The Holocene period, Climax and anthropocene

After experiencing a near-polar atmosphere,  global warming is setting up in the Alps about 10 000 years ago,  this is the Holocene or climatic optimum. this is the reclaiming plant, plants migrate from their meridional shelters to territories abandoned by ice. This is also the period where we find the first traces of human activities. The settlement of men begins to take precedence over nature. they exploit the land for agricultural purposes but also for the frame.
The Holocene  known to be stable also had some climatic variations period . According to a report from Bintz et al (1989) this period is subdivised in some others ones.

• Preboreal (10000 - 9000 BP): the transition from late glacial warming and this results in regression of herbaceous and shrub steppe taxa in favor of Betula and Pinus. Changes in vegetation during the Preboreal emphasizes improving the climate. Rising temperatures, entails a substantial increase in forest communities. Although any estimate altitudinal be difficult, it seems that the upper limit of the forest has puatteindre 1900 m in the area of intra-Alpine.
• Boreal (9000 - 8000 BP): The Boreal is the period of the peak of Corylus (hazel), which extends roughly in the floors and hill and mountain, quickly eliminating Pinus and Betula.The hazels are in competition witha "mixed oak" in which Ulmus (elm) plays an important role, rising to 1500 m above sea level. Other elements of the package deciduous Quercus and Tilia (lime) are well represented, Acer (maple) is rare, Fraxinus (ash) extends gradually. The appearance of Hedera (ivy) and Viscum (mistletoe) may reflect relatively high temperatures. In the Montane level , the Hazelnut trees still dominate, constituting a narrow belt between the Quercetum mixtum and pine forests. Groups of subalpine Pinus uncinata (pin hook), Pinus cembra (arolla pine) and Benda to develop expense of shrubland in the massive internal, the tree probably reached at the end of this period, 2 100 meters. The margins have a southern vegetation dynamics close to that of the Southern Alps. Corylus is very discreet, Quercus, Ulmus and Tilia are still poorly represented.
• Atlantic (8000 - 4700 BP):The Atlantic is a time of profound changes in the vegetation cover in the Northern Alps. The Quercetum mixtum wins in  collinean stage . Its high percentages are due mainly to the development of Fraxinus, but also Tilia and Ulmus, Acer achieved its highest values. Then, in mid- old Atlantic  Abies huge extends  in the Montane stage and has a significant wealth  on all the North Alps.The Pine Forest shows a remarkable altitudinal range, from 1 000 to 2 000 m, pushing below the "mixed oak" and spreading largely on the slopes at the expense of groups alticola Pins. The sharp increase causes a sharp rise in the upper limit. In Oisans, Abies colonized slopes above 2100m. If the massive western wetlands, Pine Forest is fast in the dry inland valleys, this training is confined to a narrow strip in shady, and the pine forests continue to dominate the intra-Alpine mountain scenery. Other distinctive features of the Atlantic are the strong increase of Alnus glutinosalincana in wetlands, the place not negligible of Taxus in the vegetation of Piemont Dauphinois and the Vercors, the expansion of Pinus cembra in the subalpine and the presence of fairly regular Alnus viridis (green alder) in altitude sites. Finally, in the external massifs. Fagus extends gradually from the 6th millennium BP. With the Neolithic, the first demonstrations of farming activities occur in the pollen diagrams. The oldest clearings, along with cereal crops, have been recognized Pluvis (210 m), Francine(288 m), Charavines-les-Bathers (495 m), Choranche(500 m), Saint-Thibaud-de-Couz (500 m) and Luitel Col (1250 m).
