Global Outlook for Ice & Snow

Rate of mass increase (billion tonnes/year)

100

4

0

8

1

2

12

-100

5

3

6

10

7

-200

11 9

-300

1992

1996

2000

2004

Jakobshavn

Observation period

Kangerdlugssuaq

Surface-elevation change (cm/year)

Helheim

-50

-25

0

25

50

Figure 6A.5: Greenland, showing rates of surface-elevation change between the late 1990s and 2003, derived by comparing satellite and aircraft laser-altimeter surveys. The graph shows rates at which the ice-sheet mass was es- timated to be changing based on satellite radar-altimeter surveys (black), airborne laser-altimeter surveys (green), airborne/satellite laser-altimeter surveys (purple), mass-budget calculations (red), temporal changes in gravity (blue). Rectangles depict the time periods of observations (horizontal) and the upper and lower estimates of mass balance (vertical). Jakobshavn, Helheim, and Kangerdlugssuaq are fast glaciers that doubled in speed recently. Sources (corresponding to numbers on rectangles): 1 and 2 Krabill and others 2000 20 and 2004 21 ; 3 Thomas and others 2006 27 ; 4 Zwally and others 2005 8 ; 5 to 7 Rignot and Kanagaratnam 2006 22 ; 8 and 9 Velicogna and Wahr 2005 23 and 2006A 24 ; 11 Chen and others 2006A 25 ; 10 Ramillien and others 2006 5 ; 12 Luthke and others 2006 26

108

GLOBAL OUTLOOK FOR ICE AND SNOW