1 (PRAA) Climate Change in the Tropical Andes

7/27/2019 1 (PRAA) Climate Change in the Tropical Andes

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Part I: The scientific basis

A report prepared by

MATHIAS VUILLE

with contributions from

RAYMOND S. BRADLEYBERNARD FRANCOUGEORG KASERBRYAN G. MARK


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Climate Change in the tropical Andes Impacts and consequences for glaciation

and water resources

Part I: The scientific basis

A report for CONAM and theWorld Bank

prepared by

MATHIAS VUILLE(University of Massachusetts)

with contributions from

RAYMOND S. BRADLEY (University of Massachusetts)BERNARD FRANCOU (IRD)

GEORG KASER (University of Innsbruck)BRYAN G. MARK (Ohio State University)

Amherst, Massachusetts, 24. January, 2007

Cover photo: Cordillera Vilcanota, June 2006 (photo credit: M. Vuille)


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TABLE OF CONTENTS

SUMMARY......................................................................................................................2

1) INTRODUCTION ...................................................................................................5

2)OBSERVED GLACIER VARIATIONS..........................................................72.1. Venezuela ..................................................................................................72.2. Colombia.....................................................................................................72.3. Ecuador .....................................................................................................102.4. Peru.............................................................................................................12

2.4.1. Quelccaya Ice Cap.......................................................................132.4.2. Cordillera Blanca ..........................................................................142.4.3. Coropuna.......................................................................................17

2.5. Bolivia.........................................................................................................182.5.1. Charquini, Chacaltaya and Zongo glaciers ..............................192.5.2. Cordillera Occidental (Sajama) ..................................................22

2.6. Summary ..................................................................................................22

3) GLACIER MASS BALANCE ............................................................................24 3.1 General characteristics of tropical glacier m. balance ...........243.2. Mass balance - climate relationships ...........................................263.3. Mass balance and large-scale forcing .........................................28

4) GLACIER SURFACE ENERGY BALANCE ..............................................314.1. Sensitivity studies .................................................................................31

4.2. The Surface Energy Balance (SEB) .............................................335) OBSERVED AND PROJECTED CLIMATE CHANGE.........................37

5.1. Observed 20thcentury climate change ........................................375.1.1. Temperature .................................................................................375.1.2. Precipitation ..................................................................................415.1.3. Humidity.........................................................................................42 5.1.4. Cloud cover ...................................................................................435.1.5. Atmospheric circulation ...............................................................44

5.2. Projected future climate change.....................................................46

6) IMPLICATIONS FOR WATER RESOURCES .........................................496.1. Antizana ....................................................................................................516.2. Cordillera Blanca ...................................................................................526.3. Cordillera Real........................................................................................57

7) CONCLUSIONS....................................................................................................59

REFERENCES............................................................................................................61


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SUMMARY

Observations on glacier extent from Venezuela, Colombia, Ecuador, Peru andBolivia give a detailed and unequivocal account of rapid shrinkage of tropical Andeanglaciers since the Little Ice Age (LIA). The retreat appears to have been non-uniform

throughout the 20th

century with periods of stronger recession interrupted by phases ofmore stable conditions or even minor readvances such as in the 1970s and at the end ofthe 20thcentury. In general however, there is clear evidence ongoing shrinkage since themid-1970s. Many smaller, low-lying glaciers are completely out of equilibrium withcurrent climate and may disappear within a few decades. Mass balance records fromBolivia and Ecuador similarly show a very coherent picture, with a generally negativemass balance, which appears to be driven by the same background forcing throughout theregion. Superimposed on this negative trend are interannual variations with occasionalperiods of equilibrated or even positive mass balance, in particular during prolonged LaNia events, such as between 1999 and 2001.

Glaciers grow or shrink as a reaction to changes in their mass balance, an obvious

reaction being the advance or retreat of their tongues. The mass balance describes whereand how a glacier is gaining or losing mass due the predominance of accumulation orablation processes, which in turn are determined by climatic variables such astemperature, precipitation, solar radiation, humidity, etc. Because of the lack of apronounced thermal seasonality but a clear differentiation between dry and wet seasons,tropical glacier mass balance and its sensitivity to climate change are fundamentallydifferent from mid- and high latitude glaciers. Inner tropical glaciers have a very negativemass balance below their Equilibrium Line Altitude (ELA) due the exposure of theablation zone to melt and sublimation 365 days per year. In the subtropics ablation ismuch reduced, mostly because sublimation dominates over melt due to generally lowhumidity. Because of these characteristics the ELA is generally close to the 0C-line in

the inner tropics and hence the ELA will react sensitive to changes in temperature. In theouter tropics the ELA is usually located considerably above the 0C-line, and atemperature increase will not have such an immediate effect. The ELA instead is moresensitive to changes in precipitation and humidity, which determines the ratio of meltingto sublimation.

In general the glacier mass balance in the outer tropics reflects the variability inwet season accumulation and ablation, while mass turn over is minor during the dryseason. In the inner tropics on the other hand, mass net ablation remains quite constantthroughout the year. The most dominant forcing factor on interannual timescales isassociated with ENSO. During El Nio years subtropical glaciers experience reducedaccumulation, a lowered albedo, combined with an increase in incoming shortwave

radiation due to reduced cloud cover. All these factors contribute to a very negative massbalance. In the inner tropics the glacier response to ENSO is very similar but for differentreasons. Here the impact of El Nio is through increased air temperature, which favorsrain over snowfall, and to a lesser degree due to sporadic snowfall, insufficient tomaintain a high glacier albedo, low wind speeds, which limit the transfer of energy frommelting to sublimation, and reduced cloud cover, which increases the incoming short-wave radiation.


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Several terms of the energy balance, show pronounced seasonality with verydifferent behavior during dry and wet seasons. In the outer tropics both mass loss andrunoff show a marked seasonality, which can not be explained by the sensible heattransfer, which show little seasonality and is generally small. Instead net radiation and thelatent heat flux dominate the energy balance. Incoming energy is quite constant

throughout the year and instead it is the partitioning of this energy into melt andsublimation, controlled by humidity, which causes the much higher mass loss during thewet season. When humidity is high (wet season), the available energy is directlyconsumed by melting, while the enhanced vapor pressure gradient during the dry seasonfavors sublimation, which is energetically inefficient and therefore leads to reduced massloss. These peculiarities of the energy balance make subtropical glaciers highly sensitiveto a) changes in atmospheric humidity which governs sublimation, b) precipitation,whose variability, particularly during the rainy season induces a positive feedback onalbedo and c) cloudiness, which controls the incoming long-wave radiation. In the innertropics the absence of thermal seasonality exposes the ablation zone to oscillations of the0C isotherm throughout the year. These small fluctuations in temperature determine the

rain-snow line on the ablation zone and hence have a major impact on the albedo.Consequently air temperature significantly influences the energy balance in the innertropics, albeit not through the sensible heat flux, as commonly thought, but indirectlythrough changes in albedo and net radiation receipts.

The high sensitivity of tropical glacier mass and energy balance to climatechange, which is under way and well documented, leaves little room for doubt that theobserved glacier retreat is occurring in response to a changing climate. Temperature inthe Andes has increased by approximately 0.1C/decade, since 1939, with the bulk of thewarming occurring over the last 2 decades. Since the mid 1970s the rate of warmingalmost tripled to 0.3C/decade. The eastern slopes show a much subdued warming, whilethe warming on the Pacific side is strongest. Higher elevations have experienced anintermediate, albeit still significant warming. On average about 50-70% of the observedtemperature change in the Andes, can be attributed to a temperature increase in thetropical Pacific. Observations suggest that precipitation has slightly increased in thesecond half of the 20th century in the inner tropics and decreased in the outer tropics.However, trends at individual stations are weak and mostly insignificant. Nonetheless thegeneral pattern of moistening in the inner tropics and drying in the subtropical Andes isdynamically consistent with observed changes in the large-scale circulation. Both satelliteinformation and reanalysis data seem to suggest a strengthening of the tropicalatmospheric circulation. Changes in the inner tropics are characterized by enhanced low-level convergence, upper-level divergence, and as a result enhanced upward motion,increased convective activity, and more humid conditions. In the subtropics the oppositetrends prevail, with increased subsidence and reduced convective activity leading topotentially drier conditions.

Model projections of future climate change in the tropical Andes indicate acontinued warming of the tropical troposphere throughout the 21st century, with atemperature increase that is enhanced at higher elevations. By the end of the 21 stcentury,following the SRES A2 emission scenario, the tropical Andes may experience a massivewarming on the order of 4.5-5C. The Special Report on Emission Scenarios (SRES)A1B scenario reaches about 80-90% of the warming displayed in the SRES A2 scenario


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at the end of the century, while the more moderate SRES B1 path displays only abouthalf of the warming of SRES A2. All emission paths tend to show the same pattern ofwarming, but they differ in amplitude. Predicted changes in precipitation include anincrease in precipitation during the wet season and a decrease during the dry season,which would effectively enhance the seasonal hydrological cycle in the tropical Andes.

