Melting glaciers

Last time, we looked at two major issues from glacier melting at Himalayas. Himalayan glaciers are important, because they are sources to rivers and ‘water tower’ of Asia to support Asian population. However, these glaciers only take small proportion of glaciers worldwide. This time, we will look at the fate of glaciers over the world. Though difficult to estimate glaciers due to uncertainties in measurement, a global glacier mass is likely in the range of 114,000-192,000 Gt (IPCC, 2013).

Figure 1. Global distribution of glaciers, based on RGI regions. (Source: adopted from IPCC, 2013).

Figure 1 shows the global distribution of glaciers, divided into 19 Randolph Glacier Inventory (RGI) regions. Large proportion of glaciers are distributed in high-latitude regions, which are dominant sources contributing to sea level rise caused by glacier melting. Changes in glaciers can be measured via mass, volume, length and area. Table 1 shows these methods and their characteristics.

                             Table 1. Measurement methods, adopted from IPCC (2013)

Glaciers are sensitive to climate change (temperature and precipitation in particular), and they adjust to new equilibrium to balance the change. Over the last few decades, global glaciers have retreated overall with considerable mass loss in order to balance global warming, e.g. all RGI regions lost glacier mass during 2003-2009 with rate at –259 ± 28 Gt/yr in total (Gardner et al., 2012). Figure 2 shows the glacier mass budget at RGI regions during the period. Large mass loss occurred in Alaska (–50 ± 17 Gt/yr), Greenland periphery (–38 ± 7 Gt/yr), Arctic Canada North (–33 ± 4 Gt/yr), Southern Andes (–29 ± 10 Gt/yr), Arctic Canada North(–27 ± 4 Gt/yr) and High-Mountain Asia (–26 ± 12 Gt/yr) (Gardner et al., 2012). 

Figure 2. Global mass budget in Gt/yr, based on GRI regions. (Source: adopted from Gardner et al., 2012)

Marzeion et al. (2012) used CRU-forced model to simulate glacier mass change 1901-2010 (Figure 3), using unit SLE. SLE represents sea level equivalent to glacier mass. Their simulations showed that glacier mass gradually lost during the period, with peak loss rate occurred in 1930s caused by glacier melting at Greenland. Peak loss rate of glacier mass at Russian Arctic occurred between 1950 and 1960, as well as Arctic Canada. After low loss rate in 1970s, the rate increased again.

Figure 3. Cumulative global surface mass balances relative to the 1986–2005 mean, and rates from the CRU-forced model. (Source: adopted from Marzeion et al., 2012)

Future projection (Marzeion et al., 2012) shows gradual loss in glacier mass in all RCPs towards 2100 (Figure 4), with rate increasing until the middle of this century . After mid-century, the rates under RCP2.6 and RCP4.5 will slow down, while the rates under RCP6.0 and RCP8.5 will still increase, according to Figure 4.

Figure 4. Cumulative global surface mass balances and rates from the model forced with CMIP5 projections under 4 RCPs (RCP2.6 in red, RCP4.5 in green, RCP6.0 in blue and RCP8.5 in pink) toward the end of this century. Solid lines represent the mean of simulations from individual models in CMIP5. (Source: adopted from Marzeion et al., 2012)

 There are about 170,000 glaciers, and they have their own characteristics and climate conditions, leading to their different response to climate change with various time scales. For example, some glaciers at Alaska have retreated quickly (Gardner et al., 2012), while some at Karakoram mountain range have been stable or even have advanced (Bolch et al., 2012) Meanwhile, many glaciers are still poorly known. These result in considerable uncertainties in future projections, particularly in mountain regions like the Karakoram-Himalaya mountain range with complex glacier properties and climate conditions (IPCC, 2013).