Delphine Grancher

Irregular tropical glacier retreat over the Holocene epoch driven by progressive warming

The causes and timing of tropical glacier fluctuations during the Holocene epoch (10,000 years ago to present) are poorly understood. Yet constraining their sensitivity to changes in climate is important, as these glaciers are both sensitive indicators of climate change and serve as water reservoirs for highland regions. Studies have so far documented extra-tropical glacier fluctuations, but in the tropics, glacier–climate relationships are insufficiently understood. Here we present a 10Be chronology for the past 11,000 years (11 kyr), using 57 moraines from the Bolivian Telata glacier (in the Cordillera Real mountain range). This chronology indicates that Telata glacier retreated irregularly. A rapid and strong melting from the maximum extent occurred from 10.8 ± 0.9 to 8.5 ± 0.4 kyr ago, followed by a slower retreat until the Little Ice Age, about 200 years ago. A dramatic increase in the rate of retreat occurred over the twentieth century. A glacier–climate model indicates that, relative to modern climate, annual mean temperature for the Telata glacier region was −3.3 ± 0.8 °C cooler at 11 kyr ago and remained −2.1 ± 0.8 °C cooler until the end of the Little Ice Age. We suggest that long-term warming of the eastern tropical Pacific and increased atmospheric temperature in response to enhanced austral summer insolation were the main drivers for the long-term Holocene retreat of glaciers in the southern tropics.

A New Tree-Ring-Based, Semi-Quantitative Approach for the Determination of Snow Avalanche Events: use of Classification Trees for Validation

Abstract On forested paths, dendrogeomorphology has been demonstrated to represent a powerful tool to reconstruct past activity of avalanches, an indispensable step in avalanche hazard assessment. Several quantitative and qualitative approaches have been shown to yield reasonable event chronologies but the question of the completeness of tree-ring records remains debatable. Here, we present an alternative semi-quantitative approach for the determination of past snow avalanche events. The approach relies on the assessment of the number and position of disturbed trees within avalanche paths as well as on the intensity of reactions in trees. In order to demonstrate that no bias was induced by the dendrogeomorphic expert, we carry out a statistical evaluation (Classification and Regression Trees, or CART) of the approach. Results point to the consistency and replicability of the procedure and to the fact that the approach is not restricted to the identification of high-magnitude avalanches. Evaluation of the semi-quantitative approach is illustrated on a well-documented path in Chamonix, French Alps. For the period 1905–2010, comparison between the avalanche years recorded in a substantial database (Enquête Permanente sur les Avalanches, or EPA) and those defined with dendrogeomorphic techniques shows that the avalanche record reconstructed from tree-ring series contains 38% of the observed events.

Can we infer avalanche–climate relations using tree-ring data? Case studies in the French Alps

Abstract Dendrogeomorphology is a powerful tool to determine past avalanche activity, but whether or not the obtained annually resolved chronologies are sufficiently detailed to infer avalanche–climate relationships (in terms of temporal resolution) remains an open question. In this work, avalanche activity is reconstructed in five paths of the French Alps and crossed with a set of snow and weather variables covering the period 1959–2009 on a monthly and annual (winter) basis. The variables which best explain avalanche activity are highlighted with an original variable selection procedure implemented within a logistic regression framework. The same approach is used for historical chronologies available for the same paths, as well as for the composite tree-ring/historical chronologies. Results suggest that dendrogeomorphic time series allow capturing the relations between snow or climate and avalanche occurrences to a certain extent. Weak links exist with annually resolved snow and weather variables and the different avalanche chronologies. On the contrary, clear statistical relations exist between these and monthly resolved snow and weather variables. In detail, tree rings seem to preferentially record avalanches triggered during cold winter storms with heavy precipitation. Conversely, historical avalanche data seem to contain a majority of events that were released later in the season and during episodes of strong positive temperature anomalies.

A Response to Bradwell's Commentary on Recent Statistical Studies in Lichenometry

In his commentary ‘Lichenometric Dating: A Commentary, in the Light of some Recent Statistical Studies’, Bradwell (2009) attacks the Generalized Extreme Value (GEV) approach (Cooley et al. 2006; Jomelli et al. 2007; Naveau et al. 2007) recently employed in several lichenometric studies (Jomelli et al. 2008; Rabatel et al. 2008; Chenet et al. 2009). Bradwell judged the GEV approach as too unconventional for geomorphologists, overly complex, and incapable of bringing any added value to the field of lichenometry. Furthermore, the article raises a more general philosophical question: ‘Can statistical complexity and high precision in a “geobotanical” dating technique, fraught with high degrees of environmental variability and inbuilt uncertainty, ever be scientifically valid?’ We disagree with Bradwell’s assessment. Furthermore, we think that Bradwell does not fully recognize the assumptions made in the traditional lichenometric analyses that he recommends.