If tropical glaciers continue to retreat and eventually disappear from certaincatchments, the change in streamflow, due to the lack of a glacial buffer during the dryseason, will significantly affect the availability of drinking water, water for hydropowerproduction, mining and irrigation. In the tropical Andes the problem is exacerbated whencompared with mid latitude mountain ranges because ablation and accumulation seasonscoincide, which precludes the development of a long-lasting seasonal snow cover outsidethe glaciated areas. Glaciers are therefore the only major seasonally changing waterreservoir in the tropical Andes. Tropical Andean catchments show a high correlationbetween their capacity to store precipitation and their percentage of glaciated area. Asglaciers retreat and lose mass, they add to a temporary increase in runoff. Downstreamusers will quickly adapt to this temporary increase in water supply, which raises serious

sustainability concerns. In the Cordillera Blanca at least 10%, and potentially as much as20%, of the annual discharge stems from volume loss of stored glacier ice. Simulationsbased on different IPCC scenarios for 2050 and 2080 indicate that glacier volume in theCordillera Blanca will be significantly lowered, but glaciers in most catchments do notcompletely disappear. Simulations further suggest that the overall discharge may notchange very much, but that the seasonality intensifies significantly. Dry season runoff isreduced, in particular in the A2 scenario, but during the wet season discharge is higher,since the larger glacier-free area leads to enhanced direct runoff. In general the results ofthe A2 scenario are much more dramatic in 2050 than they are 30 years later, in 2080under the more moderate B1 scenario. These results illustrate how uncertain the futureextent of glaciation and therefore the changes in runoff really are; they clearly depend onwhich emission path we will ultimately follow.

In order to improve our knowledge and to enhance our understanding to a levelwhere useful decisions regarding adaptation and mitigation can be made, a number ofscientific and institutional improvements are necessary. These include a better equippedand denser on-site monitoring network, enhanced use of the available remote sensing andGIS technologies, more adequate modeling studies which take into account thetopographic and climatic peculiarities of the tropical Andes, a better collaboration amongthe scientists and institutions involved, and finally, a better dissemination of results tolocal stakeholders and decision-makers.


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1) INTRODUCTION

In the arid and semiarid regions of the tropics and subtropics more than 80% ofthe freshwater supply originates in mountain regions, affecting more than half of theearths population (Messerli, 2001). Much of this water is initially stored as ice in

mountain glaciers and then gradually released over time. Mountain glaciers, such as thosefound in the tropical Andes, therefore act as a critical buffer against highly seasonalprecipitation and provide water at times when rainfall is low or even absent. At the sametime these glaciers are particularly sensitive to climate change because they areconstantly close to melting conditions. They are arguably the most visible indicator ofclimate change, due to their fast response time, their sensitivity to climate variations andthe clear visibility of their reaction (glacier growth or shrinkage) to the public.

Indeed the existence and the shrinkage of glaciers in the tropical Andes are deeplyrooted in the perceptions of local indigenous people living in the Andes. For example amountain in the Cordillera Blanca that was once called sleeping lion due to the shape ofits glacier is now called lion has left (i.e. leon dormido has become leon se ha ido;

Young and Lipton, 2006). For indigenous people in the Cuzco region local myth holdsthat when the snow disappears from the tops of the mountains, it will herald the end ofthe world (Regalado, 2005). While the ramifications of disappearing glaciers may inreality not be quite that dramatic, major repercussions will be felt throughout Andeancountries, which rely on fresh water from glaciated basins during the dry season. Glaciersretain water that falls on the glacier as snow and release it later so that water is availablefor domestic, agricultural or industrial use even at times when rainfall is low or absent.

This looming threat of changes in water supply associated with tropical glacierretreat has received little attention so far, mostly because the climate change communityis very much focused on observed and projected large climatic changes at high northernlatitudes. However, these global projections rely on models, which due to their coarse

resolution, are inadequate to resolve the steep topography of long and narrow mountainchains, such as the Andes. As a consequence, climate change at high tropical locales isnot well simulated in these models. Indeed, when considering the rate of warming in thefree troposphere (e.g. Bradley et al., 2004, 2006) rather than at the surface, it becomesevident that warming in the tropical Andes is likely to be of similar magnitude as in theArctic, and with consequences that may be felt much sooner and which will affect a muchlarger population.

This report provides the scientific basis regarding climate change and glaciers inthe tropical Andes. It is meant to facilitate a discussion amongst policy makers and localstakeholders, who are in charge of implementing adaptation and mitigation strategies.Such measures, designed to alleviate the magnitude and impacts of climate change, can

only be put in place with an accurate and detailed knowledge on how future climatechange will affect glaciological and hydrological systems in the Andes. Therefore it isimportant to first summarize the current state of knowledge regarding climate change,tropical glaciology and potential future impacts on Andean water supply. This report firstdocuments observed historical changes in glaciation in all tropical Andean countries(section 2). A detailed description of how glaciers interact with and respond to changes inclimate by adjusting their mass balance (section 3) and how the energy received at theglacier surface is consumed by melting and sublimation processes (section 4) is given


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next. A review of observed changes in climate during the 20 th century as well asprojections of how climate might change during the 21stcentury (section 5) is needed toput the observed cryospheric changes into a climatic context. Only after a thoroughreview and discussion of all these factors is it possible to assess the potentialramifications of the observed and projected future glacier retreat for glacier discharge and

downstream water supplies (section 6). Finally this reports ends with some concludingremarks and some recommendations as to how the scientific network in the Andes shouldbe expanded (section 7).


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2) OBSERVED GLACIER VARIATIONS

In the following sections a review of glacier fluctuations in the tropical Andes isprovided, for Venezuela, Colombia, Ecuador, Peru and Bolivia. Although Chile also hasa few glaciers which can be considered tropical in the broadest sense in its northernmost

corner, these glaciers are all located along the border with Bolivia in the CordilleraOccidental, and they are therefore covered in the Bolivian chapter.We limit our analysis to fluctuations since the Little Ice Age (LIA) when glaciers

in most of the tropical Andes reached a maximum extent. Most of the discussionhowever, will be restricted to the 20th century, with a special emphasis on the last 3-4decades. Since the 1970s much more detailed information has become available, thanksto the initiation of several glacier monitoring programs on selected glaciers and theavailability of various new satellite products. This chapter is purely descriptive, showingwhere and when glaciers changed in size, length and volume (mass), but it does notaddress the climatic causes or the potential impacts of the observed change. These issueswill be dealt with in later sections.

2.1. Venezuela

Glaciers in Venezuela are currently restricted to only 3 peaks in the Sierra deMerida: Pico Bolivar (5002 m), Humboldt (4942 m) and Bonpland (4839 m). Combinedthe 5 remaining cirque glaciers cover less than 2 km2and extend down to elevations of4450 m. Glaciers at these locations have been continuously retreating during historicaltimes and are currently not in equilibrium with the modern climate. A comparison ofearly sources (reports and paintings) with present-day conditions indicates a rapid glacierretreat during the last 100 years. Reports by Schubert (1992, 1998) show that these

glaciers have lost more than 95% of their glacier-covered area since the mid-19

th

century.Their extent was estimated at approximately 10 km2in 1910 and about 3 km2in 1952. Ofthe 10 glaciers mapped in 1952, 4 have completely or almost completely disappeared, 1has disintegrated into firn patches, and the remaining 5 are substantially smaller.

2.2. Colombia

Glacier monitoring in Colombia grew mostly out of concern for natural hazards.The combination of glaciers, active volcanoes and earthquakes poses a significant threat.The 1985 lahar on Nevado del Ruiz was one of the deadliest ever recorded, with 23,000

deaths and most of Armero, a city of about 25,000, disappearing under a mudflow(Hoyos-Patino, 1998). In June 1994, an avalanche originating in the Nevado del Huilakilled at least 1,500 people (Hoyos-Patino, 1998). Nonetheless, glaciology is still a veryyoung science in Colombia and all information on glacier change is based on glacierlength observations. Mass-balance studies are just starting and results from theseprograms are not yet available (Ceballos et al., 2006).

During the LIA, glaciers in Colombia advanced to about 4200 m a.s.l. in the southand to about 4600 m a.s.l. in the north, occupying a total area of about 374 km 2(Florez,


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1992). Dramatic glacier recession took place in the 20thcentury, most pronounced fromthe mid-1980s onward. However, various figures have been given for the total extent ofthe remaining glacierized area in Colombia. Jordan et al. (1989) estimated a total of 246glaciers on 9 mountains, with a total glacierized area of 109 km2. This work was based onfieldwork and aerial photography dating from 1957 to 1978. Hoyos-Patino (1998)

measured the extent of ice-and-snow areas on Landsat-MSS images from the early 1970'sand determined a total area of 104 km2. Thouret et al., (1996) gave a range from 100 to112 km2for the presently glacierized areas of Colombia. In 2003, according to Ceballoset al. (2006), glacier termini had retreated to 4700 4900 m a.s.l. and the total glacierizedarea had shrunk to 55.4 km2.

