Palaeogeography, Palaeoclimatology, Palaeoecology

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1 Palaeogeography, Palaeoclimatology, Palaeoecology 281 (2009) Contents lists available at ScienceDirect Palaeogeography, Palaeoclimatology, Palaeoecology journal homepage: Fluctuations of glaciers in the tropical Andes over the last millennium and palaeoclimatic implications: A review Vincent Jomelli a,, Vincent Favier b, Antoine Rabatel c, Daniel Brunstein d, Georg Hoffmann e, Bernard Francou f a CNRS-IRD, Great Ice, Maison des Sciences de l'eau, 300 Avenue Jeanbrau, Montpellier, France b CEAZA, Universidad de la Serena, Casilla 599, La Serena, Chile c CNRS, EDYTEM, Chambery, France d CNRS, LGP, Meudon, France e CEA, LSCE, Gif sur Yvettes, France f IRD, Great Ice, Quito, Ecuador article info abstract Article history: Received 23 June 2007 Accepted 18 October 2008 Available online 9 April 2009 Keywords: Tropical Andes Glacier fluctuations Last millennium Palaeoclimatic conditions Solar activity Volcanism El Niño-Southern Oscillation (ENSO) The aim of this paper is to document the evolution of glaciers in the tropical Andes (between 10 N and 16 S) for the last millennium, based on moraines dated by lichenometry and radiocarbon, lake sediment records and historical documents. Viewed collectively, several glacial advances occurred synchronously. The first advance is dated around AD The maximum glacial extent (MGE), defined as the furthest downvalley extent recorded synchronously by the majority of glaciers, occurred around in Bolivia and Peru (the outer tropics) and around AD 1730 in Ecuador, Colombia and Venezuela (the inner tropics). Subsequently, during the 18th and 19th centuries, glaciers retreated continuously with only minor synchronous readvances. In the outer tropics, minor glacial advances occurred around 1730, 1760, 1800, 1850, and In the inner tropics, synchronous minor advances occurred around 1760, 1820 and Between the MGE and the early 20th century, glaciers lost about 30% of their total length. The retreat was slow between the 17th and 18th centuries but then became more marked. Use of the accumulation area ratio (AAR) method or historical observations in the different cordilleras revealed an increase in the equilibrium line altitude (ELA) of about 300 m from the MGE onward. Palaeoclimatic hypotheses, based on glaciological models run in different countries, suggest a cool and humid period in the 16 18th centuries followed by a colder and drier period in the 19th century. The reduction of glaciers observed from the middle of the 19th century is due to increasingly warmer conditions than before. Here, quantitative estimates are proposed to explain the evolution of the glaciers. In Venezuela, results indicate for the period that mean air temperatures were 3.2±1.4 C cooler and precipitation was about 22% higher than at present. In Ecuador, temperatures of between 0.8 and 1.1 C lower than today, and between 25% and 35% higher accumulation than today, appear to have occurred in the 18th century, followed by a short drier but colder period at the beginning of the 19th century. In Bolivia, the MGE could be a consequence of a decrease in temperature of 1.1 to 1.2 C, and a 20 to 30% increase in accumulation or an increase in cloudiness of about 1 2/10. We discuss not only external climatic forcing but also the coincidence between glacier expansion and a decrease in solar irradiance. Dating uncertainties, however, have made the role of volcanism in glacier fluctuations impossible to determine. Finally, we discuss the relationships between ENSO and glacier fluctuations in recent centuries, which do not match directly with current knowledge of modern teleconnections between tropical Pacific SSTs and variations in glacier mass balance Elsevier B.V. All rights reserved. 1. Introduction In the last few decades, significant effort has been made to understand the climate of recent centuries, within different regions and at different Corresponding author. CNRS-IRD, Great Ice Maison des Sciences de l'eau 300, Avenue Jeanbrau Montpellier, France. Tel.: ; fax: address: jomelli@cnrs-bellevue.fr (V. Jomelli). timescales (Bradley and Jones, 1993, 1995; Villalba, 1994; Bradley et al., 2002; Villalba et al., 2003; Jones and Mann, 2004), in order to discriminate between natural forcing and the role played by anthropogenic activity in the variability of the current climate (IPCC, 2007), and to validate ocean-atmosphere models covering a long period of time. However, the spatial distribution of paleoclimate proxies is heterogeneous. While observational and multi-proxy reconstructed data are available for the mid- and high-latitudes of both hemispheres (dendroclimatology, historical documents, weather data), data concerning the /$ see front matter 2009 Elsevier B.V. All rights reserved. doi: /j.palaeo

2 270 V. Jomelli et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 281 (2009) tropics are extremely scarce (Jomelli et al., 2008b). Yet the tropical climate has effects that reach far beyond the limits of the tropics (Vuille et al., 2000), and thus an understanding of the climate of the tropics is key in understanding world climate (McGregor and Nieuwolt, 1998). Furthermore, this area is the dominant driver of atmospheric circulation, and intermittent tropical phenomena, such as ENSO, influence the climate over very large areas, including the mid-latitudes. Tropical glaciers are considered to be especially sensitive to climate change (Francou et al., 1995a,b; Kaser, 2001; Thompson, 2000; Thompson et al., 2006), and thus offer an attractive proxy for palaeoclimatic conditions. They are also a major water resource for Andean communities, even if they represent only 0.15% of global ice cover (Kaser et al., 1996a; Vergara et al., 2007). In the Andes, climate reconstructions based on glaciers have predominantly used two approaches: analysis of ice core records (see the