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.
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.
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.
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.
- 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.
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.