Today six different mountain ranges still have glacier coverage, while eightNevados (formerly glacierized areas) have lost their glaciers entirely during the 20thcentury (Ceballos et al., 2006). The currently glaciarized mountains are: the SierraNevada de Santa Marta, Sierra Nevada del Cocuy and Nevados del Ruiz, de Santa Isabel,del Tolima and del Huila. The glacierized areas on these mountains vary from 1 to 20km2.

The regions most affected by glacier shrinkage are the Sierra Nevada de SantaMarta and the Nevado de Santa Isabel (Figure 1). In the former, a 50% area loss has beenobserved in the last 20 years, while the latter showed a 50% loss in just 15 years(Ceballos et al., 2006). Sierra Nevada del Cocuy and Nevados de Tolima and del Ruizhave also been greatly affected, losing 3545% of their glacier area in the last 1517years of the 20th century. Glaciers on Nevado del Huila showed a comparativelymoderate shrinkage, with about a 20% loss in the last 20 years (all estimates fromCeballos et al., 2006).

Figure 1: Glacier extent on Nevado de Santa Isabel as seen from aerial photography in 1959 and 1996 anda Landsat Thematic Mapper satellite image in 2002 (from Ceballos et al., 2006).

Glacier length measurements conducted since the 1980s reveal a similar picture ofrapid retreat. On Nevado de Santa Isabel, glacier termini retreated 170250 m in only 15years (1988 2003; Figure 2), which translates into an annual retreat rate of 1117 m. Inthe Sierra Nevada del Cocuy, where the total glacier area is about six times larger than onNevado de Santa Isabel, the retreat at some glacier tongues was nearly 500 m in the last


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18 years (Figure 3). This is a very large value since the total lengths of individual glaciersin the Cocuy region are about 11.5 km (all estimates from Ceballos et al., 2006).

Figure 2: Cumulative retreat of different glacier tongues of Nevado de Santa Isabel

(from Ceballos et al., 2006).

Figure 3: Cumulative retreat of different glacier tongues in Sierra Nevada del Cocuy (from Ceballos et al.,

2006).

The small elevation range of most Colombian glaciers makes them particularly

vulnerable to climate change and therefore future disappearance of several glaciers isprojected for the next few decades (Hoyos-Patino, 1998; Ceballos et al., 2006). Inaddition, edge effects may become increasingly more dominant, especially for the smallerice caps, such as on Nevado de Santa Isabel (Figures 1-2). The extraordinary loss ofalmost 40% of its area in only 6 years from 1996 to 2002 supports this hypothesis andmakes a further rapid shrinkage likely (Ceballos et al., 2006). The larger and compact icecaps on the active volcanoes Nevados del Ruiz and del Huila have experienced slightly


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(Ruiz) and significantly (Huila) smaller glacier retreat in the last 20 years (Ceballos et al.,2006).

2.3. Ecuador

Ecuador's glaciers are situated closer to the equator than any other Andeanglaciers and are therefore considered typical for examples of continental tropicalglaciation (Hastenrath, 1981; Jordan and Hastenrath, 1998). The glaciers are restricted tothe highest peaks, mostly of volcanic origin, and do not contain large contiguous icefields, such as those found in Peru, or Bolivia. Instead the glaciers occur as ice caps thatfeed numerous outlet glaciers and are confined to the limited summit areas (Jordan andHastenrath, 1998).

The glaciers in Ecuador are located on two mountain chains, the CordilleraOccidental and the Cordillera Oriental. According to Jordan and Hastenrath (1998) 4mountains are glacierized in the Cordillera Occidental, and 13 in the Cordillera Oriental.

Glaciers are more common in the Cordillera Oriental because it is exposed to themoisture supply from the Amazon basin. The total glacierized area in Ecuador in 1998was 97.21 km2, of which 21.92 km2was located in the Cordillera Occidental and 75.29km2in the Cordillera Oriental (Jordan and Hastenrath, 1998).

Various historical sources indicate a rather extensive glaciation from the 1500's tothe first part of the 1800's, followed by a drastic ice recession starting around the middleof the past century and continuing to the present time (Hastenrath, 1981).

More information is available for the last few decades, thanks primarily to thetropical glacier monitoring program, spearheaded by the French IRD (Pouyaud, et al.,1995). Detailed measurements on Antizana 15 glacier, including monitoring of glacierlength changes and glacier mass balance have been conducted since 1994 (Semiond et

al., 1997; 1998). This ice cap, located only 40 km east of Quito is of special interestgiven the use of its glacial runoff for the capitals water supply (Francou et al., 2004).Aerial photogrammetry, starting in 1956 has been used to put the on-site monitoring,which started in 1995, in a longer-term perspective (Francou et al., 2000; 2004). Resultsshow that the glacier retreated 7-8 times faster between 1995 and 2000 than during theprevious period 1956-1993 (Figure 4; Francou et al., 2000). This period was influencedby strong El Nio events and a later study (Francou et al., 2004) subsequently confirmedthe strong sensitivity of the glacier mass balance on Antizana (Figure 5) to ENSOextremes.

Aerial photography was also used on Cotopaxi volcano to reconstruct glacierextent and recession since the mid 1950s (Jordan et al., 2005). Results from this study

show that glaciers on Cotopaxi were almost stagnant between 1956 and 1976 and thenlost approximately 30% of their surface area between 1976 and 1997 (Figure 6). Thecalculated total mass (thickness) loss on selected snouts of Cotopaxi between 1976 and1997 equals 78 m, or 3-4 m w.e. yr-1, consistent with similar values obtained on Antizana.


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Figure 4: Antizana Glacier 15: terminus fluctuations over the last four decades (from Framcou et al.,

2000).

Figure 5: Monthly and cumulative mass balance on Antizana 15a glacier (ablation zone) between 1995 and

2002 (from Francou et al., 2004).


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Figure 6: Cotopaxi glaciers in orthophoto (left) and retreat between 1976 (red) and 1997 (blue). (fromJordan et al., 2005).

2.3. Peru

The Peruvian Andes contain the largest fraction of all tropical glaciers, andglaciers in Peru are among the best studied in the tropical Andes. In 1988 the total icecovered area was estimated at 2,600 km2(Morales-Arnao, 1998, see Figure 7).

Figure 7: Glacierized mountain ranges in Peru (from Morales-Arnao, 1998).


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The largest single glacier is the Quelccaya ice cap, located in the CordilleraVilcanota, with 8 of the next 11 largest glaciers located in the Cordillera Blanca(Morales-Arnao, 1998). We refrain from giving a detailed description of every single

glacierized mountain region in Peru and refer the reader to the compilation in Morales-Arnao (1998), which gives a nice regional overview and a detailed description of allglacier massifs. Here we focus primarily on the two glacierized areas, which havereceived the most attention and where by far the most scientific research has been done,the Quelccaya Ice cap and the Cordillera Blanca.

2.4.1. Quelccaya Ice Cap

The Quelccaya ice cap is situated in the Cordillera Vilcanota in the eastern branch(Cordillera Oriental) of the Peruvian Andes (Figure 7). It a low lying (summit elevation

of only around 5,700 m) glacierized plateau near the drop-off to the wet Amazon basinand is among the few large ice plateaus in the tropics. In 1998 its size was estimated at 54km2(Hastenrath, 1998). The Quelccaya ice cap has been the target of detailed research byProf. Lonnie Thompson and his group from Ohio State University for over 20 years. Themain aim of this research is of paleoclimatic nature (reconstructing climate of the past byanalyzing ice cores drilled on the summit). In parallel, however, the extent of a largeoutlet glacier to the west, Qori Kalis, has been monitored annually. This outlet glacier hasretreated throughout the entire monitoring period and a large lake formed in its formersnout area (Fig. 8). Brecher and Thompson (1993) first noted the accelerated recession,with a retreat rate that was nearly 3 times as fast between 1983 and 1991 as between 1963and 1978, and a rate of volume loss, which was nearly seven times as great. More recentanalyses show that the retreat rate was 10 times faster (~60 m yr -1) between 1991 and2005 than in the initial measuring period, 1963-1978 (Thompson et al., 2006). However,the recent retreat rate may not reflect climate forcing alone, but also depend on the glacierbed geometry.

Figure 8: Retreat of Qori Kalis glacier (from Thompson et al., 2006)


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2.4.2. Cordillera Blanca

The Cordillera Blanca is the worlds most extensively glacier-covered tropical

mountain range (Morales-Arnao, 1998), with several peaks above 6,000 m (Figure 9). Atotal of 722 individual glaciers were recognized in the Cordillera Blanca based on airphotos from 1962 to 1970, covering an area of 723.4 km2 (Ames et al., 1989). 530glaciers were identified as west-sloping (covering an area of 507.5 km2) and 192 glaciers(covering an area of 215.9 km2) were facing east in the assessment by Ames et al.,(1989). The estimated ELAs were generally higher in the west than in the east, reflectingthe east-west precipitation gradient. The majority of glacierized watersheds in theCordillera Blanca discharge toward the Rio Santa, the second largest river in Perudischarging into the Pacific. Four hydroelectric power plants are situated along the riverbetween the Cordillera Blanca and the Pacific coast (Mark, 2007).