review on ice cores in this volume) or analysis of glacial moraine deposits created during advances or retreats. The second approach was used in this study. As moraines show the previous positions reached by the glacier in specific climatic conditions, it is possible to estimate the length of the glacier and other glaciological parameters, such as the equilibrium line altitude (ELA) or mass balance, which can then be used to reconstruct climate. The temporal resolution is lower than that obtained from ice core records and depends on the number of moraines preserved in the glacier foreland, as well as the method of dating. Generally, uncertainties associated with dates are limited to between one and five decades. Using glacial moraines as indicators of climate change also requires that the time lag between mass balance fluctuations and the movements of the snout of the glacier have to be taken into account. Additionally, the glacial moraine landscape is, by nature, discontinuous and subsequent glacial advances destroy the moraines deposited during older, lessextensive advances, meaning the geomorphic record is incomplete. Fortunately, the moraines in the Andes are well preserved (Fig. 1) due to reduced activity of the periglacial processes, a superficial (b20 cm) freeze thaw cycle (Francou et al., 2001), and also because of the specific glacial evolution that we describe in this article. A limited number of studies have reconstructed glacier fluctuations from dated moraines for last the millennium. In most cases, the last millennium was not the main objective of the study, but was part of a larger timescale analysis, for example the Last Glacial Maximum or the Neoglacial period (Clapperton, 1983; Schubert and Claperton, 1990; Seltzer, 1990; Seltzer et al., 1995). Investigations have predominantly been carried out in Peru (71% of tropical glaciers) by Mercer and Palacios (1977), Clapperton (1981) and Rothlisberger (1987), who provided the first radiocarbon dates for glacial moraines. Rodbell (1992) used lichenometry to improve the chronology of glacier fluctuations in the Holocene. In Bolivia, pioneering studies on Holocene glacier fluctuations started in the 1980s and were based on radiocarbon dating (Gouze et al., 1986; Seltzer, 1992). In Ecuador, pioneering studies were made by Hastenrath (1981), who published an extensive review of the evolution of glaciers. In Colombia and Venezuela, glaciological observations have been made by various authors from the end of the 19th to the beginning of the 20th century, to document glacier fluctuations during recent centuries. However, these did not provide a clear chronology (Schubert, 1972; Herd, 1974). One possible explanation for the limited interest in glacier fluctuations over the last millennium is the lack of a reliable dating method, which has prevented the establishment of accurate chronologies. Moreover, the Little Ice Age in the tropical Andes has historically been identified from an ice core record, rather than from moraine landscapes (Thompson et al., 1986). In recent years, progress in lichenometry (Cooley et al., 2006; Naveau et al., 2007) has made it possible to improve the chronology of glacier fluctuations in the last century. As a result, new papers, based on a large number of glaciers, have been published. New investigations have also been carried out in Peru (Solomina et al., 2007; Jomelli et al., 2008a) and Ecuador (Jomelli et al., 2007b). Rabatel et al. (2005, 2006, 2008) published the chronology of glacier fluctuations over recent centuries in Bolivia using this dating technique. In addition, recent studies based on lake sediment records enabled palaeoclimatic conditions inferred from glacier fluctuations to be further extended (Polissar et al., 2006). In order to use glacier fluctuations, estimated from glacial landforms, as a paleoclimatic proxy, it is very important to understand the role of the climatic parameters that control the ablation and accumulation processes of tropical glaciers. Fortunately, a substantial number of studies have clarified the relationships between glacier fluctuations and current climate based on mass and energy balances (Wagnon et al., 1999; Kaser, 2001; Ramirez et al., 2001; Wagnon et al., 2001; Sicart, 2002; Francou et al., 2003; Wagnon et al., 2003; Favier et al., 2004a; Francou et al., 2004; Sicart et al., 2005). The main goals of this paper are to i) document tropical glacier fluctuations for the last millennium and ii) summarize the inferred climate hypotheses that can explain this evolution. This work Fig. 1. LIA moraines of South Charquini glacier in Cordillera Real, Bolivia. Dates were provided by lichenometric analyses and uncertainties were estimated from the GEV approach.

3 V. Jomelli et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 281 (2009) Table 1 Characteristics of data used in this study. Country Cordillera References Number of glaciers considered Proxy of glacier fluctuations Dating method Venezuela Merida Polissar et al., Lake sediment concentration 14C and 210pb Colombia Santa Marta Schubert and Claperton, Moraine 14C Ruiz-Tolima Herd, Moraine 14C Ecuador Oriental Occidental Jomelli et al., 2007a 15 Moraine Lichens Peru Blanca Broggi, Historical documents Rothlisberger, Moraine 14C Rodbell, Moraine 14C-lichens Solomina et al., 2007; Jomelli et al., 2007a 20 Moraine Lichens Seltzer, Moraine 14C Vilcanota Mercer and Palacios, Moraine 14C Mercer, Moraine 14C Bolivia Apolobamba Gouze et al., Moraine 14C Real Rabatel et al., 2005, 2006, 2008 Jomelli et al., 2008a 13 Moraine Lichens Quimsa Cruz Rabatel et al., 2005, 2006, Moraine Lichens comprises a review of papers published since the 1970s. Observations made in Venezuela, Colombia, Ecuador, Peru and Bolivia are included (Table 1). Although rock glaciers are frequent in the dry Andes, they were not taken into account as they behave differently with respect to climate (Francou et al., 1999). Glacier