Are Debris Floods and Debris Avalanches Responding Univocally to Recent Climatic Change – A Case Study in the French Alps

Debris flow is a dominant mass movement process in mountain areas all over the world and is a significant natural hazard. A classical distinction is made between a debris flood (DF) corresponding to a rapid, surging flow of water, heavily charged with debris in a steep channel, and a debris avalanche (DA) corresponding to a rapid or extremely rapid shallow flow of partially or fully saturated debris on a steep slope without confinement in an established channel (Hungr, 2005). In mountain areas like the Alps, the increase in human activity has resulted in increased risks of natural hazards such as debris flows. There is thus a growing demand for hazard zoning and debris flow protection. However, debris flows are caused by complex interactions between local topography, weather and sediment properties, making the understanding of debris flow activity very difficult. Because anticipated changes in climate may alter the dynamics of slope processes and the frequency or magnitude of extreme events, understanding the mechanisms that link climate and debris flow activity is the first step in any attempt at forecasting. Consequently, many studies have focused on the meteorological conditions that trigger debris flows in different environmental conditions in northern Europe (Innes, 1985; Rapp, 1995; Nyberg and Rapp, 1998) and in the Alps (Haeberli et al., 1990; Zimmerman & Haeberli, 1992; Rebetez et al., 1997). Triggering thresholds based on analyses of intense rainy events or long duration precipitation have been proposed for different spatial scales (Caine 1980; Guzzetti et al., 2008). An increase in temperatures and changes in the amount and frequency of rainfall have been observed in different mountain regions in the last few decades. Such changes in climate conditions could have an impact on the intensity and/or frequency of debris flows. However, only a few authors conducted detailed analyses of the impacts of climate change on DF activity to check the validity of this hypothesis. In British Columbia, Canada, Jakob & Lambert (2009) predicted an increase in the total number of debris flows by the end of the century due to increases in precipitation. From tree-ring series Stoffel & Beniston (2006) clearly show that the debris-flow frequency at Ritigraben (Swiss Alps) increased in the 1866–1895 period that followed the maximum extent of LIA glaciers and that events occurred most often in the early decades of the 20th century.

Perception of the risk of tsunami in a context of high-level risk assessment and management: the case of the fjord Lyngen in Norway

BackgroundNorth Norway, the banks of the fjord Lyngen are highly exposed to a rockslide tsunami hazard. However, the local municipality believes that the coastal community is well-informed about the risk and ready to evacuate, should a warning be issued. Accordingly, the social survey we conducted in this municipality was a matter of exploring three main questions: is the Lyngen population well-informed about the tsunami risk in general and about the potential evacuation time in particular? Is the local population as confident as the local municipality hope? Is there enough information on the tsunami risk for tourists, given their growing number?.ResultsThe survey shows that the local population has a clear perception of the tsunami hazard, but that warning and evacuation conditions are not sufficiently well-known, despite the local and national communication work. Moreover confidence in the municipal authorities seems to be imperfect, although confidence concerning hazard surveillance is higher than confidence in the information provided on risk and management. As often, tourists are less informed on natural hazards or evacuation conditions.ConclusionsThe municipal authorities have to improve the information locally delivered. Authorities must also disseminate information to the tourists, especially on a possible evacuation during their stay, so as not to raise anxiety or trigger a decline in the areas touristic appeal.

Territorial Accessibility and Decision-Making Structure Related to Debris Flow Impacts on Roads in the French Alps

The Alps are highly impacted by debris flows that cause major problems for companies and transport networks located in the valley bottoms. One such event occurred in the Rif Blanc catchment and affected the road network in the French Alps, as well as adjacent areas across the Italian border, for several days in June 2012. This article presents two independent approaches to vulnerability assessment. Based on investigations conducted during a survey of local authorities following the event, we compared theoretical risk management and real crisis management in terms of decision making and modes of intervention. Functional vulnerability and territorial consequences were analyzed using a best travel time model of accessibility. We show that a bottom-up approach is practiced in case of actual management planning with a central coordination of general council. Conversely theoretical crisis management shows prefect as the key actor supported by several other state institutions. Our analysis also revealed that a debris flow event with a local impact on the road network has territorial consequences at a regional scale. This study contributes to the discussion about how to minimize the vulnerability of alpine transport networks prone to debris flows. Our results could serve as a decision support tool for public authorities.