Figure 9: Glaciers in the Cordillera Blanca ((from Morales-Arnao, 1998).


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The initiation of glacier monitoring networks in the Cordillera Blanca was in partmotivated by several catastrophic outbursts and floods from proglacial lakes and iceavalanches. In May 1970 the city of Yungay was destroyed by an ice avalanche fromHuascaran Norte, with an estimated 18,000 people dead (Patzelt, 1983; Portocarrero,

1995; Ames, 1998; Morales-Arnao, 1998; Mark, 2007). The recent glacier retreat in theCordillera Blanca has lead to the formation of a number of proglacial lakes dammed bymoraines, many of which appear to be increasing in size (Ames, 1998).

Glacier shrinkage in the Cordillera Blanca probably started at the end of the LittleIce Age (LIA), in the middle of the 19thcentury (Ames, 1998; Kaser, 1999 and referencestherein; Kaser and Osmaston, 2002; Solomina et al., 2007). There is evidence for anadvance in the mid 1920s at least in parts of the Cordillera Blanca, followed by a rapidretreat in the 1930s (Ames, 1998; Kaser, 1999). More detailed information becomesavailable only later in the 20th century thanks to better maps, aerial photographs andeventually a regular on-site monitoring. Kaser et al. (1990) showed, based on aerialphotographs and ground measurements that a general retreat occurred over the period

1940-1990. Much of the glacier shrinkage and tongue retreat may have occurred in thetime between 1930 and 1950, when a significant rise of the ELA was observed (Kaserand Georges, 1997). The general retreat of the 20th century was again interrupted byminor advances in the late 1970s, which Kaser and Georges (1997) associated withgenerally cooler and wetter conditions. On glacier Artesonraju a retreat of 1140 m wasobserved between 1932 and 1987 and glacier Broggi retreated an average of 17.4 m yr -1between 1932 and 1994, despite a slight advance around 1977 (Ames, 1998). GlacierPucaranra similarly lost 690 m in length between 1936 and 1994 and glacier Urushrajuretreated 675 m since 1932 (Ames, 1998). A similar analysis on glacier Yanamareyshowed that its tongue retreated by 350 m between 1948 and 1988 (Hastenrath and Ames,1995) and an average of 8.9 m yr-1between 1932 and 1994 (Ames, 1998). Today glacierretreat on Yanamarey continues unabated with an estimated 20 m yr-1 (average 1977-2003), four times the speed of 5 m yr-1 observed between 1948 and 1977 (Mark et al.,2005). The loss of ice volume from 1948 to 1982 was estimated at 22x106m3 and at7x106m3 between 1982 and 1988 (Hastenrath and Ames, 1995). On Huascaran-Chopicalqui the glacier extent decreased from 71 km2in 1920 to 58 km2in 1970 (Kaseret al., 1996a). This amounts to an increase in the ELA of ~95 m. Mark and Seltzer(2005a) estimated a glacier volume loss of 57x106m3 between 1962 and 1999 in theQueshque massif of the southern Cordillera Blanca. This translates into a glacier thinningof 5 22 m and an estimated ELA rise of 25-125 m, depending on the aspect of theglacier.

A comprehensive overview over the entire Cordillera Blanca was provided byGeorges (2004) based on new and revised data, including new estimates from SPOTsatellite images. The revised numbers suggest an overall decline in glacierized area from~ 850-900 km2during the LIA to 800-850 km2 in 1930, 660-680 km2 in 1970 and 620km2in 1990. The ice coverage at the end of the 20thcentury was estimated to be slightlyless than 600 km2. Georges (2004) also indicated that some glaciers had again stoppedtheir retreat or even slightly advanced between 1999 and 2002, most likely as aconsequence of persistent cold conditions in the tropical Pacific. Slightly differentnumbers were published by Silverio and Jaquet (2004). On the basis of Landsat TM data


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they detected 643 km2of glacierized area in 1987, and 600 km2 in 1991. As shown byRaup et al. (2006) these differences are mostly due to different methodologies anddefinitions as to what should be included as glacier (e.g peripheral snow fields, snowcovered ground above the glacier accumulation zone, stagnant debris-covered ice, etc.).Raup et al. (2006), in a detailed study of the Huandoy-Artesonraju massif found a

reduction in glacier size of 20% between 1962 and 2003, and a rise of the glacier terminiof approximately 60 m (Figure 10).

Figure 10: Hypsometric distribution of glacierized area in the Huandoy-Artesonraju massif in 1962 and

2003. The total glacier area is reduced by 20% and the shift of the glacier to higher elevation is

evident (from Raup et al., 2006).

While variations in glacier length and size are quite easy to observe, they are notthat easy to interpret because the underlying climate signal may be delayed and filteredthrough glacier flow dynamics. Mass balance measurements, on the other side, are muchharder to obtain, but provide a direct climate signal at the glacier surface. Mass-balancemeasurements were started in the Cordillera Blanca in 1966 on Pucahirca glacier andthen extended between 1977 and 1983 by the glaciology department of Electroper tothree other glaciers, Uruashraju, Yanamarey, and Santa Rosa (Kaser et al., 1990). Thesemeasurements were mainly restricted to the ablation areas. Kaser et al. (2003) were laterable to reconstruct the annual mass balance history from the Cordillera Blanca back to1953 based on glacial runoff data. Their data show a general negative trend, characterizedby strong mass loss in the 1950s and 60s, interrupted by a short period of mass gain in theearly 1970s. Afterwards the negative mass balance resumed again (Figure 11).


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Figure 11: Cumulative glacier mass balance and individual glacier terminus variations in the CordilleraBlanca (from Kaser et al., 2003).

2.4.3. Coropuna

Studies on glacier changes in Peru outside the Cordillera Blanca and theQuelccaya regions are virtually non-existent. One of the few exceptions is a recentanalysis of glacier size and thickness change on Coropuna in the Cordillera Ampato in

southwestern Peru, based on a comparison of older aerial photography with recent ShuttleRadar Topography Mission (SRTM) and Advanced Spaceborne Thermal Emission andReflection Radiometer (ASTER) satellite data (Racoviteanu et al., 2007). In their studythe authors found a significant decrease in glacier extent from 82.6 km2in 1962 to 60.8km2 in 2000. The glacier thickness appears to have changed as well, with a glacierthickening in the summit region and a thinning at lower elevations (Figure 12).


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Figure 12: Glacier thickness change between 1955 and 2000 on Coropuna based on aerial photography

derived maps (1955) and SRTM data (2000). The thinning at lower and thickening at higher

elevations is apparent (from Racoviteanu et al., 2007).

2.5. Bolivia

Glaciers in Bolivia are restricted to the highest peaks of the Andes. They can befound in two main mountain ranges, the Cordillera Occidental along the western borderwith Chile and the Cordilleras Apolobamba, Real, Tres Cruces and Nevado Santa VeraCruz in the east (Jordan, 1998). Glaciers in the Cordillera Occidental are limited toNevado Sajama (6,542 m) and its neighboring volcanoes, with a total surface area in1998 of about 10 km2 (Jordan, 1998). They consist of small summit ice caps on extinctvolcanoes, which due to the dry conditions have the highest minimum elevation on earth,with an ELA several hundred meters above the 0C isotherm (Messerli et al., 1993). Mostglaciers, however, are located in the eastern Cordilleras and consist of ice caps, valley

and mountain glaciers. Their extent was estimated at 550 km2 in 1998 (Jordan, 1998).Because of the limited precipitation, no glaciers exist today in southern Bolivia.

The first compilation of a glacier inventory in Bolivia was started in 1980 (Jordanet al., 1980; Jordan, 1983; 1985). In the early 1990s the French IRD (formerlyORSTOM) started a detailed monitoring program on several glaciers in the CordilleraReal, namely glaciers Charquini, Chacaltaya and Zongo (Pouyaud et al., 1995). Resultsfrom these studies are discussed in more detail below.