fluctuations in the 20th century are also not analysed in detail here because major results and reviews have already been published (Kaser et al., 1990; Jordan, 1991; Ames and Francou, 1995; Kaser et al., 1996b; Kaser and Georges, 1997, 1999; Kaser, 1999; Francou et al., 2000; Georges, 2004; Jordan et al., 2005; Mark and Seltzer, 2005; Machguth, 2006). In section two, we describe the status of current knowledge of the climate parameters that control contemporary fluctuations in tropical glaciers. Then, in section three, we describe the different glaciological methods and dating techniques that have been used to estimate glacier fluctuations. The major glacial advances documented in different countries, the changes in length, surface area and ELA, and the climatic hypotheses inferred from these data are described in section four. Finally, we discuss the relevance of these climatic hypotheses. 2. Current knowledge of tropical glaciers and climate relationships Use of glacier fluctuations as a climate proxy first requires identification of the significant climatic parameters controlling their behaviour, i.e. establishing a transfer function between current climate and glacier variations. Tropical glaciers are divided into two main groups according to climatic conditions. Within the inner tropics, stable humidity and temperature imply accumulation and ablation occur simultaneously throughout the year. Within the outer tropics, notable accumulation only occurs during the wet season, which is also a period of enhanced ablation (Kaser, 2001). Understanding glacier atmosphere interactions is particularly complex in the tropics as accumulation and ablation processes are synchronous and have to be separated for their interpretation (Hastenrath and Ames, 1995; Kaser, 2001). Moreover, in the tropical Andes, in contrast to mid-latitude glaciers, calibration between mass balance and climatological parameters over a long period is difficult because data are not available. Tropical glaciers are generally described as very sensitive climatic indicators that reflect the effects of global warming (Hastenrath and Kruss, 1992; Thompson, 2000; Vuille et al., 2003; Ceballos et al., 2006). Significant correlations between mass balance and re-analyzed air temperature at 500 hpa have been observed (e.g. Francou et al., 2003, 2004). These correlations make sense because temperature is correlated with almost every energy flux at a daily timescale (Kuhn, 1993; Sicart, 2002, p. 207). In addition, air temperature controls the phase of the precipitation in the ablation area, particularly in the inner tropics (Favier et al., 2004a; Francou et al., 2004). This is why glaciers in Ecuador are particularly sensitive to temperature changes. In contrast to mid-latitude glaciers, considering Positive Degree Day models or assuming air temperature as a primary parameter (Hostetler and Clark, 2000; Thompson, 2000; Taylor et al., 2006) is an oversimplified approach that does not adequately account for the complex physical processes at the interface between the atmosphere and the glacier surface (Mölg et al., 2006). Indeed, although temperature in the tropics is generally correlated with fusion, this link is not usually direct and is also incomplete. Seasonal temperature variations in the tropics are small and climate seasonality is primarily due to changes in air humidity, precipitation and moisture advection. As a consequence, changes in precipitation and more generally, moisture explain an important component of tropical glacier fluctuation. This is supported by studies on the physical processes of accumulation and ablation (melting/ sublimation). Indeed, understanding the relationship between climate and ablation requires knowledge of the Surface Energy Balance (SEB). Estimation of the ratio of energy used for sublimation to that of melting is crucial in distinguishing glacier response under humid or dry tropical climates (Kaser, 2001). In general, studies of the SEB are carried out over short time periods (e.g., Hastenrath, 1978; Hardy et al., 1998; Wagnon et al., 2003). However, long-term field campaigns performed in Bolivia (Wagnon et al., 1999, 2001; Sicart et al., 2005), Peru (Juen, 2006) and Ecuador (Favier et al., 2004a,b) revealed that in the tropics the SEB in the ablation areas is dominated by net short-wave radiation (S). As S is closely related to cloud cover and surface albedo, albedo appears to be one of the main variables controlling the amount of energy available at the surface of the ablation area of all tropical glaciers. With its direct effect on accumulation and its positive feedback on albedo, solid precipitation is, therefore, a key meteorological parameter in explaining the mass balance variability of Andean tropical glaciers (Wagnon et al., 2001; Francou et al., 2003; Favier et al., 2004b; Francou et al., 2004). Following this reasoning, mass balance is thus closely related to the total amount, the annual frequency, and the distribution of precipitation (Wagnon et al., 2001; Francou et al., 2003; Favier et al., 2004a; Francou et al., 2004; Sicart et al., 2005). This relationship is particularly clear in glaciers of the outer tropics, where liquid precipitation rarely occurs. The control of mass balance by precipitation is corroborated by other studies that used hydrological data collected from partially glacierised watersheds (Ribstein et al., 1995; Kaser et al., 2003). Finally, moisture also controls glacier fluctuations (Hastenrath and Kruss, 1992; Wagnon et al., 1999; Kaser, 2001; Ramirez et al., 2001; Francou et al., 2003, 2004; Sicart et al., 2005) becauseofitsimpacton incoming longwave radiation (LWi) (Sicart et al., 2005) and on the turbulent latent heat flux (LE) (Wagnon et al.,1999; Favier et al., 2004a,b), both of which are very important in explaining the seasonality of ablation, particularly in the outer tropics (Wagnon et al., 1999; Sicart et al., 2005).

