for these regions since the risk for water shortage will increase with a warming climate. In summary, the full range of responses to climate change should be addressed by a nested approach, whereby discharge is computed with high resolution, e.g. hourly, while glacier size changes are adjusted only on annual to decadal time scales. To assess glacier sensitivity to climate change, models are often forced by simple linear shifts in climate data records (e.g. Kuhn and Batlogg 1998; Braithwaite and Zhang 1999). Such studies are useful to investigate sensitivities, but preclude the prediction of the real responses to climate change for a specific catchment, since future climate changes will not be homogeneous. Hence, the direct use of results from climate models is preferable in impact studies. General Circulation Models are currently too coarse in resolution, but major advances in regional climate modelling have been made to capture the effects of complex mountain topography in a more realistic way. For glacier hydrology, it is important to consider seasonal variations in predicted climate changes to account for seasonality in snow accumulation and melt. We conclude that more sophisticated and detailed modelling studies are required to predict the full range of climate change impacts on glacier discharge on all time scales. We emphasize the need to model short-term as well as long-term effects including consideration of glacier size changes and direct use of results from climate models for scenario runs. Such studies should adopt more holistic views and an

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