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2.5.1. Charquini, Chacaltaya and Zongo glaciers

While early studies focused on a better understanding of glacier mass balance andthe dominant influence of ENSO on interannual variability of mass balance and runoff

(e.g. Francou et al., 1995a, 1995b, 1998; Ribstein et al., 1995b), subsequent studiesstarted to focus on longer-term behavior and trends. Rabatel et al. (2005, 2006) mappedand dated LIA moraines on the Charquini massif and were able to show that themaximum glacier extent had been reached at the second half of the 17 th century.Afterwards glaciers started to retreat almost unabated until today (Figure 13). Thisinterpretation is consistent with the current decay of a rock glacier in southern Bolivia,which by the time of the LIA was still ice covered but now shows severe signs ofdegradation (Francou et al., 1999). Glaciers on Charquini have by now lost between 65-78% (depending on aspect) of their LIA size and the ELA rose by approximately 160 mbetween the LIA maximum extent and 1997 (Rabatel et al., 2006). The authors furthershowed that recession rates have increased by a factor of 4 over the last decades and that

Charquini glaciers experienced an average mass deficit of 1.36 m w.e. yr

-1

between 1983and 1997. This led the authors to speculate that all glaciers in the Cordillera Real locatedbelow 5300 m are in imbalance with current climate and may completely disappear in thenear future (Rabatel, 2006).

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Figure 13: Change in ELA determined from dated moraines on Charquini using the AAR method. Current

AAR was assumed constant over time. For the last 50 years, glaciers have been generally

unbalanced and consequently situated at higher elevation than these estimates. The zero

represents the average altitude of the ELA over the whole period (modified from Rabatel et al.,

2006).


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Ribstein et al. (1995b), based on a reconstruction of glacier discharge since theearly 1970s, showed that Zongo glacier is losing more mass than is replenished byprecipitation. 11 out of the 17 years analyzed showed an extremely negative hydrologicbalance.

In 2000 Francou et al. (2000) reported first results from the glacier mass balance

studies on Chacaltaya. They showed that the glacier had lost 62% of its mass between1940 and 1983 and that its remaining size in 1998 was only 7% of the extent in 1940. Inthe early 1990s the glacier still functioned as a small ski resort. The results by Francou etal. (2000) further suggested 3-5 times higher ablation rates in the 1990s than in previousdecades, with an average loss of 1400 mm w.e. yr-1, and they made the prediction that theglacier might completely disappear within 10 years. Chacaltaya is a very small and low-lying glacier, and therefore particularly vulnerable to climate change. The retreat of suchsmall glaciers is accelerated once they reach a critical size, below which edge effectsbecome important. At the edge of tropical glaciers air temperature above surroundingrocks can exceed 20C (Francou et al., 2003), and hence advection of warm air above theglacier can become very important. Nonetheless Chacaltaya must be considered

representative of many glaciers in the region, since more than 80% of all glaciers in theCordillera Real are less than 0.5 km2in size (Francou et al., 2000). Ramirez et al. (2001),based on new data from ground penetrating radar (GPR) concluded that the glacier hadlost 40% of its thickness in only 6 years from 1992 to 1998, as well as two thirds of itstotal volume. The low elevation of the glacier essentially meant that it had lost itsaccumulation zone, as the ELA had moved above the uppermost reaches of thecatchments (Ramirez et al., 2001). According to the calculations by the authors, a 200 mlowering of the ELA would be required to stabilize the glacier. Francou et al. (2003) tooka closer look at mass balance variability and trends on both Chacaltaya and Zongo glacierand showed that both glaciers featured very similar interannual variability and longerterm trends, although data from Zongo is from the ablation zone only (Figure 14).

Figure 14: Cumulative mass balance from Zongo and Chacaltaya glaciers, Bolivia between 1991 and 2001

(from Francou et al., 2003).

The negative trend on Chacaltaya was interrupted briefly in 1993, 1996 and 2000. On theother hand the glacier lost a third of its entire volume (6 m w.e.) in an exceptional 18


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month period between 1997 and 1999 (Figure 14). Today the glacier has essentiallydisappeared and disintegrated into a few small stagnant ice fields (Coudrain et al., 2005;Figure 15).

Figure 15: Evolution of the limits of Chacaltaya Glacier from 1963 to 2005. The outer border is probably

dating from the second part of the 17th

century, as on Charquini Sur glacier, where this morainewas dated by lichenometry ( from Berger et al., 2006).

2.5.2. Cordillera Occidental (Sajama)


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Outside the Cordillera Real very little work on glacier variations has been done.Arnaud et al. (2001), based on aerial photographs and Landsat data, observed as steadyrise in the snowline of a glacier on Sajama in the western, arid Cordillera Occidental,between 1963 and 1998. The number of images analyzed, however, was limited and thedates analyzed were strongly influenced by interannual variability due to ENSO, both of

which inhibited the authors from drawing any general conclusions regarding thesignificance of the observed rise in snowline.

2.6. Summary

The previous sections on glacier size and mass balance variations fromVenezuela, Colombia, Ecuador, Peru and Bolivia give a detailed and unequivocal accountof rapid and accelerated retreat of tropical Andean glaciers since the LIA. While theclimatic forcing behind this retreat may have varied over time and may not necessarily bethe same everywhere, evidence for a coherent regional pattern is beginning to emerge.

Measurements of glacier area and length on 10 glaciers in Ecuador, Peru and Bolivia forexample provide a clear picture of a region-wide glacier shrinkage, which appears to haveaccelerated over the last few decades (Francou and Vincent, 2007; Fig. 16).

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Figure 16: Change in length and surface area of 10 tropical Andean glaciers from Ecuador (Antizana 15a

and 15b), Peru (Broggi, Pastoruri, Urushraju, Cajap) and Bolivia (Zongo, Charquini,Chacaltaya) between 1930 and 2005. (from Francou and Vincent, 2007).

Mass balance records from Bolivia and Ecuador similarly show a very coherentpicture, with a generally negative mass balance, which appears to be driven by the samebackground forcing throughout the region (Figure 17). Superimposed on this generalretreat are interannual variations with occasional periods of equilibrated or even positive


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mass balance, in particular during prolonged La Nia events, such as between 1999 and2001.

cumulative

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Figure 17: Comparison of cumulative and annual mass balance on glaciers in Bolivia and Ecuador. Notethat the hydrological year is September-August in Bolivia, and January-December in Ecuador

(from Berger et al., 2006).

In order to understand what is causing this retreat and what factors may have beenresponsible in the past, we first have to review how tropical glaciers respond to climatechange. This entails a review of mass and energy balance studies (sections 3 and 4) whichwill discuss the peculiarities of tropical glaciers and how their response and theirsensitivities to climate differ from mid- or high latitude glaciers. Only then, incombination with a review of observed 20thcentury climate change in the Andes, can weobtain a complete picture of tropical glacier climate interactions and attempt to

understand what has caused this dramatic retreat.


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3) GLACIER MASS BALANCE

3.1 General characteristics of tropical glacier mass balance

The glacier mass balance provides the most immediate link between a glacier and

its surrounding climate. The mass balance describes where and how a glacier is gainingor losing mass due the predominance of accumulation or ablation processes, which inturn are determined by climatic variables such as temperature, precipitation, solarradiation, humidity, etc. (e.g. Kaser, 2002).

Because of the lack of a pronounced thermal seasonality (temperatures stay moreor less constant throughout the year) but a clear differentiation between dry and wetseasons (one each in the outer tropics, almost constant precipitation near the equator),tropical glacier mass balance and its sensitivity to climate change is fundamentallydifferent from mid- and high latitude glaciers (see Kaser and Osmaston (2002) for adetailed review). Because of these peculiarities of tropical climate, accumulation isconfined to one wet season (outer tropics) or occurs throughout the year (inner tropics),

while ablation can take place all year long. So unlike mid-latitude glaciers whereaccumulation and ablation are separated into a winter accumulation and a summerablation season, in the tropics ablation and accumulation can occur at the same time(Figure 18). Also, because temperature does not change much throughout the year,ablation occurs predominantly in the ablation zone, below the ELA and accumulation isrestricted to regions above the snow-rain line, which remains more or less constantthroughout the year (e.g. Kaser, 1995; Kaser et al., 1996; Kaser and Georges, 1999).

Figure 18: Schematic comparison of glacier mass balance in mid latitudes, inner and outer tropics (from

Kaser and Georges, 1999).


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Due to these differences in the mass balance, inner tropical glaciers have a muchstronger vertical net mass balance gradient. This vertical mass balance profile describesthe distribution of the specific net mass balance with altitude along a vertical profile fromthe glacier snout to its highest point (Kaser, 1995; Kaser et al., 1996). Figure 19 showstypical vertical mass balance profiles for the mid latitudes, the inner tropics and the

subtropics. It is immediately apparent that inner tropical glaciers have a much highernegative mass balance (higher ablation), than mid-latitude glaciers below their ELA. Thisalso means that the vertical distance between ELA and the glacier snout is generally lessin the inner tropics. This characteristic of inner tropical glaciers is mostly due to the lackof thermal seasonality and the exposure of the ablation zone to melt and sublimation 365days per year (Kaser, 1995). In the subtropics on the other hand ablation is much reduced,mostly because sublimation dominates over melt due to generally low humidity (Kaser,2001). The profiles in Figure 19 are simulated using some idealized assumptions, such as100 days of ablation in mid-latitudes and 365 days in the inner tropics, and having allavailable energy consumed by sublimation instead of melting in the subtropics (seesection 4). Nonetheless they compare very favorably with actual measured mass balance

profiles (Kaser, 2001).

