Satellite geodesy has revolutionized the way we can observe the Earth's surface and gravity field and changes to these properties. For the past few decades, different satellite geodetic measurements have provided an unprecedented level of detail on changes to the polar ice sheets and helped us understand how these ice masses are contributing to sea-level rise.

The Greenland and Antarctic ice sheets contain enough ice to raise global sea levels by around 7 m and 58 m, respectively (Vaughan et al., 2013). They are huge bodies of ice that sit on Earth's bedrock, are 1000s of kilometers wide and, at their centers, around 3 to 4 km thick.

Although they may appear somewhat static, ice sheets are in fact dynamic in nature. In general terms, an ice sheet gains ice due to the accumulation of snowfall on its surface. Ice then slowly flows from the center to the edges of the ice sheet, where it is lost through melting and from the discharge of outlet glaciers (e.g. icebergs that break off). The reality is, however, rather more complicated than this because of both complex internal ice sheet processes and complex interactions between the ice sheets and other parts of the climate system.

SURFACE ELEVATION: Surface elevation changes for Antarctica and Greenland over the period 2003–2007. Borrowed from Pritchard et al. (2009).

Scientists will often refer to the mass balance of an ice sheet when discussing ice sheet changes. A negative mass balance means that the ice sheet is losing more ice than it is gaining – meaning the ice sheet is shrinking and contributing to global sea-level rise. Conversely, a positive mass balance means the ice sheet is growing causing sea level to fall.

Modern Geodetic observations

Observations from satellite geodesy show that both the Greenland and Antarctic ice sheets are in a state of negative mass balance and are contributing to global sea-level rise. A combined estimate of Greenland and Antarctic ice mass loss indicates they contributed 11.2 ± 3.8 mm to global sea-level rise between 1992 and 2011 (Shepherd et al., 2012).

Although these quantities may sound small, they in fact represent significant changes to the ice sheets and climate system. A 1 mm rise in sea level is the equivalent of ~360,000,000,000 tonnes of ice melt!

GLOBAL SEA-LEVEL: The cumulative contribution of Greenland (green) and Antarctica (blue) to global sea-level change over 1992–2011. Data from Shepherd et al. (2012).

The satellite geodetic measurements indicate that the rate of ice loss from Greenland has increased since 1992. The area of the ice sheet experiencing summer melt has also got larger. Ice loss from Greenland is split approximately 50:50 between surface melting and outlet glacier discharge (van den Broeke et al., 2009).

In contrast, in Antarctica, although surface melting occurs, very little of this water makes to the sea and instead refreezes on the ice sheet. Increased ice loss from Antarctica is instead almost all due to the acceleration of outlet glaciers on its periphery. Evidence points towards warmer ocean temperatures being a major driver of these changes (e.g. Pritchard et al., 2012).

References

H. D. Pritchard, R. J. Arthern, D. G. Vaughan, L. A. Edwards (2009) Extensive dynamic thinning on the margins of the Greenland and Antarctic ice sheets. Nature, 461, 971–975.

H. D. Pritchard, S. R. M. Ligtenberg, H. A. Fricker, D. G. Vaughan, M. R. van den Broeke, L. Padman, 2012: Antarctic ice loss driven by ice-shelf melt. Nature, 484, 502–505.

A. Shepherd et al. (2012). A reconciled estimate of ice-sheet mass balance. Science, 338, 1183–1189.

M. van den Broeke, et al. (2009). Partitioning recent Greenland mass loss. Science, 326, 984–986.

D. G. Vaughan, J. C. Comiso, I. Allison, J. Carrasco, G. Kaser, R. Kwok, P. Mote, T. Murray, F. Paul, J. Ren, E. Rignot, O. Solomina, K. Steffen, T. Zhang. (2013). Observations: Cryosphere. In: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.