4 272 V. Jomelli et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 281 (2009) In summary, tropical glaciers are very sensitive to changes in both moisture and temperature. However, although tropical glaciers react to current climatic variability (e.g., ENSO), glaciers of the inner tropics should be much more sensitive to temperature than glaciers of the outer tropics, which depend to a greater extent on precipitation deficit, particularly between December and February (Favier et al., 2004b). 3. Data and methods 3.1. Documenting glacier fluctuations The glaciers that were considered in this review were predominantly small (ranging from 0.2 to 5.5 km 2 ) and displayed different geomorphic characteristics, particularly in the elevation of the accumulation area, which ranges between 5000 and 6400 m. Most of the glaciers studied are located in the Cordilleras Blanca and Vilcanota (Peru), the Cordilleras Real and Quimsa Cruz (Bolivia), and the Cordilleras Occidental and Oriental (Ecuador; Table 1; Fig. 2). Recently, investigations were undertaken in the Cordillera de Merida (Venezuela). The location of these glaciers enabled the authors to take into account climatic variations observed between the inner tropics with more or less continuous precipitation throughout the year and the outer tropics, which are characterised by one dry season (April September, when subtropical conditions prevail), and one wet season (October March, when tropical conditions prevail) (Kaser, 2001). The location of these glaciers also reflects the shape and setting of the tropical Cordilleras, which form a significant barrier to the dominant and persistent easterly atmospheric flow separating the wet Amazon side from the dry Pacific side to produce pronounced windward and leeward effects Reconstructing glaciological parameters: glacier length, surface area, volume and equilibrium line altitude (ELA) Variations in glacier length and the ELA are the dominant glaciological parameters used to infer climatic hypotheses. In Peru and Ecuador, reconstructions of variations in length were based on a GPS survey of the moraines, combined with analyses of aerial and terrestrial photographs, historical documents, and old topographic maps. In general, historical documents from before the late 19th century that report fluctuations in tropical glaciers are rare (Wagner, 1870). In Ecuador, however, many explorers made landscape drawings (La Condamine, 1751; de Humboldt, ; Boussingault, 1849; Wolf, 1892), engravings (Whymper, 1892) or paintings (Stübel, 1897; Meyer, 1907), which provide accurate illustrations for accounts of their adventures or their scientific reports (see Hastenrath, 1981; García and Francou, 2002; Francou, 2004). For this review, these illustrations were not used to directly map the past extent of a glacier but were considered as qualitative estimates. Additionally, substantial information is available from the first glaciological expeditions at the beginning of the 20th century (Troll, 1929; Sievers, 1914; Kintzl, 1942; Broggi, 1943), when the first cartographic expedition, organised by the Austrian German Alpine Club, explored the Cordillera Blanca and the Cordillera Real. From the 1930s on, glaciological observations and terrestrial photographs documented changes in glacier length. In Bolivia, changes in length were supplemented with analyses of variations in surface area and volume (Rabatel et al., 2006). Glacier volume was reconstructed for the main moraine stages using moraine height and proglacial margin shape. Variations in the volume of a glacier between two consecutive moraine stages enabled computation of the mean mass loss, i.e. the mean mass balance for the period concerned (Rabatel et al., 2006). In Venezuela, glacier topography was reconstructed by defining the glacial limits, calculating ice thickness along the glacier centerline, and contouring the glacier surface. The glacier centerline was defined by connecting the lowest points in the topographic cross-sections of the glacier surface (Polissar et al., 2006). Fig. 2. Location map of the tropical glaciers reviewed in this paper. In Bolivia and Peru, the ELA was reconstructed for each moraine stage using the accumulation area ratio method (AAR) and a ratio of 0.65, based on current measurements on glaciers. In Ecuador, ELA fluctuations were estimated from the method of Area Altitude balance ratio (AABR), also known as the Balance Ratio method (e.g., Benn and Evans, 1998; Benn et al., 2005). This method accounts for both mass balance gradients and reconstructed glacier hypsometry. The AABR is defined as the ratio of the mass-balance gradients of ablation and accumulation. The assumption of a constant AABR value enables estimation of palaeo-ela values (e.g., Osmaston, 2005; Stansell et al., 2007). The vertical profile of the mass balance is specific to a region and specific mass balance profiles emphasise the relationship between a glacier and the regional climatic setting (Kaser, 2001; Kaser and Osmaston, 2002). Gradients were computed from direct mass balance measurements made on different glaciers between 1995 and The mass balance in the accumulation area was estimated using snow pits, while ablation was measured every month using ablation stakes (Francou and Pouyaud, 2004). In Ecuador, fluctuations in the ELA were also estimated from historical data that provide a description of the snow-line altitude (Francou, 2004). At this latitude and on conic volcanoes (i.e., with limited snow accumulation) this limit fits well with the altitude of the equilibrium line of glaciers. The accuracy of the data was assessed through comparison of altitude measurements from the high summits (made by the French Academicians) and the modern estimation obtained with a differential GPS at the end of the 20th century. For example, the French Academicians La Condamine and Bouguer in the middle of the 18th century estimated the elevation of the Chimborazo volcano to be 6276 m only ~10 m higher than the modern estimation of 6268 m obtained from a differential GPS at the end of the 20th century (García and Francou, 2002). Depending on the document, a confidence degree betterthan 100 m can be assumed forthe resulting data (Hastenrath, 1981; Francou, 2004). These measurements are relevant for the central valley (Illiniza or the western slope of Antizana, for instance). However, one should keep in mind that, like today, the ELA 0 (at equilibrium) could have varied in space depending on the position of

5 V. Jomelli et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 281 (2009) glaciers in the Andean chain, and particularly their exposure to the humid fluxes coming from the Amazonian basin. As is true of current glaciers, past ice caps could not have had tongues extending far from the ELA 0,and it is thus logical to consider their lower reaches as the ELA 0 with an uncertainty of ±50 m. Indeed, current glacier limits on small ice-capped volcanoes coincide with the ELA with an uncertainty of b100 m, implying that glaciers are in balance with climatic conditions. Field experiments on Ecuadorian glaciers support this finding. Both AAR and AABR methods assume that modern and past glacier characteristics are similar. This assumption, however, has not been rigorously verified under different climate conditions, which introduces uncertainty in paleo-ela reconstructions Dating glacier fluctuations A variety of dating methods have been used to estimate glacier fluctuations. In Venezuela, the timing of glacier fluctuations was estimated indirectly with continuous decadal-scale lake-sediment records from two watersheds in the Cordillera de Merida. Increased glacierisation of catchments enhances clastic sedimentation in proglacial lakes, leading to higher concentrations of fine-grained magnetic minerals that can be visually identified by colour changes and quantified by measuring magnetic susceptibility (Polissar et al., 2006). In the other countries, the timing was based on moraine dating using lichenometry, radiocarbon dating of peat bogs (Gouze et al., 1986) or remains of Polylepis trees that were killed by glacial advances (Rothlisberger, 1987). Historical documents, for instance old paintings, were also used, predominantly as a relative dating method (Fig. 3). Peruvian, Bolivian and Ecuadorian moraines were mainly dated by lichenometry and dates can thus only be considered as minimum ages. Lichenometry was developed in the 1950s by the botanist Beschel (1961) to date paleo-glacial extent in the Alps. The method involves measuring the diameter of lichens (symbiotic association between an algae and a fungus) on dated and undated surfaces, and identification of a transfer Fig. 3. Altar glacier in Ecuador painted by Troya in 1872 (a) and by Meyer in 1903 (b). At the end of the 19th century the snout of the glacier was located several hundred meters upslope from the MGE moraines clearly visible in the background. The glacier is currently located at the high part of the cirque in the shadow of the rock faces.