Figure 19: Modeled vertical mass balance profile at a mid-latitude, subtropical and tropical glacier. Note

that the 0C-line may differ from the ELA (above the ELA in the tropics, below the ELA in theouter tropics; from Kaser and Georges, (1999)).

Because of the differences in their vertical mass balance profiles, the sensitivity ofthe ELA to climate change is quite different in the humid inner tropics (e.g. Ecuador) asopposed to the drier subtropics (e.g. Bolivia). In the inner tropics the ELA is generallyclose or slightly below the 0C-line and hence the ELA will react to changes intemperature because a temperature increase would have immediate impacts on ablation,but also change the accumulation due to a shift in the rain-snow line. In the dry outertropics, where the ELA is usually located considerably above the 0C-line, a temperatureincrease will not have such an immediate effect. Instead the ELA will be more sensitiveto changes in the vertical gradient of accumulation and hence changes in precipitation,


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but also to humidity changes, which determines the ratio of melting to sublimation(Kaser, 2001).

Glacier terminus variations are often used to assess how glaciers respond toclimate, as they are very easy to monitor (see previous section). The terminus of a glacieris determined by the balance between net ablation and downward mass flux. Where the

two are equal, defines the end point of the glacier. Because of the strongly negative massbalance in the ablation zone and the close proximity to the ELA, glacier tongue variationsare generally more rapid and more pronounced in the tropics (Kaser, 1995).

The unique mass balance characteristics of tropical glaciers of course havesignificant ramifications for the catchments downstream hydrology. In contrast to midlatitudes, the runoff regime is smoothed in glacierized catchments, especially in the outertropics, where glacial melt water often provides the only runoff during the dry season(see section 6.).

3.2. Mass balance - climate relationships

In the following we discuss mass balance characteristics from a few regions in thetropical Andes, where continuous measurement programs are under way. Beforediscussing these results in detail, it should be emphasized that most mass and energybalance studies, due to logistical reasons, stem from glaciers that are easily accessible andsafe to work on. In effect this means that they are usually rather small and relatively low-lying. This in turn may affect the results to some degree, as there are indications thatsmall and low-lying glaciers have more negative mass balances and have experiencedgreater loss than colder glaciers, located in higher, inaccessible terrain (Francou et al.,2005; Kaser et al., 2006). In addition mass balance measurements are often limited to theablation zone, because of safety concerns, but also because the lack of a clear annual

stratigraphy makes accumulation measurements very difficult (e.g. Kaser and Georges,1999). Finally the limited sampling density (estimates for entire glaciers are often derivedform a limited number of stakes) can lead to inaccurate or biased results, especiallyduring extremely negative or positive years (Sicart et al., 2006). These limitations shouldbe kept in mind during the following discussion.

On Zongo and Chacaltaya glaciers in Bolivia mass balance measurements withstake networks were initiated in the early 1990s (Francou et al., 1995a, b). As shown byFrancou et al. (2003) there is a strong seasonality associated with mass balancevariability; that is, the largest differences from year to year occur during the summermonths OctoberApril. This implies that the accumulation and ablation seasons coincide,as discussed in the previous section. During the dry and cold winter months May

September on the other hand, mass balance is always near the equilibrium and does notdisplay any significant variations from year to year (Figure 20, left). Accordingly, theannual mass balance largely reflects the variability in summertime accumulation andablation. The largest fraction of year-to-year mass balance variability on Chacaltaya canbe attributed to the three summer months DJF, which alone account for 78% of the totalvariance of the annual mass balance (Francou et al., 2003).

Continuous monthly mass balance measurements from the ablation zone ofAntizana 15 glacier in the Andes of Ecuador between January 1995 and December 2002


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(Francou et al., 2004) reveal quite a different picture. Here, on seasonal timescales, meanablation rates remain at a quite constant level all year round (Figure 20, right), althoughthe periods FebruaryMay and September show much larger variations from year to year.The large variability during those months can be explained by the dominant influence ofENSO on the glacier mass balance and the large differences that occur in the seasonal

cycle during the two opposite phases of ENSO (see next section).

Figure 20: Average monthly mass balance on left) Chacaltaya (1991-2001,modified from Francou et al.,

2003) and right) Antizana (1995-2002, modified from Francou et al., 2004). Vertical bars indicate

1 standard deviation.

On interannual time scales Francou et al.(2003) found a clear inverse relationshipbetween mass balance on Chacaltaya and temperature (Figure 21, left). Periods ofnegative (positive) mass balance values coincide with positive (negative) temperatureanomalies. This is in seeming contradiction to earlier statements that glaciers in the outertropics are more sensitive to changes in precipitation and humidity than temperature.However, as shown by Francou et al. (2003) temperature in this region is stronglycorrelated with humidity, cloudiness and precipitation, especially on interannualtimescales. Since temperature integrates all the fluxes, it appears to be significantlycorrelated with mass balance on longer timescales, but the apparent correlation betweentemperature and mass balance does not reflect the real physical processes present at theglacier surface (Francou et al., 2003).

On Antizana, on interannual time scales mass balance is equally closely related totemperature variations (Figure 21, right). Here however, the impact of temperatureappears to be more direct. Higher temperatures in the inner tropics, where Antizana islocated, directly influence the mass balance through changes in the snow-rain line, whichreduces accumulation in the lower zone of the glacier, exposes the snout to rain asopposed to snow and thereby lowers the albedo (Francou et al., 2004). Besides increasedtemperature, weak and sporadic snowfall, insufficient to maintain a high glacier albedo,low wind speeds, which limit the transfer of energy from melting to sublimation, andreduced cloud cover, which increases the incoming short-wave radiation, were additionalfactors found by Francou et al. (2004), which negatively affect glacier mass balance inthe inner tropics.

JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC

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Figure 21: Apparent anti-correlation between mass balance and temperature on interannual time scales.

Left: Chacaltaya glacier (1992-2001, modified from Francou et al., 2003) and right Antizana 15glacier (1995-2002, modified from Francou et al., 2004).

3.3. Mass balance and large-scale forcing

Francou et al. (1995a, 1995b, 2000), Ribstein et al. (1995a), Wagnon et al. (2001)and Ramirez et al. (2001) first identified the significant role played by ENSO, with ElNio years featuring a strongly negative mass balance and La Nia events producing anearly balanced or even slightly positive mass balance. These results do not come as asurprise since climate in the tropical and subtropical Andes is significantly influenced byENSO on interannual timescales (Vuille, 1999; Vuille et al., 2000a, b; Garreaud et al.,2003; Vuille and Keimig, 2004), with La Nia years tending to be wet, while dryconditions usually prevail during El Nio years. In conjunction with dry conditions, theAndes also experiences above average temperatures during El Nio events (Vuille andBradley, 2000; Vuille et al., 2003). On average, near-surface summer temperatures are

0.71.3C higher during El Nio as compared to La Nia (Vuille et al., 2000a). Tropicalglaciers, such as Chacaltaya, thus do not only experience a deficit of summerprecipitation and consequently reduced accumulation and a lowered albedo during ElNio events, but are also exposed to higher temperatures and an increase in incomingshortwave radiation due to reduced cloud cover (Wagnon et al., 2001; Francou et al.,2003). Indeed as shown in Figure 22, Chacaltaya mass balance is significantly correlatedwith tropical Pacific sea surface temperatures (SSTs), in a way which is reminiscent ofthe canonical ENSO mode. The causal mechanism linking tropical SSTA with glaciermass balance in the Andes is the same as for precipitation, described in previouspublications (Vuille et al., 2000a, Garreaud et al., 2003). This result is of relevancebecause it shows that glacier mass balance is linked to SSTA in the tropical Pacific and

that this linkage is being translated through changes in precipitation. It follows that massbalance anomalies on Chacaltaya are largely governed by climatic conditions in thetropical Pacific domain (Francou et al., 2003). This is consistent with observationsindicating an accelerated negative mass balance and glacier retreat in many tropicalAndean locations after the mid 1970s, concurrent with the 1976/77 Pacific climate shift.

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Figure 22: Correlation of Chacaltaya mass balance with tropical SST. White contours indicate 95%

significance levels (modified from Francou et al., 2003).

Results from Antizana similarly indicate a strong dependence on ENSO. Over the8-year period investigated, mass balance was negative all year round during El Nioperiods but remained close to equilibrium (positive anomalies) during La Nia events(Figures 23-24).

Figure 23: Correlation of Antizana mass balance with tropical SST. White contours indicate 95%significance levels (modified from Francou et al., 2004).

