Antarctica (see Figure 6A.3) 31 billion tonnes of ice per year
8
or a net gain of about
27 billion tonnes per year
9
. The difference between these
Measurements by satellite techniques based on gravity in- estimates from totally independent techniques reflects the
dicate mass loss at a rate of 138 ± 73 billion tonnes per year uncertainties in these difficult measurements; neverthe-
during 2002–2005, mostly from the WAIS
6
. That is equiva- less, on balance, they indicate a recent shift to a net loss
lent to a rise in global sea level of 0.4 ± 0.2 mm per year, or of Antarctic ice and suggest that losses may be accelerat-
10–30% of the global rate measured since the 1950s (see ing. Similar conclusions result from studies of Antarctic
Chapter 6C), and is in good agreement with recent mass- Peninsula glaciers, indicating that they are melting much
budget estimates
10
. However, two interpretations of satel- faster than previously predicted and are probably already
lite radar altimetry pointed to a much smaller loss of about contributing significantly to sea-level rise
11
.
Rate of mass
increase (billion tonnes/year)
100
0
6
4
5
1
2
-100
-200 7
3
-300
1992 1996 2000 2004
Observation period
Surface-elevation change (cm/year)
-50 -25 0 25 50
Figure 6A.3: Antarctica, showing rates of surface-elevation change derived from satellite radar-altimeter measurements
3
. The graph
shows rates at which the ice-sheet mass was estimated to be changing based on radar-altimeter data (black), mass-budget calcu-
lations (red), and satellite gravity measurements (blue). Rectangles depict the time periods of observations (horizontal) and the
upper and lower estimates of mass balance (vertical).
Sources (corresponding to numbers on rectangles): 1 Rignot and Thomas 2002
4
; 2 Ramillien and others 2006
5
; 3 Velicogna and Wahr 2006a
6
; 4 Chen and
others 2006a
7
; 5 Zwally and others 2005
8
; 6 Wingham and others 2006a
9
; 7 Rignot and others 2007
10
CHAPTER 6A ICE SHEETS 105