6 274 V. Jomelli et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 281 (2009) function between lichen size and age. As the growth of the lichens depends on climatic conditions, as well as other parameters like lithology, a transfer function was derived in each country. However, since the pioneering work of Beschel (1961), several approaches have been used to date glacial moraines of tropical glaciers. Some authors used the five largest lichens method (see Rodbell, 1992 and Solomina et al., 2007 for example), while others used the generalized extreme value method (GEV) (see Jomelli et al., 2008a for example). This distinction is important for two reasons: 1) a comparison between the two methods revealed differences in dates (Jomelli et al., 2007a); 2) the GEV method is the only one that can provide uncertainty values for estimated dates (Jomelli et al., 2007a). In the first approach (the five largest lichens), the sampling method consists of measuring the largest lichen colonizing a boulder, and repeating the experiment for many different boulders on each moraine. This information is summarised by computing the average of the five largest lichens measured on selected boulders for each moraine. In the GEV approach, the diameter of the largest lichens is described, not only by computing averages (as was previously the case), but also by modelling the entire distribution of all lichen measurements. Because only the largest lichen on each boulder is used, the distribution of the lichen data (maxima) cannot be normal but instead follows a specific, generalized extreme value distribution (GEV; Naveau et al., 2005, 2007), whenever the sample size is large enough. Based on this procedure, a Bayesian model is built, and growth function parameters are treated as random variables with prior distributions (Naveau et al., 2007). The empirical distribution for each parameter is then computed with a Monte Carlo Markov chain (MCMC) procedure. Confidence intervals for the age of the undated moraine are computed from the mean and from the variance of computed age distributions. Finally, in order to reduce error estimates, measurements corresponding to dated and undated surfaces are compiled into the same data set and analysed together. Some dated surfaces used to derive the growth curves display considerable uncertainty, which is then included in the statistical analysis Glacier climate models Climatic parameters inferred from glacier fluctuations in the tropical Andes (usually changes in temperature or precipitation) have been computed using pioneer equations derived from Hastenrath (1984) and Kuhn (1989), and adapted to palaeoclimatic and tropical conditions (Seltzer, 1994; Kaser, 2001). These formulae used either ELA fluctuations estimated with different techniques or mass balance variations and were applied to the glaciers in the different countries concerned. In Ecuador, climate hypotheses proposed to explain glacial evolution during the LIA were based on a simple area altitude balance ratio (AABR) model. ELA values provided by historical records were used as independent data and were compared with AABR values. Moreover, in order to infer precipitation and/or temperature with ELA values (Kaser et al., 1990), the equation given by Greene et al. (2002) was used (see below). This equation represents a statistical average for glaciers at a global scale (65 glaciers) including tropical glaciers (13 glaciers). The equation was calibrated assuming annual precipitation from the climatology of Shea (1986) and freezing height computed for each site from the NCEP-NCAR reanalysis (Kalnay et al., 1996). Finally, data for both freezing height and precipitation were interpolated bilinearly to the locations of the ELA data points. This method needs to be evaluated carefully for individual glaciers (Stansell et al., 2007). However, it was suitable for the ELA measurements performed on Antizana Glacier 15 between 1995 and 2005 (Jomelli et al., 2007b). According to Greene et al. (2002), ELA can be expressed as follows: ELA = :01FH 0:51P where FH is the freezing height value (m asl.) and P is the mean annual precipitation (mm). Temperature variations were inferred from the ð1þ freezing altitude taking into account a lapse rate of 6.5 C km 1.This value corresponds to a mean of the values commonly used in the tropical Andes (e.g., Stansell et al., 2007). In Venezuela, temperature and precipitation were derived from ELA fluctuations using the accumulation area ratio (AAR) (Polissar et al., 2006). Changes in temperature and precipitation associated with variations in the ELA were estimated by combining equations for mass and heat balances. Variations in the ELA were used to estimate changes in accumulation amounts, temperature, radiative balance or any combination of these parameters. In Bolivia, the model derived from variations in the ELA was combined with the balance sensitivity analysis (Rabatel, 2005; Rabatel et al., 2006, 2008) proposed by Hastenrath (1984) and Hastenrath and Ames (1995). This required interpretation of mass-balance variations in terms of energy modulation. Assuming that all the energy available at the glacier surface is used for melting, possible variations in the climatic parameters that can produce such an energy modulation can be independently deduced. In view of current climate glacier relationships, the interpretation of variations in glacier length or the ELA is relevant if it takes into account changes in both temperature and precipitation (e.g. Benn et al., 2005) instead of only temperature. These relationships are the basis of current modelling approaches in the tropical Andes. However, models assuming temperature and precipitation variations still do not enable a complete description of the glacier climate interaction. Future work should use more detailed approaches that make assumptions of the annual and daily cycles of meteorological variables (e.g. Kull and Grosjean, 2000; Kull et al., 2007). This method requires excellent reconstruction of paleohypsometry and dynamics of glaciers, which were not available for all the glaciers in the present study. Although a link between temperature and ablation