• Subboreal (4700 - 2700 BP): From the  end of the Atlantic , around 5000 BP, Fagus begins a continuous increase in the higher collinean and lower montane, between 400 and 1200m, . However, this species has never transgress the limits of current area that is both the south. In the Mountane stage, beech could be associated to the fir tree, it is also possible that the distributionof these two species resulted from the influence of local  and ecological factors. Abies colonizing cool, moist slopes, and Fagus hot and dry slopes. The competition factor between different tree species only may explain the regression of hardwood forests. At low altitude,elm, ash and linden and oak disappear,may be promoted by clearing the  final Neolithic and and the Bronze Age, becomes the constitutional main groups killing hill and forest.
  In the subalpine zone of massive internal Alnus viridis, Larix and Picea, which appeared in the Atlantic are quite regularly but moderately represented. The moors and montane grasslands are
  colonized by Arolles. In Oisans, for against the Pinus cembra pollen is rare. The upper limit
  Procedure of the forest rises over this chronozone, In Maurienne, it probably reached2 400-2 500 m, well above the stage. Clearing of the Neolithic Age and the Bronze, more or less marked depending on the site,reflect the growing influence of man oncover, the intensity of settlement and the colonization of territories with little or no popular before.
• Subatlantic (2700 BP to Present):Temporary colonization by the tree orBeech at the top of hill and at the  transition Subboreal-subatlantic, report wetter conditions and cooler. Moreover,  many authors have hypothesized aclimatic deterioration at the end of the Bronze Age.Until then mainly determined by the great migrations of arboréenes species that  punctuate the forest dynamics and postglacial climate change, the composition and physiognomy of the plant cover in the Northern Alps  changes depending on the needs of people. The evolution of the vegetation is deeplyupset, at all altitudes, by clearing, agricultural and pastoral practices,logging and the introduction of new species.  In collinean area ,  succesive clarifications  of oak woods have indirectly but undoubtedly favored the development of hornbeam, Walnut and Chestnut. Their introduction dates, traditionally reported to the Roman times, are not known. The major eventis the expansion of Picea in the Subalpine basis, to the detriment of the Pine Forest and Arolles, Pins hook and larches.The extension of this came later - around 2000 BP in the central area at the medieval time in Oisan- was probably favored by the increasing human pressure. A high altitude, a consequence of  the cembra degradation   and other groups  in subalpine forest , the secondary stands of Larix, moors in Ericaceae or Juniperus (Juniper), of Alnus viridis bush to spread on surfaces deforested. Finally, the chronology of the various episodes of the Little Ice Age in the MassifDes Ecrins was established on the basis of a comparison of dendroclimatic and pollen data.

