Rockfalls and climate in the permafrost-affected rockwalls of the Mont Blanc massif L. Ravanel and P. Deline EGU 11, Vienna, Austria, April 2011

Rockfalls and climate in the Mont Blanc massif – L. Ravanel and P. Deline – EGU 2011, Vienna ► Many smaller rockfalls during the 2003 summer heatwave in the Alps B. Sergent Les Drus, August 2003 V = m 3 Context: many rockfalls in permafrost-affected rockwalls in the last two decades ► Large recent rock falls/avalanches in high mountain areas worldwide Punta Thurwieser, 2002 V = 2.5 M m 3 Fossati Increase in the number of rockfalls: real or perceived? What correlation between rockfalls and global warming? 1

Rockfalls and climate in the Mont Blanc massif – L. Ravanel and P. Deline – EGU 2011, Vienna Study area: the Mont Blanc massif 40 km 14 km Seen from the West North face of the Grandes Jorasses (4208 m a.s.l.) Aiguille du Midi (3842 m a.s.l.) ► Large permafrost area ► Steep granitic slopes ► Very active morphodynamics ► Good accessibility ► Important vulnerabilities 2

► Overview: Identify morphological and colour changes and date the corresponding rockfalls in two areas of the massif (Drus, Aig. de Chamonix) ► Material: ~ 150 photos West face of the Grands Charmoz (3445 m a.s.l.) ► Period: End of the Little Ice Age = 150 years M1: Comparison of photographs Rockfalls and climate in the Mont Blanc massif – L. Ravanel and P. Deline – EGU 2011, Vienna 3 methods to collect and process historical and current data on rockfalls ► Results: 50 rockfalls (500 to m 3 ) 3 Bisson brothers, 1862

Rockfalls and climate in the Mont Blanc massif – L. Ravanel and P. Deline – EGU 2011, Vienna ► Period: 2003 M2: Analysis of a satellite image ► Results: 182 rockfalls (100 to m 3 ) ► Overview: Rockfalls identified in the entire massif from their supraglacial deposits ► Material: SPOT-5 image 051/257 taken at the end of the 2003 Summer heatwave 4

Rockfalls and climate in the Mont Blanc massif – L. Ravanel and P. Deline – EGU 2011, Vienna M3: Network of observers ► Results: 139 rockfalls (100 to m 3 ) 5 ► Period: 2007 to 2009 (2010 data not already processed) ► Overview: A network of observers allows to survey rockfalls in the central area (57%) of the massif ► Material: Data from observations checked and completed on the field during each fall North face of Les Droites (4000 m a.s.l.)

Rockfalls and climate in the Mont Blanc massif – L. Ravanel and P. Deline – EGU 2011, Vienna Relationship between climate and rockfalls: at a century scale °C / decade Chamonix, 1040 m a.s.l., data Météo France °C / decade R. / decade M1: Comparison of photographs (50 R.) R. have various frequencies End LIA – mid-20th c.: great stability Strong activity since two decades Concomitance between R. and hot periods 6

Rockfalls and climate in the Mont Blanc massif – L. Ravanel and P. Deline – EGU 2011, Vienna Relationship between climate and rockfalls: at a year scale M1: Comparison of photographs (50 R.) MAAT > mean Summer period Particularly hot or canicular period M2: Analysis of a satellite image (182 R.) 7 0°C isotherm

Rockfalls and climate in the Mont Blanc massif – L. Ravanel and P. Deline – EGU 2011, Vienna M3: Network of observers (139 R.) – The example of 2009 (72 R.) Aiguille du Midi, 3842 m a.s.l. Data: Météo-France Thermal factor: major role in rockfall triggering Concomitance between occurrence of rockfalls and warmest periods 8

Total N S Total N S Altitude (m a.s.l.) Rockfalls and climate in the Mont Blanc massif – L. Ravanel and P. Deline – EGU 2011, Vienna Rockfall altitude as indicator of climate conditions 255 m 175 m 235 m 320 m 9 The relationship between temperature and altitude confirms the role of climate in rockfall triggering

Rockfalls and climate in the Mont Blanc massif – L. Ravanel and P. Deline – EGU 2011, Vienna Conclusions 10 ► Hypothesis: global warming is affecting the stability of rock mountain via the permafrost degradation 3 methods of rockfall identification in the Mont Blanc massif nearly 380 surveyed rockfalls Permafrost degradation = probably the main triggering factor IPCC, 2007 Strong climatic (thermal) control on the rockfall triggering degradation measured and consistent with global warming ► Now almost all the rockfalls occurred in the context of permafrost (according to the TEBAL model)

Thanks!