is not supported by direct physical ablation processes, correlations between temperature and ablation are generally significant at an annual or a decadal timescale. Nevertheless, calibration of positive degree day models (PDD) requires very long mass balance time series, which are not often available in the Andes. Additionally, the use of the PDD model is sometimes controversial within the tropics. Complete physical modelling of ablation processes including climatic regionalisation (Gerbaux et al., 2005) should be encouraged. In summary, the reconstructed temperature and precipitation data presented here should be considered as estimations of variations with regard to current means. Isolating the most important climatic parameters that control glacier fluctuation in the current period is an important aspect, but the response time of the glacier must also be taken into consideration. Tropical glaciers are considered to be fast-responding glaciers (Kaser et al., 2003) because they are characterised by a strong activity index (Kaser, 1999; Kaser and Osmaston, 2002). Hence, for several glaciers, a significant correlation has been observed between fluctuations in the annual mass balance and snout position. Generally the uncertainties of the dating methods used in the different studies (typically 30 to 80 years) are equivalent to, or larger than, the time lag of the glaciers concerned. 4. Results 4.1. The chronology of major glacial advances Three analogous phases of glacier fluctuations have been identified in the different countries based on numerous publications: An early advance ending around 1350, a maximum glacial extent ending between the 17th and 18th centuries, and a phase of general glacial retreat interrupted by only minor glacial advances The early glacial advance Dating of glacial landforms and lake sediments revealed an early glacial advance at the beginning of the last millennium in the tropical Andes (Fig. 4). In the Cordillera Vilcanota (Peru), Mercer and Palacios (1977) dated glacial moraines around 630±65 years BP (i.e. AD 1289

7 V. Jomelli et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 281 (2009) Fig. 4. Glacial advances in the tropical Andes over the last millennium from 10 N to 16 S recorded synchronously for most of the glaciers in each region. Solid black triangle = maximum glacial extent (MGE), empty triangle = minor glacial advance (dating uncertainty relative to the size of the triangle). 1437; 2 sigma ranges). These observations were corroborated by lichenderived ages of 1330±29 from the Cordillera Blanca (Peru), where the remnants of moraines of this glacial advance were observed (Jomelli et al., 2008a). However, these dates have to be interpreted with caution due to the small number of moraine deposits analysed (4 moraines). Nevertheless, there was a marked difference in the size of the lichens between the moraines from the 14th century and those dating from the late Holocene, suggesting a lack of glacial extension of comparable intensity for several centuries. In the Cordillera Real (Bolivia), the moraines at the base of the snout of three glaciers (i.e., Glacier Telata) correspond to this 14th century glacial stage. However, as is true for the Cordillera Blanca (Peru), this 14th century glacial stage is absent in most valley glaciers, suggesting that younger glacial advances extended further than those that occurred in the 14th century. In the inner tropics, moraines corresponding to this glacial stage have not been found. In Ecuador, moraine records did not reveal a glacial advance in the early 14th century (Hastenrath,1981; Jomelli et al., 2007b), suggesting that this glacial advance in the 14th century was shorter than, and thus destroyed by subsequent advances, or this extent did not take place. This first explanation is true of most glacial valleys in the outer tropics. Conversely, analysis of continuous decadal-scale lake-sediment records in Venezuela, indicate that a glacial advance occurred between AD 1180 and AD 1350 (Polissar et al., 2006). Between the 14th and the 17th centuries, evidence of glacial advances is limited, except in Ecuador where colonial archives suggest that glacial advance near Quito were less extensive during the 16th century than those of the 17th to 19th centuries (Hastenrath,1981). In the Cordillera de Merida (Venezuela), fine-grained lake minerals revealed a glacial advance between AD 1450 and 1590 (Polissar et al., 2006). In addition, Clapperton (1983) reported that glaciers on the Ruiz-Tolima volcanic complex in Colombia began their late Holocene advance after AD The maximum glacial extent In several cordilleras in Peru and Bolivia, the maximum glacial extent (MGE) for the last millennium defined as the furthest downvalley extent recorded synchronously by the majority of glaciers (32 glaciers) occurred in the 17th century. In the Cordillera Blanca (Peru), moraines corresponding to this glacial advance were dated at ca. 1630±27 using the GEV method (Jomelli et al., 2008a). Moraines dated at the end of the 16th century by Solomina et al. (2007) using the five largest lichens approach were dated again using the GEV method and also correspond to this 17th century glacial stage. In the Cordillera Real and Cordillera Quimsa Cruz (Bolivia), the glacial advance recorded on 13 glaciers using lichenometry ended ca. 1657± 20 to 1686±20, depending on the glaciers (Rabatel, 2005; Rabatel et al., 2005, 2006, 2008). In Ecuador, lichenometry (GEV method) was used to date terminal moraines at the base of 15 glaciers, and it was observed that the MGE occurred at two different periods, depending on the altitude of the glaciers (Jomelli et al., 2007b). On glaciers covering volcanoes above 5700 m asl. (i.e., Chimborazo and Antizana volcanoes), terminal moraines were dated to be from the beginning of the 18th century (1720±16), which is in agreement with analysis of historical documents by Hastenrath (1981). On glaciers located on summits below 5400 m asl, the MGE was recorded at the beginning of the 19th century (1830±14) (Jomelli et al., 2007b). In Venezuela, two glaciers advanced occurred between 1640 and 1730 (Polissar et al., 2006), synchronously with other previously maximum glacial advances described in Peru, Bolivia and Ecuador. In Colombia, the lack of accurately dated moraines for this time period prevents us from drawing any conclusions. In their review of Quaternary glaciations in the northern Andes, Schubert and Claperton (1990) report a 14 C date of AD 1650 from basal peat sampled from a pond located behind the terminal moraine of the Cataca glacier in the Sierra Nevada de Santa Marta The deglaciation phase After the MGE, glaciers in the outer and inner tropics underwent a continuous retreat that was interrupted by only minor advances. In the 18th century, two glacial advances occurred synchronously in the