References :http://www.persee.fr/web/revues/home/prescript/article/bspf_0249 7638_1989_num_86_2_9362
Consulté le 16 novembre 2011

The Alps at the time of ice

"Over the past millennia and centuries , glaciers have continued to evolve, changing volume, shape, color, forward or backward. These changes, sometimes significant, are closely linked to climate fluctuations. In cold regions where they are preserved, the different layers of ice and snow a glacier component trap over time chemical and biological elements that deliver valuable information on the successive states of the atmosphere over a long period. Ice and glaciers and offers essential tools for scientists to reconstruct past climates and better understand the current warming of the planet." (Francou et Vincent, 2007)
The last major ice age began in the Alps at the end of quaternaire.About 70 000 years ago, begins the Würm glaciation, which reaches its maximum  25 000 years ago.
At that time, ice covered all the Alpine valleys. Indeed, the Mont Blanc glacier covered the whole Arve valley to Geneva, the Rhone Glacier in Switzerland reached then the gates of the city of Lyon (about 15 km, at the current airport) . The Rhone glacier was joined by some parts of the glacier Isère (Grenoble).

Mont Blanc valley  and the main glaciers during the Würm (Geo- Alpes, 2010)

This extreme development of Glaciers, particularly in France and Switzerland explained in part by the topography of this area of the Alps. The slopes are less steep and glacial cirqueswider. Ice has no problem extending into the valleys, forming a thick and solid layer.
In the "Research on the Ice Age and the former extension of glaciers of Mont Blanc from the Alps to the Jura" Charles Martin explains that during this period, temperatures in Geneva should have decreased by 4 or 5 ° C (from 10 to 5 degrees), this way and due to the altitudinal gradient (the air temperature decreases by 1 ° C every 188 meters) the eternal snow line being lowered then 2700 m to 1955 meters. The bottom of the glacier also be lowered by 750 meters and therefore reach the Swiss plain. This plus the fact that the glacier of Mont Blanc (glacier of the Arve) is located in an area large circuses explains the fact that the glacier have been able to reach the town of Geneva from Chamonix.
the ice age left its mark in the Alps and in particular in France in the Alps in the north. Glaciers are large "erosion machines" and deep valleys post coolers are formed after the retreat of the ice like the Grésivaudan valley (Albertville, Grenoble) which now forms a deep U-shaped valley.


Grésivaudan valley  from le Touvet ( Wikipedia, 2011)

The first result of glaciation of such importance is the decline in sea level (120 meters lower than today during the Würm). This lowering of sea level causes a decrease in the surface of the continental shelf (richest  lifezone). Furthermore, the thermal gradient becomes higher, this leads for each species to a trophic zone which decreases thus increasing the competition between species.
Phase of ice to survive, a species subject to too much cold for them, should descend to the plains  and find new ecologic shelters. They must do so especially since they are sensitive to cold, or survive in smaller populations and sometimes less dense in areas of refuge less affected by cold.
During the last ice age, it does not appear to have been much loss of overall species on the planet, but for species with low dispersal ability, the cold has caused the local extinction of many populations in metapopulations then existing, with the corollary reduction of genetic diversity in some groups, these effects "negative" for biodiversity can be mitigated by the dewatering of the continental shelves helped by lower sea levels. Thus there were new areas that have reconnected habitats almost disjoint (set out for birds and marine mammals and some species) during interglacial phases (eg the current France was reconnected to the current United Kingdom during the last ice age, allowing large mammals (including mammoths) to move from one area to another across the current floor of the Channel and the Strait of the Pas-de-Calais. the genetic consequences of climatic oscillations, and glaciations in particular, are important. (wikipedia, 2011)
The late glaciation period has deeply shaped the Alps as we know it today.

A report on glacier retreat

http://www.maxisciences.com/glacier/recul-des-glaciers-un-reportage-sur-les-consequences-du-changement-climatique_art642.html

A French / English video about Glacier retreat in Swizerland. Its financial and environmental consequences.

Glacier retreat and consequences

According to a Méteo France study published in 2005, temperature in the French Alps has increased over the last four "more than the rest of France." Indeed, as temperatures on the French territory increased 1 degree on average (against 0.6 in the world). Altitude to 1800 meters in the winter, temperatures have increased by 1 to 3 degrees.

In addition, a climatologically study of the Alps from 1958 to today, also shows that "the most recent years appear to be largely deficient" in the snow.
According to researchers at the Centre for the Study of Snow Météo-France (Grenoble), there have been these last 40 years of "snowy winters of 1975-1985 and a deficit of white gold from 1987 to 1993." (Proof is that the winter of 2011 was particularly low snow).

Glaciers are a predominant part of the Alps, and because of climate change, these people may need to disappear the next 100 years according to glaciologists from the Laboratory of Glaciology and Geophysics of the Environment (CNRS-Université Joseph Fourier, Grenoble) and the Federal Institute of Technology Zurich (ETHZ). Indeed, the glaciers are experiencing a general decline since 1850 due mainly to lower precipitation snow (especially since 1980) in 1970, the Alps Count 5 150 glaciers in an area of ​​2909 square kilometers, which already corresponds to a decrease in 35% compared to 1850. Since then, the melting of glaciers has accelerated particularly after the 2003 heat wave that began ice surface 5 to 10%.
It is estimated that the Alpine glaciers have lost half of their area since 1850 and could completely disappear within 100 years if it is based on the IPCC's global models, with a temperature rise of between 3 to 5 degrees April and September.

