(source: modified from Zemp et al. 2019).
Fora color version of this figure, see www.iste.co.uk/mercier/climate.zip
2.5.2. Regionally: paraglacial risks
At the scale of mountain ranges, the melting of glaciers and areas with permafrost has consequences for the paraglacial sedimentary cascade (see Chapter 5), periglacial dynamics (see Chapter 6), hydrology of Arctic rivers (see Chapter 7), and water supply in Central Asia (see Chapter 9).
On the other hand, the shrinking of glaciers also brings with it paraglacial risks for the populations living on the margins of these glaciated areas.
Figure 2.14. Paraglacial hazards induced by melting glaciers. For a color version of this figure, see www.iste.co.uk/mercier/climate.zip
(source: design D. Mercier; drawing F. Bonnaud, Sorbonne University, 2019)
Glacier melt induces the formation of lakes in the immediate periphery of glaciers, with meltwater often retained by natural dams formed by frontal moraines (see Figure 2.14). This glacial melting and these lakes feed rivers that flow downstream in valleys to the seas and oceans. When glaciers calve and large areas of ice fall into lakes (see Figure 2.14, step 1a), waves can form (see Figure 2.14, step 2) and breach the frontal moraines (see Figure 2.14, step 3). The flows then become torrential with a mixture of water and sediment, resulting in highly turbid, torrential lava flows that can affect localized issues at the periphery of the flow area (see Figure 2.14, steps 4a and 4b). As a result, infrastructure such as roads, bridges, airstrips, hangars, dykes, houses, etc., can be damaged by these lava flows (see Figure 2.14, Zone B).
The problem of draining water pockets remains a threat in the Alps since the dramatic accident in Saint-Gervais in 1892, which claimed 175 victims downstream from the Tête Rousse glacier. These Glacial Lake Outburst Floods (GLOFs) are present in many mountains, in the Andes, in the Himalayas2 (Westoby et al. 2014).
In Iceland, these floods, called jokulhlaups (literally “the running glacier”) are associated with the melting of glaciers under the effect of global warming but can also be exacerbated by sub-glacial volcanic activity. In North America, the Alaska Climate Adaptation Science Center (AK CASC) is funding research on the Mendenhall Glacier to better understand its dynamics and the risks induced by its flash floods in order to model its dynamics and predict flooding by monitoring, and among other things, the current water level in the lake, its spatial extent and its bathymetry.
The melting of glaciers also induces landslides on the slopes because the pressure exerted during the glacial sequence is replaced by a decompression of the walls (Mercier 2016). Thus, rock volumes can move down the slopes and end their course in proglacial lakes (see Figure 2.14, step 1b), potentially generating waves, breaches and torrential lava, or directly affecting the infrastructures below the uplift zones. Melting permafrost in the walls, or increased rainfall, may also cause debris flows that may also end their course in the proglacial lake, leading to the same chain of events (see Figure 2.14, step 1c). These gravitational hazards are therefore all linked directly or indirectly by melting glaciers and are potential hazards to downstream urbanized areas (see Figure 2.14, zones A and B).
2.6. Conclusion
The contemporary melting of the terrestrial and marine cryosphere is a reality observed at high latitudes as well as in the mountains. Although this melting has had many precedents in the history of our planet, the consequences of this loss of ice and snow cover have repercussions on different spatial scales, from the global to the local level.
Some of the consequences will be global, such as sea level rise related to the melting of the Earth's cryosphere. Although the melting of the marine cryosphere does not induce sea level rise, the melting of the Arctic sea ice does have an impact on the North Atlantic thermohaline circulation and thus has implications for the general atmospheric circulation. Locally, the disappearance of glaciers induces changes in the morphogenic dynamics that cause hazards, potentially dangerous for populations, and in plant colonization.
Moreover, climate predictions for the late 21st Century and for the centuries to come converge to affirm that the melting of the various components of the cryosphere will continue (IPCC 2019). This long-term evolution is doubly logical, on the scale of the current global warming and on the scale of the interglacial period in which humanity has been living for thousands of years.
2.7. References
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Mercier, D. (2016). L'Arctique face aux crises géomorphologiques paraglaciaires. In L'Arctique en mutation, Joly, D. (ed.). EPHE, Paris.
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Westoby, M.J., Glasser, N.F., Brasington, J., Hambrey, M.J., Quincey, D.J., Reynolds, J.M. (2014). Modelling outburst floods from moraine-dammed glacial lakes. Earth-Science Reviews, 134, 137-159.
Zemp, M., Nussbaumer, S.U., Gärtner-Roer, I., Huber, J., Machguth, H., Paul, F., Hoelzle, M. (2017). Global glacier change bulletin No. 2 (2014-2015) [Online]. Available at: https://wgms.ch/downloads/WGMS_GGCB_02.pdf.
Zemp, M., Huss, M., Thibert, E., Eckert, N., McNabb, R., Huber, J., Barandun, M., Machguth, H., Nussbaumer, S.U., Gärtner-Roer, I., Thomson, L., Paul, F., Maussion, F., Kutuzov, S., Cogley, J.G. (2019).