8 276 V. Jomelli et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 281 (2009) Fig. 5. Evolution in length from the MGE of four glaciers in the tropical Andes (Charquini in Bolivia; Akillpo in Peru; Meyer at Chimborazo and Carihuayrazo in Ecuador at high and low elevations, respectively). Cordillera Blanca (Peru) and in the Cordillera Real (Bolivia), in the 1730s and in the 1760s respectively. In Ecuador, a glacial advance was recorded in 1748±16 in glaciers located on high summits. In the 19th century, moraine deposits revealed three glacial advances in the 1800s, 1850s and 1870s (Rabatel, 2005; Rabatel et al., 2008; Jomelli et al., 2007b). The buried soil horizon published by Rothlisberger (1987) at 440±185 years BP (i.e. AD , 2 sigma ranges) from the base of Ocshapalca Glacier in Cordillera Blanca, suggests that the advance that occurred some time later could correspond to this stage. In Venezuela, a minor glacial advance was recorded between 1800 and 1820 in two watersheds (Polissar et al., 2006). In Colombia, Schubert and Claperton (1990) report the existence of a moraine dated in the 1700s located 50 m higher than the MGE at the base of the Cataca Glacier (Santa Marta), suggesting that ice cover was approximately 2.7 times greater than current. Kaser (1999) concluded that glaciers in the Cordillera Blanca advanced in the mid 1920s, prior to rapid retreat between 1930 and 1950, followed by weaker retreats. Similarly, in the 1970s several glaciers advanced, with subsequent rapid retreats (Mark and Seltzer, 2005). According to Rabatel et al. (2006), the glacier retreat that continued throughout the 20th century in Bolivia occurred in three stages: (1) a major retreat starting in the late 19th century; (2) a relative slowdown between the 1910s and the 1930s with a marked increase after the 1940s; and (3) a very strong retreat during the 1980s and 1990s. Reconstruction of glacier volume by photogrammetry using aerial photographs taken in 1956, 1963, 1974, 1983 and 1997 showed that the mean mass balance of the five glaciers of a small summit in the Cordillera Real was two- to three-fold more negative during the and periods than during the period (Rabatel, 2005). 17th century and the middle of the 18th century (Fig. 5). This loss subsequently increased to reach about 15% in the 19th century. In addition, these authors estimated a reduction in area of about 15% between the MGE and the end of the 19th century for the Bolivian glaciers (Rabatel et al., 2006). Additionally, between 1665 and 1870, ice loss was about 0.10 m we yr 1. Between 1870 and 1910, the mass balance was on average 0.40 m we yr 1.Fortheperiod , deficits were less pronounced, with an average of 0.25 m we yr 1.In the last two decades of the 20th century, the loss increased significantly to 1.44 m we yr 1 (Rabatel et al., 2006). These results should be interpreted with caution as some periods can include phases of readvances and thus only represent the estimated average value for the period. In Peru, the glacial retreat displays the same values as in Bolivia. From the MGE to 1938, Hastenrath and Ames (1995) measured a retreat of the Yanamarey Glacier snout (Cordillera Blanca) of ca. 950 m. More generally, twenty Peruvian glaciers lost between 12% and 17% of their area in the 18th century (estimated from the MGE to the end of the 20th century), with a rate of about 17 20% in the 19th century (Jomelli et al., 2008a). Ecuadorian glaciers showed the same trend. From the MGE to the middle of the 20th century, a survey of 15 glaciers located on the slopes of high and low altitude summits showed they lost about 30± 5% and 36±5% of their total length respectively. As in other countries, the loss was greater during the 19th century than before (Fig. 5) ELA fluctuations In the Cordillera Real (Bolivia), Rabatel et al. (2008) estimated an increase in the altitude of the ELA determined geometrically at ca. 150 m with the accumulation area ratio method (AAR, ratio 0.65), with a total range of m depending on the glacier, between the MGE and the late 20th century. However, assuming an AAR 0 (the ELA value for a glacier in equilibrium for the period) in the late 20th century, the ELA 0 increased by about 300 m (Fig. 6). In the Cordillera Blanca, Jomelli et al. (2008a) observed a similar trend estimated with the same AAR method and ratio. Considering that the terminal moraines of the 14th century stage are close to those of the 17th century, the ELA of AD 1350 was probably at the same elevation as in the 17th century. From the maximum 17th century extent to the late 19th century, the ELA increased by about 108±30 m. The ELA also 4.2. Evolution of glaciological parameters Evolution of the size of glaciers Few papers provide estimations for the evolution of glaciological parameters in the last millennium. Variation in length is the only parameter that can be compared between at least three countries. In the Cordillera Real (Bolivia), Rabatel et al. (2006) observed that, throughout the period from the mid 17th century to the end of the 19th century, all five glaciers in their study retreated a distance of between 950 and 1400 m. Variations in length were not homogeneous in recent centuries. Glaciers lost about 5% of their total length between the middle of the Fig. 6. Fluctuations in the ELA estimated with the AAR method in the Cordillera Blanca (diamonds) in Bolivia (circles) and in Ecuador (triangles). Uncertainties correspond to the values obtained from different glaciers.