Modeling of the Rhone glacier showing:- Its extension in 1850 (white line)- Its size in 1973 (red line)- Its decline with a rise of 3 ° C (blue)Credit: Zemp and al

Two studies by researchers including the Laboratory of Glaciology and Geophysics of the Environment (LGGE, CNRS / Université Grenoble (1)) show that global warming has a strong impact on mountain glaciers.
The first study concerns the St Sorlin glacier (3400m). A simulation of the evolution of this glacier in the twenty-first century, carried out under the B1 scenario of the IPCC future emissions of greenhouse gases, shows that despite a relatively optimistic climate scenario (+1.8 ° C by 2100), it should have disappeared in 2060, suggesting a similar fate for all the small glaciers of the Alps located in low to medium altitude.

Mer de glace (Chamonix, France), 1850

The second study focuses on the glacier of the Dome du Goûter (Mont Blanc, 4250 m). Temperature measurements made ​​in its ice show his remarkable recent warming and up to 60 meters deep and thus the existence of an atmospheric warming at those altitudes.

                                                                             

             Mer de glace (Chamonix, France), today

A simulation carried out under different scenarios of global warming shows that the cold high-altitude glaciers could be tempered in this century. Glaciers glaciers with a temperature of -11 degrees at depth  will have their temperature increase around 0 degrees (If warming reaches the base of the hanging glaciers  it could affect their stability dangerously)

The consequences of climate change on glaciers warming will be particularly important for human activities such as Michael Zemp said "it is in the densely populated mountain regions, such as the Alps, which must consider the consequences of widespread melting of glaciers, the water cycle, water economy, tourism and natural hazards. ".

Consequences of glaciers retreat could be numerous and dangerous. Indeed their disappearance causes a local imbalance with consequences on mountain ecosystems.

•  Breaks in pockets of water : Emptying a bag can be progressive (the outlet is sometimes impossible to detect) or abrupt (usually when the hydrostatic pressure developed in the reservoir exceeds that of the surrounding ice). This pressure then expels a plug of ice in large size. When the catastrophe of Saint Gervais, in 1892, is estimated as the Tête Rousse glacier was about 30 m in diameter. The sudden release of large amounts of water can then generate a debris flow, as was the case in Tete Rousse. (Today, the threat is importance above St Gervais, the bag of water threatening the village is regularly drained despite this, it is filled with water .It is closely monitored because it is very dangerous for everyone in the Valley) .
• The formation of proglacial lakes: These lakes are formed downstream of the glacier where runoff accumulates behind a dam, usually moraine. When the glacier retreats, the ancient moraine dam plays this role. When it is destabilized, the lake drains.

                

                                      

                                    

                                                    diagrams Cemagref ( Grenoble, France)

• Serac falls sometimes cause triggering of avalanches.
• Destabilization of land sub-and peri-glacial: We may think that the glaciers concourrent stabilisation (by frost and gravity) of a number of land surface. Major melting of the glaciers and thawing of the land involved could lead to landslides. Also with a massive thawing permafrost (permanently frozen ground, especially on the shady side above 2500m) which keeps the soil in place. A large solifluction could release massive amounts of CO2 contained in the soil and cause landslides very important and dangerous for people, flora and fauna below
• Reduced water supply lakes and short lower water: In summer it is the glaciers that support the water level of lakes and water courses below. Disappearance of glaciers will cause a progressive drying up of mountain aquifers, springs, water courses, lakes. This would pose problems for water supply of population and for their electricity supply (reservoirs turbine EDF).
• Changes in climate: The enormous mass is colder than the glacier affects the local climate balance. Upsetting this balance could affect the micro-climate itself and on the ecosystem (species extinction).
• Impacts on Tourism: The history of mountain people is closely linked to their mountains and glaciers. Their predicted demise atriste and strengthens the feeling of misunderstanding with respect to policy, unable to act. More physically, one-half of summer resorts are now closed. The mountain guides are a great number of their race glacial become simple high altitude walks, in addition, many glaciers are rapidly becoming impassable due to ice and deep crevasses that outcrop.

Many scientists are currently working on climate change impact on glaciers and its consequences. It is actually a big deal in mountain areas such as the Alps.