Ice loss from ice sheets 1: Greenland ice sheet

A ice sheets is a enormous glaciers that cover an area over 50,000 km². Two.ice sheets are Greenland ice sheet (7 × 10⁶ km²⁾ and Antarctica ice ice sheet (1.4 × 10⁷ km²). They, together,  contain about 99% freshwater ice over the world. 2/3 of sea level rise is contributed by ice melting from the two sheets (IPCC, 2013), the major source of freshwater to the ocean. Figure 1 shows how the two ice sheets form and flow.

Figure 1. Form and flow of Greenland and Antarctica ice sheets. (Source: adopted from  LIMA, NASA)

Greenland

Change in mass of ice sheets is the net mass gain over precipitation and melt and runoff, and it can be measured by mass budget, repeated altimetry and temporal variations in Earth gravity field (IPCC, 2013). Over the last few decades, Ice mass loss has occurred in Greenland with decline rate accelerated: the average loss rate had accelerated from -121 Gt/yr over 1993-2010 to -229 Gt/yr over 2005-2010 (IPCC, 2013). Up till now, the highest ice loss occurred in 2012, as simulated by model MARv3.5.2 (Figure 2). In this year (2016), the loss is estimated about -144 Gt/yr extra with respect to 1981-2010 average loss, according to the model. The melt season occurred earlier than previous years. 

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Figure 2. Anomalies of surface mass balance, snowfall and runoff form the Greenland ice sheet, simulated by model MARv3.5.2.(Source: adopted from Laboratoire de Climatologie et Topoclimatologie).

Fettweis et al. (2012) used the MAR model to estimate the contribution of surface mass balance from the Greenland to global sea level rise in the future. The model is a regional climate model (RCM) forced by general circulation models (GCMs) from CMIP5 and ICE2SEA under scenario RCP4.5 and RCP8.0. Their study showed that models used in the study simulated continuous ice loss from Greenland within this century (Figure 3), positive correlated to temperature rise but with non-linear relationship (Figure 4). 

Figure 3. Annual changes in surface mass balance, snowfall, rainfall, runoff and contributions to sea level rise. (Source: adopted from Fettweis et al. (2012))

Figure 4. Annual changes under RCP4.5 and RCP8.0. (Source: adopted from Fettweis et al. (2012))