Spatial Impacts of Climate Change. Denis Mercier

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On a global scale: rising sea levels

      At this scale, the most important consequence of the melting of the cryosphere is sea level rise. In addition to the thermal expansion of the oceans, the main sources of this sea level rise are the melting of the Greenland ice sheet and the Antarctic ice sheet, the contribution of mountain glaciers and permafrost. It was 18 cm during the 20th Century, and the various IPCC scenarios envisage a rise of around 60 to 100 cm by the end of the 21st Century (IPCC 2019).

      However, we should not think in terms of this deadline alone, but rather that the rise of the seas and oceans will continue over the coming centuries as part of the melting of continental ice that has begun since the beginning of the Holocene interglacial period in which we live.

      Thus, an increase (rise) of 5 m will surely be recorded by 2300. The consequences for low-lying coastal areas such as estuaries, tidal marshes, deltas, etc. will affect the economic activities and human occupation of millions of citizens (see Chapter 4).

      A recent assessment by Zemp et al. (2019) shows that glaciers alone lost more than 9 billion tons of ice between 1961 and 2016, raising water levels by 27 millimeters (see Figure 2.13).

      With more than 3,000 Gt, the Alaska Glaciers (ALA) have contributed the most to sea level rise. The glaciers of Southwest Asia (ASW, green circle) were the only ones to record an increase in mass.

Map depicts the regional share of glaciers in sea-level rise from 1961 to 2016.

      Figure 2.13. Regional share of glaciers in sea-level rise from 1961 to 2016. The cumulative change in regional and global glacier mass (in gigatons, 1 Gt = 1,000,000,000 tons) corresponds to the size of the circles. The synthesis is based on 19,000 glaciers worldwide

      (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

      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.

Schematic illustration of paraglacial hazards induced by melting glaciers.

      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)

      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).

      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.

      AMAP (2017). Snow, Water, Ice and Permafrost in the Arctic (SWIPA). Arctic monitoring and assessment programme, Oslo.

      Francou, B. and Vincent, C. (2011). Les Glaciers à l'épreuve du climat. IRD, Paris.

      IPCC (2019). Special report on the ocean and cryosphere in a changing climate [Online]. Available at: https://www.ipcc.ch/srocc.

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