While the response to ENSO-related climate variability is very similar onAntizana to what is observed on glaciers in Bolivia, the seasonal dependence and thephysical mechanisms linking ENSO with mass balance variations are very different


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(Favier et al., 2004a). Unlike in Bolivia, the impact of El Nio is primarily throughincreased air temperature, which favors rain over snowfall, but to a lesser degree also dueto weak and sporadic snowfall, insufficient to maintain a high glacier albedo, low windspeeds, which limit the transfer of energy from melting to sublimation, and reduced cloudcover, which increases the incoming short-wave radiation. La Nia events on the other

hand are characterized by colder temperatures, higher snowfall amounts, and to a lesserdegree, more constant winds, factors which increase albedo and sublimation andtherefore preclude melting at the glacier surface (Francou et al., 2004).

Figure 24: Antizana mass balance anomalies stratified by ENSO events. Individual monthly measurements

(small circles), the mean (large circles), and 1 standard deviation (vertical bars) are indicated(modified from Francou et al., 2004).

In the Cordillera Blanca, mass balance is also dependent on the phase of ENSO,but the influence is generally not as strong. Kaser et al. (2003) found a significantpositive relationship between the Southern Oscillation Index (SOI) and reconstructedglacier mass balance using a 41 year long mass balance time series, but they also pointedout that this relationship does not hold in all years. Vuille et al. (2007) were able to showthat this relationship is relatively weak because the Cordillera Blanca is located in an

intermediate zone, where the ENSO influence on temperature and precipitation is not asstrong as to the north on Antizana (temperature) or to the south on Chacaltaya(precipitation).


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4) GLACIER SURFACE ENERGY BALANCE

The previous chapter discussed how climate affects the mass balance of tropicalglaciers through changes in accumulation and ablation (melting and sublimation). Tounderstand how these changes are transmitted from the atmosphere to the glacier,

however, requires detailed knowledge of the surface energy balance (SEB) at the glacier-atmosphere intersection. The SEB describes the amount and direction of the variousenergy fluxes from and to the glacier surface. A detailed SEB can only be establishedthrough accurate measurements on the glacier itself, which requires the installation andmaintenance of automated weather stations (AWS) for a period long enough to properlyunderstand diurnal and seasonal cycles, as well as interannual variations. Such detailedmonitoring programs are relatively new and the results from these programs will bediscussed in section 4.2. First we review some previous studies which are based onsimpler sensitivity calculations (section 4.1.).

4.1. Sensitivity studies

The first energy balance studies focused on quantifying the amount of energyneeded to produce an observed change in glacier mass balance, or the changes required tostabilize a retreating glacier. For example Hastenrath and Ames (1995b) estimated theaverage surface lowering on Yanamarey glacier in the Cordillera Blanca between 1977and 1988 to be 1.5 m yr-1. Given the latent heat of melting,Lm= 3310

4J kg-1, the energyrequired to produce such a change corresponds to a positive energy balance (directedtoward the glacier) of 16 W m-2. Sublimation, which is much less energy-efficient (latentheat of sublimation Ls = 28410

4 J kg-1) was not considered a factor in this study. Intheory, of course, the easiest way to stabilize the glacier would be to increase

precipitation by the amount of imbalance, in this case, 1.5 m. This, however, is acompletely unrealistic change. Alternatively Hastenrath and Ames (1995b) calculatedthat a cloudiness increase of 10%, a temperature reduction of 2C, a specific humiditydecrease of less than 1 g kg-1 or any combination of the above, could lead to astabilization of the glacier. On Urushraju glacier (also in the Cordillera Blanca) Ames andHastenrath (1996) used the same sensitivity study approach to diagnose the energyimbalance of the glacier between 1977 and 1987. The results were very similar to whatthey previously found on Yanamarey, with an average surface lowering of 1.04 m yr-1,equivalent to an energy surplus of 12 W m-2. Their calculations suggested that the energydecrease required to stabilize Urushraju could be produced by a cloudiness increase ofless than 10%, a temperature decrease of 1.5C, a decrease in specific humidity of less

than 1 g kg-1

or any combination of these (Ames and Hastenrath, 1996).Kaser et al. (1996a) performed a similar sensitivity study in the Cordillera Blanca,calculating the energy surplus responsible for the observed ELA rise of 95 m between1920 and 1970. In addition they compared their results with actual observed changes inclimate to assess what combination of climatic changes was most likely responsible forthe ELA shift. According to their results either a temperature increase of 0.51C, aprecipitation decrease of 1177 mm, a global radiation increase of 1.076 MJ m -2d-1or anincrease in evaporation of 157 mm yr-1could have caused the change in ELA position.


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Comparing their results with actual observed changes in climate, Kaser et al. (1996a)concluded that a temperature increase could only explain about half of the observed risein ELA and that precipitation changes were too small to have had a significant impact.The remaining half of the observed ELA change was therefore attributed to a combinedeffect of the remaining factors. Similarly Kaser and Georges (1997) attributed the

observed ELA increase in the Cordillera Blanca between 1930 and 1950 only in part to atemperature increase, but more so to decreased humidity and its indirect effects ofreduced precipitation, reduced cloud cover and hence increased incoming radiation.Kaser and Georges (1997) also observed large differences in ELA rise across theCordillera Blanca from east to west. They argued that this differential ELA rise wasinconsistent with a temperature forcing, which should have had a similar impactthroughout the region. Vuille and Bradley (2000) and Vuille et al. (2003), however, latershowed that temperature trends in the 20 century have indeed been vastly different on theeastern and western Andean slopes (see section 5.1).

On Chacaltaya glacier Ramirez et al. (2001) estimated the average melting(assuming no sublimation) since 1983 to be equal to an average increase in heat supply of

10 W m

-2

. According to their calculations it would take a ~200 m drop in the ELA tomake up for this increase in heat supply and to again stabilize the glacier. Such a scenariowould bring the ELA down to a level near 5220 m, close to the average ELA in theperiod 1940-1963, before accelerated glacier recession began (Francou et al., 2005).

Mark and Seltzer (2005a) performed a similar sensitivity analysis on NevadoQueshque, also in the Cordillera Blanca, to assess the causes of the observed ice thinning(353 mm yr-1) between 1962 and 1999. Assuming that 20% of the mass loss was due tosublimation and the remaining 80% due to melt, they concluded that 9.3 W m -2 wasrequired to produce the observed mass loss. According to their results this energy surpluswas most likely caused by a temperature rise of 1C, combined with a specific humidityincrease of 0.14 g kg-1, as this scenario was most in line with actual observed changes inclimate over this period. They also pointed out that this estimate was inconsistent withthe observed average ELA rise of only 72m, which they explained with the fact that theglaciers are not in equilibrium and lag behind the climate forcing.

Another, rather simple but effective way to qualitatively attribute observedchanges in glacier extent to various climate forcings, is to assess how the glaciergeometry changes over time and where glaciers tend to lose the most mass. For exampleif changes in cloudiness and hence solar radiation were the main cause of changes inglacier extent, one would expect to see a differential thinning in accordance with theradiation geometry of the glacier (e.g. Mark and Seltzer, 2005a, b). While tropicalAndean glaciers do indeed reside preferentially in locations that receive less shortwaveradiation than non-glacierized areas at similar elevation (Klein et al., 1996), there are nostudies indicating that a preferential thinning has taken place on tropical Andean glaciers,which could be attributed to changed radiation receipts. If glaciers react to increasedtemperatures on the other hand, one would expect to see larger mass loss at lowerelevation glaciers, and a mass loss dependence on the glacier hypsometry, that is thevertical distribution of glacier mass and area. Mark and Seltzer (2005a, b) indeeddetected such changes on Nevado Queshque in the Cordillera Blanca, which lead them tothe conclusion that the observed mass loss was primarily caused by a temperatureincrease.


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4.2. The Surface Energy Balance (SEB)

While the sensitivity studies discussed in the previous section are very interestingand insightful in their own right, they fall short of clearly attributing observed changes in

glacier mass and extent to one or several factors. There is always a variety of potentialcombinations which could explain the energy imbalance at the glacier surface. The wayto solve this problem is to actually measure all the relevant fluxes at the surface over along enough period of time. The energy balance of a melting ice surface can be writtenas:

