Greenland Ice Sheet Contribution to Sea Level Rise

A critically important area of research in the polar regions is the contribution of ice sheets to Sea Level Rise (SLR) (IPCC 2013 Chapter 13). Accurate calculations of ice mass loss from large geographical areas of the climate system such as the Greenland Ice Sheet (GIS), West Antarctic Ice Sheet (WAIS) or the East Antarctic Ice Sheet (EAIS) requires substantial international cooperation, hence studies of this magnitude can only be undertaken every few years.

In light of the upcoming IPCC 6th Assessment Report, a second phase of the programme designed to update our understanding of this issue is being undertaken, named the Ice sheet Mass Balance Inter-comparison Exercise (IMBIE).

The most update research indicates that if the GIS were to melt entirely, sea level would rise by 7.36m. This post will summarise our contemporary knowledge of one of the most challenging research areas in polar science – specifically ice mass loss from the GIS (SLR from Antarctica will not be discussed in this post).

Greenland Ice Sheet contribution to Sea Level Rise in IPCC AR5

Observed global mean SLR from 1900 to 2013 is shown in Figure 1. The IPCC estimate for SLR between 1901 and 2010 is 0.19 (0.17 to 0.21) m (IPCC 2013 pp. 11). Rate of SLR is believed to have increased over this time, culminating in an estimated rate of 3.2 (2.8 to 3.6) mm yr^-1 between 1993 and 2010. This is also the period with the greatest certainty in observations of SLR due to increased accuracy and coverage of satellite observations. For the same period, GIS contribution is estimated as 0.33 (0.25 to 0.41) mm yr^-1 – approximately 10%.

Figure 1: ‘Global mean sea level relative to the 1900-1905 mean of the longest running dataset, and with all datasets aligned to have the same value in 1993, the first year of satellite altimetry data. All time-series (coloured lines indicating different data sets) show annual values, and where assessed, uncertainties are indicated by coloured shading’. Caption quoted from the source.

Source: IPCC 2013, Figure SPM.3 Panel (d).

Whilst the loss of sea ice does not contribute to SLR, the loss of land based ice does contribute directly to SLR (Figure 2). Variations in the mass of the entire GIS are used to keep track of ice exchange between the ocean and land.  Figure 2 is taken from a comprehensive study by Sheperd et al. (2012), which shows the conversion between mass loss and SLR from the GIS (Figure 2 also show the contribution of Antarctica). Sheperd et al. (2012) estimate that the GIS lost 2700 ± 930 Gt of ice between 1992 and 2011, although as the time series in Figure 2 shows, the rate of this change is not linear and increases substantially over time. Linear mass loss between 1992 and 2000 is calculated to be 51 ± 65 Gt yr^-1, compared to 263 ± 30 Gt yr^-1 between 2005 and 2010 (Sheperd et al. 2012).

Figure 2: ‘Cumulative changes in the mass of (left axis)… …GrIS and AIS and the combined change of the AIS and GrIS (bottom), determined from a reconciliation of measurements acquired by satellite RA, the IOM, satellite gravimetry, and satellite LA (Lidar Altimetry). Also shown is the equivalent global sea-level contribution (right axis), calculated assuming that 360 Gt of ice corresponds to 1mm of sea-level rise’. Caption quoted from the source.

Source: Sheperd et al. 2012, Figure 5, bottom panel.

Methodology for Calculating Mass Loss

There are a variety of methods used to calculate the mass loss of an ice sheet on the scale of the GIS. Only by synthesising these methods can a robust estimate of total ice loss be attained. Here is a quick overview of each method:

• Input-Output Method (IOM): Increases and decreases in ice are calculated separately – allowing them to be analysed discretely. Ice mass loss is calculated at the catchment basin scale using direct observations of sublimation, meltwater and glacier outflow  (). Snowfall (accumulation – ice mass increase) is derived from regional climate models.

• Radar Altimetry (RA) and Lidar Altimetry (LA): Satellites (specifically the European Space Agency’s (ESA) CryoSat-2) and aeroplanes are used to measure variations in the surface elevation of the GIS. Changes in ice sheet volume are converted to changes in ice sheet mass using models of ice density.

• Gravimetry (GRACE satellite): The Gravity Recovery And Climate Experiment (GRACE) satellite measures the gravitational force from the GIS, therefore directly measuring changes in ice mass. A significant source of uncertainty for this method is correcting for Glacial Isostatic Adjustment (GIA) of the underlying crust.

Sheperd et al. (2012) compares these methods in Figure 3. A key weakness of the combined dataset is inconsistency between the time periods of each methodology, which can be seen in Figure 3. The IOM has much greater uncertainty bounds than the alternative techniques, and estimates a greater rate of ice mass loss. Each method calculates a different value for ice mass loss, and the methods are all subject to considerable uncertainty. However, using multiple methods is critical to distinguishing robust trends in mass loss over the Anthropocene – and estimating GIS contribution to SLR.

Figure 3: Rate of mass change of the GIS for three of the methods described, including uncertainty. Rates of mass balance derived from ICESat LA were computed as time varying trends. The gravimetry and RA mass trends were computed after applying a 13-monthmoving average to the relative mass time series. Caption adapted from source.

Source: Sheperd et al. 2012, Figure 4, top-right panel.

To summarise, SLR from the GIS is one of the greatest challenges facing science in the polar regions. IPCC observational ensemble estimates show a substantial and increasing annual SLR contribution from the GIS throughout the Anthropocene.