9 V. Jomelli et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 281 (2009) increased significantly (more than 150 m in some cases) between 1930s and the end of the 20th century. In Ecuador, historical analysis revealed an increase in the snow line assimilated with ELA at ca m from the 18th to the end of the 20th century. In the middle of the 18th century, the lower limit of the snow line in the Central Valley was ca. 4700±50 m asl. According to descriptions provided by La Condamine (1751), Bouguer (1748) and Humbolt (1804), it appears to have increased by approximately 50 m in Observations made between 1869 and 1873 gave an estimated value of 4800±50 m, indicating that the altitude of the snow line increased by ca. 50 m from the beginning of the 19th century. The permanent snow line was located 4830 m asl at the beginning of the 20th century (Meyer, 1907), then reached about 4950 m asl in the 1980s (Hastenrath, 1981). Finally, recent studies at Antizana (Cáceres et al., 2005) showed that the current ELA 0 is located at 5030 m asl in the eastern cordillera (Maisincho et al., 2007). In Venezuela, using the AAR method, Polissar et al. (2006) estimated the altitude of the ELA of two glaciers at the MGE to be approximately m lower than today Climatic hypotheses derived from glacier fluctuations Climatic conditions at the MGE Given the limited number of observations available, the climatic conditions that prevailed between the 13th and 14th centuries in Bolivia and Peru cannot be estimated using moraines. In Venezuela, the high concentration of clastic sediments observed in two sediment cores between AD 1300 and 1550 was interpreted as a higher precipitation/ evaporation rate (Polissar et al., 2006), which was supported by higher abundances of Cyperaceae (sedge) pollen. Between the 17th century and the beginning of the 19th century, climatic conditions estimated from glacier fluctuations in the outer and inner tropics indicate a cooler and wetter period than today. Indeed, in order to explain the formation of the LIA moraines at ca m in the cordilleras of Peru, and Bolivia in the middle of the 17th century, and at ca m in Ecuador in the early 18th century, the glacier mass balance had to be in equilibrium with the climate at lower elevation (several hundred meters) than today. Current physical ablation processes suggest that greater precipitation than today could explain their glacial apposition. In addition, colder temperatures may have contributed to a lower snow/rain limit, reduced ablation and hence glacier expansion. In Bolivia, Kaser's model (2001) suggests that to increase mass balance and induce the pronounced glacier advance of the 17th century, precipitation had to increase by 20 30% compared to present conditions. Hastenrath and Ames' (1995) mass balance sensitivity analysis was in agreement, showing that cloudiness had to increase by 1 2/10. Independently, both models showed that temperature had to decrease by K (Rabatel, 2005; Rabatel et al., 2006, 2008). In Ecuador, MGE modeling was used to propose climatic hypotheses for glaciers located on high summits to explain the formation of the terminal moraines at 4400 m asl at the beginning of the 18th century. The ELA was assessed from BR values, and accumulation at the summit was deduced from the gradient of the mass balance in the accumulation area. Finally, the temperature was obtained with Eq. (1) (Jomelli et al., 2007b). If the mean temperature was 0.8 C to 1.1 C lower than today, the model suggests that at the beginning of the 18th century precipitation at the summit was about 30% higher than today. This cool and wet period deduced from modelling glaciological parameters in Bolivia and in Ecuador appears to qualitatively agree with results obtained from other proxies. Analyses of different ice core records (Quelccaya and Huascaran in Peru, and Sajama and Illimani in Bolivia) show relatively depleted δ 18 O values indicating cooler and wetter conditions from the end of the 16th century until the beginning of the 19th century, defined as the LIA by Thompson et al. (1986). The minimum δ 18 O between AD 1650 and AD 1750 is synchronous with the maximum glacial advance recorded from glacial moraines (Rabatel et al., 2005; Solomina et al., 2007; Jomelli et al., 2008a; Rabatel et al., 2008) (Fig. 7). Thus, Peruvian and Bolivian glacier advances in the 17th century were triggered both by a decrease in temperature and an increase in snow accumulation Interpreting glacier retreat after the MGE Glacier modeling allows us to assess to what extent climatic conditions would have changed after the MGE to trigger the glacier recession observed throughout the tropical Andes. In the Bolivian Andes, glaciers began to recede after 1740 (Rabatel, 2005; Rabatel et al., 2008). Glacial retreat was moderate, but continuous, until about In this region of the Andes, there is no evidence to suggest that temperature increased during the mid-18th century, as it did in Europe and in other parts of the Northern Hemisphere (Chuine et al., 2004; Moberg et al., 2005). Consequently, Rabatel et al. (2008) assume that the recession of the Bolivian glaciers after 1740 was mainly a consequence of continuously drier conditions than in those prior to These authors interpreted the reconstructed ELA and mass balance using Kaser's model and Hastenrath and Ames' mass-balance sensitivity analysis, and calculated that such dry conditions resulted in a decrease of about 20% from the current mean accumulation rates on glaciers. This finding is consistent with hypotheses derived from other proxies in favour of a drier climate in the second part of the LIA (i.e. ~1740 ~1870) (Liu et al., 2005). From the late 19th century to the early 20th century, the accelerated glacier recession observed in the Central Andes is not consistent with temperatures measured on the world scale, which did not increase significantly before the first half of the 20th century (IPCC, 2007). The significant rise in the ELA on glaciers can thus only be interpreted in terms of variations in precipitation. Such variations could have encompassed a decrease of 15 20%, compared with the present. Results reported by Thompson et al. (1985) for the Quelccaya ice cap support the occurrence of drier conditions in the period between 1870 and Using a network of weather stations distributed throughout the intertropical zone, Kraus (1955) also identified an abrupt decrease in precipitation in the period between 1870 and In Ecuador, modelling revealed differences in temperature and precipitation that may have induced the time lag between the LIA maximum of low- and high-altitude glaciers. At the beginning of the 19th century, when high-altitude glaciers underwent a minor advance during a period of overall retreat, low-altitude glaciers underwent a marked glacial advance, which created the outermost moraines. Results suggest that temperatures must have been very cold for a short period (colder than 0.8 C during one or two decades) just before the beginning of the 19th century and precipitation must have been less than in the 18th century (possibly 10% less), inducing a decrease in the ELA value of about 70 m for both high- and low-altitude glaciers (Jomelli et al., 2007b). For low-altitude glaciers, the increase in the size of the accumulation area counterbalanced the decrease in accumulation amounts, whereas the marked decrease in accumulation had more impact on the mass balance of the high-altitude glaciers. However, models assuming temperature and precipitation variations do not fully describe the glacier climate interaction. Hence results should be interpreted with caution. Nevertheless, the mass balance method clearly suggests that, following a humid period, a short, cold and dry period would explain the time lag between the LIA maximum of low and high-altitude glaciers. Finally, from the middle of the 19th century, the general glacial retreat of both high- and low-altitude glaciers was due to a combination of increasingly warmer and drier conditions than the mean moisture conditions observed during the 18th century. In Venezuela, Polissar et al. (2006) estimated changes in temperature and precipitation associated with ELA values computed with the AAR method by combining equations for the mass and heat balances of glaciers. The results suggest that the 300 to 500 m lowering of the ELA for the whole period ( ) is equivalent to a C drop in