SW

(1-)+LW+LW+H+ C+Ls S+LmM= 0

whereSW is the incoming shortwave radiation, the surface albedo (reflectivity), LWand LW the long wave radiation (emission) toward and from the glacier surface, H thesensible heat flux, C the subsurface conductive heat flux and Ls S and LmM the mass

consuming terms, withLs andLmthe latent heat of sublimation and melt respectively andS and M the rate of sublimation and melt. All energy fluxes are considered positive ifdirected toward the surface and negative if they are directed away from it. All these termsin turn depend on a number of climatic factors. SW for example depends largely oncloud cover, is controlled by the amount and timing of snowfall and LWdepends onatmospheric humidity and cloud cover. The mass consuming term Ls S is controlled bythe vapor pressure gradient between the glacier surface and the air above, which in turn isrelated to air humidity and wind speed. The terms of the equation also show pronouncedseasonality on tropical and especially subtropical glaciers, with very different behaviorduring dry and wet seasons. SWfor example is reduced during the wet season because ofincreased cloud cover and is very high due tofrequent snowfall. At the same timeLW

and LW

almost cancel each other and sublimation is reduced because of the highhumidity and low wind speeds. This effectively means that most of the surplus energy isconsumed by the very energy-efficient melting. As a consequence high melt and highaccumulation during the wet season lead to a large mass turnover (e.g. Sicart et al., 2003;Kaser et al., 2005). During the dry season on the other hand, the LWbalance is negative(away from the surface) as the large vapor pressure gradient leads to enhancedsublimation, which is 8 times more energy-intensive than melting. As a result melting isreduced and mass turnover limited. These peculiarities of the tropical glacier energybalance make tropical glaciers highly sensitive to changes in atmospheric humidity whichgoverns sublimation, precipitation, whose variability, particularly during the rainy seasoninduces a positive feedback on albedo, and cloudiness, which controls the incoming long-

wave radiation (Francou et al., 2003).Some of the first energy balance studies were performed on Quelccaya Ice cap inthe 1970s (Hastenrath, 1978, 1997). They were however very limited in terms ofinstrumentation and the length of operation. Hastenrath (1978), based on a few daysworth of shortwave and longwave radiation measurements during austral winter,concluded that due to the high albedo and constant subfreezing temperatures virtually noenergy was available for ablation. The limited sublimation that took place during the daywas approximately compensated by nighttime deposition (Hastenrath, 1997). Hence it


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was concluded that net ablation only took place at lower elevations of the ice cap(Hastenrath, 1978, 1997). More recent detailed energy balance measurements with anAWS installed permanently on the summit, however, paint a very different picture, withtemperature above freezing and melting taking place throughout much of the year (D.Hardy, 2006, pers. comm.), However, it is difficult to assess exactly, how much of the

difference between the early studies and todays measurements is due to a much moresophisticated, year-round monitoring at the summit, and how much is due to an actualchange in climate between the mid 1970s and today.

Wagnon et al. (1999a, b) provided the first in-depth SEB study from a tropicalglacier based on measurements from an AWS installed on Zongo glacier. They were ableto show that mass loss and runoff show a marked seasonality, which can not be explainedby the sensible heat transfer, which shows little seasonality and is generally small.Instead net radiation and the turbulent fluxes dominate the SEB (Figure 25). They alsodemonstrated that the incoming energy is quite constant throughout the year and thatinstead it is the partitioning of this energy into melt and sublimation, controlled byhumidity, which causes the much higher mass loss during the wet season. When humidity

is high, the available radiative energy is directly consumed by melting (wet season),while the enhanced vapor pressure gradient during the dry season favors sublimation,which is energetically inefficient and therefore leads to reduced discharge. Thecalculations based on the measurements of the various terms were verified by comparisonwith mass balance measurements from a stake network, sublimation measurements usinglysimeters and proglacial stream discharge measurements. As shown by Wagnon et al.(1999a), the SEB based calculations compared favorably with the ablation, sublimationand runoff measurements, increasing the confidence in the results.

In summary, the initial studies on Zongo showed how important the humidity ison these subtropical glaciers, as it determines the partitioning of the available energy intomelt and sublimation. As shown by Wagnon et al. (1999a), sublimation consumed 63%of the total available energy during the hydrologic year 1996-97 to produce only 17% ofthe mass loss. This shows how sensitive tropical glaciers are toward changes in specifichumidity and that increasing humidity is the most effective climate change scenario toenhance glacier retreat, at least in the subtropics. In addition, as emphasized by Sicart etal. (2005), long-wave radiation, directly linked to humidity but also to cloud cover, isanother key factor of the energy balance, as it equally drives the seasonal changes ofenergy available for melt.

On interannual time scales the glacier mass balance is strongly influenced byENSO (see section 3.3). Using the same AWS measurements from Zongo, Wagnon et al.(2001) demonstrated that the much larger mass loss during the El Nio 1997/98 ascompared to previous year was not due to the admittedly higher temperatures (and hencehigher sensible heat flux), but due to a significant precipitation deficit. Below averageprecipitation, very common during El Nio in the Bolivian Andes (e.g. Vuille, 1999;Vuille et al., 2000a; Garreaud et al., 2003) leads to low-albedo bare ice exposed for amuch longer time of the year and over a much larger glacier surface (the ELA was 450 mhigher in 1997/98 than during the previous year). Absorption of shortwave radiation wastherefore greatly enhanced, translating into a dramatic increase in net all-wave radiation.As a consequence glacier melting was significantly enhanced and measured runoff at theglacier snout was two thirds higher than normal (Wagnon et al., 2001). These results


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were later applied to the much smaller Chacaltaya glacier and the results there confirmedthat cloudiness, which controls the incoming long-wave radiation, precipitation, whichhas a strong feedback on albedo, and humidity, which is responsible for sublimation, arethe key variables explaining variations of the glacier mass balance (Francou et al., 2003).

Figure 25: Daily cycle of the various energy balance terms on Zongo glacier during the dry (above) andwet seasons (below). Net radiation (green is the main term of the energy balance in both seasons,

but during the dry period it is almost entirely consumed by sublimation (red). During the wet

season this term is much reduced, leaving a lot of energy available for melting (from Wagnon et

al., 1999b).

In the inner tropics near the equator the various terms of the energy balance varysomewhat differently. Measurements of the SEB on Antizana glacier in Ecuador revealthat it is equally governed by net radiation and hence albedo (Favier et al., 2004b).However, because of the absence of thermal seasonality, the ablation zone is exposed tooscillations of the 0C isotherm throughout the year. These small fluctuations intemperature determine the rain-snow line on the ablation zone and hence have a major


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impact on the albedo. Consequently air temperature significantly influences the energybalance in the inner tropics, albeit not through the sensible heat term of the SEB equation,as commonly thought, but indirectly through changes in albedo and net radiation receipts(Favier et al., 2004a, b; Francou et al., 2004).

As already outlined in section 3.3., interannual variations of the SEB on Antizana

are strongly affected by the ENSO cycle. During El Nio, increased air temperature,which favors rain over snowfall, weak and sporadic snowfall, insufficient to maintain ahigh glacier albedo, low wind speeds, which limit the transfer of energy from melting tosublimation, and reduced cloud cover, which increases the incoming short-waveradiation, are the main factor affecting the SEB and causing high melt rates. La Niaevents on the other hand are characterized by colder temperatures, higher snowfallamounts, and to a lesser degree, more constant winds, factors which increase albedo andsublimation and therefore preclude melting at the glacier surface (Francou et al., 2004).

For reasons discussed in section 3, most SEB studies stem from melting surfacesin the glacier ablation zone. To date there is only limited information available on theenergy balance from the accumulation zone at high-altitude, where no melting takes

place. Wagnon et al. (2003) performed some experiments and measured the SEB for ashort time in austral winter near the summit of Illimani, at 6340 m in the Cordillera Real.Their results show that here the net radiation balance is mostly negative due to aconstantly high albedo and the reduced LW, resulting from generally clear skies. Thelatent heat flux was always negative, which indicates significant sublimation. Wagnon etal. (2003) estimated the mass loss due to sublimation at ~ 1 mm w.e. d-1 at this highelevation site.


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5) OBSERVED AND PROJECTED CLIMATE CHANGE

The glacier sensitivity and SEB studies from the previous section have outlinedhow climate change can affect glaciers in the tropical Andes by altering the various termsof the energy budget. To accurately attribute glacier retreat to a particular climate forcing,

however, requires detailed knowledge and understanding of the climatic changes thathave actually taken place in the 20thcentury. In this section we will first review observed20th century climate changes (section 5.1.) and then discuss what the most likelyscenarios of future climate change in the 21stcentury look like (section 5.2.).

5.1. Observed 20thcentury climate change

5.1.1. Temperature

Probably the most detailed analysis of near-surface temperature trends was

presented by Vuille and Bradley (2000), based on a compilation of 268 station recordsbetween the 1N and 23S. Their results showed that near-surface air temperature hassignificantly increased over the last 60 years. Their analysis documented a temperaturerise of 0.10 - 0.11C/decade, since 1939, with the bulk of the warming occurring over thelast 2 decades. Indeed the rate of warming almost tripled since the mid 1970s (0.32 0.34C/decade (Figure 26). Vuille et al. (2003a), based on an updated data set, laterconfirmed these results and showed that other data sets (CRU05, and ECHAM4 modeldata) show similar warming trends in the Andes.

Figure 26: Annual temperature deviation from 1961-90 average in the tropical Andes (1N-23S) between1939 and 1998. Black line indicates 11-yr running mean (modified from Vuille and Bradley,

2000).


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One noteworthy result of the Vuille and Bradley (2000) and Vuille et al. (2003a)studies was the astonishing dependency of the temperature trend on elevation, and evenmore so on the location of the stations considered. Stations located on the eastern slopeshowed a much subdued warming, with trends that were close to zero and insignificant atthe lowest elevations, while the warming on the Pacific side

1 (PRAA) Climate Change in the Tropical Andes

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