Why does sea level change?

Recent contributions

Ocean Thermal Expansion
Contributions from non-polar Glaciers
Contributions from the Ice Sheets

Global warming from increasing greenhouse gas concentrations is a significant driver of both increases in ocean mass and ocean thermal expansion as components of recent and future sea level rise.

Ocean Thermal Expansion

From 1955 to 1995, earlier estimates of ocean thermal expansion is estimated to have contributed about 0.4 mm/year to sea level rise, less than 25 per cent of the observed rise over the same period. For the 1993 to 2003 decade, when the best data are available, thermal expansion was estimated to be significantly larger, at about 1.6 mm/year for the upper 750 m of the ocean alone, about 50 per cent of the observed sea level rise of 3.1 mm/year. Over the last few years we have been working hard on providing improved estimates of ocean thermal expansion.

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Contributions from non-polar Glaciers

Kaser et al and others estimate the melting of glaciers and ice caps (excluding the glaciers surrounding Greenland and Antarctica) contributed to sea level rise by about 0.4 mm per year from 1961 to 1990 increasing to about 1.0 mm per year from 2001-2004. (Kaser, G., J.G. Cogley, M.B. Dyurgerov, M.F. Meier and A. Ohmura (2006), Mass balance of glaciers and ice caps: Consensus estimates for 1961-2004, Geophysical Research Letters, 33, L19501, doi:1029/2006GL027511

Meier et al state that mass loss from glaciers is dominating the eustatic component of sea level rise in the 21st century, providing 1.1 mm/year of the total eustatic contribution of 1.8 mm/year in 2006. (Meier, M.F>, M.B. Dyurgerov, U.K. Rick, S. O'Neel, W.T. Pfeffer, R.S. Anderson, S.P. Anderson and A.F. Glazovsky (2007), Glaciers Dominate Eustatic sea level Rise in th 21st Century, Science, 317, 1064-1067

Dyurgerov and Meier glacier estimate.

The figure below shows the time series of glacier contributions to global sea level (top panel) and the cumulative effect (bottom panel) from 1961-2004.

Plot of glacier contributions to global sea level

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Contributions from the Ice Sheets

The ice sheets of Greenland and Antarctica have the potential to make the largest contribution to sea level rise, but they are also the greatest source of uncertainty. Since 1990 there has been increased snow accumulation at high elevation on the Greenland ice sheet, while at lower elevation there has been more widespread surface melting and a significant increase in the flow of outlet glaciers. The net result is a decrease in the mass of the Greenland ice sheet - a positive contribution to sea level rise. For the Antarctic Ice Sheet, the uncertainty is greater. There are insufficient data to make direct estimates for the preceding decades. At present, the mass gain of the Antarctic Ice Sheet due to increased thickening of the East Antarctic Ice Sheet does not appear to compensate for the mass loss due to the increased glacier flow on the Antarctic Peninsula and the West Antarctic Ice Sheet. Modelling studies suggest that the Antarctic Ice Sheet is still responding to changes since the last ice age and that this may also be contributing to sea level rise.

Antarctica

Map of Antarctice showing surface elevation change and estimates of recent masss loss
The figure shows rates at which the ice-sheet mass was estimated to be changing based on radar-altimeter data (black), mass-budget calculations (red), and satellite gravity measurements (blue). Rectangles depict the time periods of observations (horizontal) and the upper and lower estimates of mass balance (vertical). Measurements by satellite techniques based on gravity indicate mass loss at a rate of 138 ± 73 billion tonnes per year during 2002-2005, mostly from the West Antarctica Ice Sheet. That is equivalent to a rise in global sea level of 0.4 ± 0.2 mm per year, or 10-30% of the global rate measured since the 1950s, and is in good agreement with recent massbudget estimates. However, two interpretations of satellite radar altimetry pointed to a much smaller loss of about 31 billion tonnes of ice per year or a net gain of about 27 billion tonnes per year. The difference between these estimates from totally independent techniques reflects the uncertainties in these difficult measurements; nevertheless, on balance, they indicate a recent shift to a net loss of Antarctic ice and suggest that losses may be accelerating. Similar conclusions result from studies of Antarctic Peninsula glaciers, indicating that they are melting much faster than previously predicted and are probably already contributing significantly to sea level rise.

Data sources (corresponding to numbers on rectangles)

1	Rignot, E. and Thomas, R. (2002). Mass balance of polar ice sheets. Science, 297(5586), 1502-1506
2	Ramillien, G., Lombard, A., Cazenave, A., Ivins, E., Remy, F. and Biancale, R. (2006). Interannual variations of the mass balance of the Antarctic and Greenland ice sheets from GRACE. Global and Planetary Change, 53, 198-208
3	Velicogna, I. and Wahr, J. (2006). Measurements of time-variable gravity show mass loss in Antarctica. Science, 311(5768), 1754-1756
4	Chen, J., Wilson, C., Blankenship, D. and Tapley, B. (2006). Antarctic mass rates from GRACE. Geophysical Research Letters, 33, L11502
5	Zwally, H.J., Giovinetto, M.B., Li, J., Cornejo, H.G., Beckley, M.A., Brenner, A.C., Saba, J.L. and Yi, D. (2005). Mass changes of the Greenland and Antarctic ice sheets and shelves and contributions to sea level rise: 1992-2002. Journal of Glaciology, 51(175), 509-527
6	Wingham, D., Shepherd, A., Muir, A. and Marshall, G. (2006). Mass balance of the Antarctic ice sheet. Transactions of the Royal Society of London Series A, 364, 1627-1635
7	Rignot, E., Arthern, R., Bamber, J., van den Broeke, M., Davis, C., Li, Y., van de Berg, W.J. and van Meijgaard, E. (2007). A net source of ocean mass from coastal thinning of Antarctic glaciers. In print

Source: UNEP: Global outlook for Ice & Snow: Antarctica, showing rates of surface-elevation change derived from satellite radar-altimeter measurements. (June 2007). In UNEP/GRID-Arendal Maps and Graphics Library. Retrieved 01:43, August 28, 2007 from http://maps.grida.no/go/graphic/antarctica-showing-rates-of-surface-elevation-change-derived-
from-satellite-radar-altimeter-measurements.

Greenland

Mass-balance estimates for Greenland show thickening at high elevations since the early 1990s at rates that increased to about 4 cm per year after 2000, consistent with expectations of increasing snowfall in a warming climate. However, this mass gain is far exceeded by losses associated with large increases in thinning of the ice sheet near the coast. Total loss from the ice sheet more than doubled, from a few tens of billions of tonnes per year in the early 1990s, to about 100 billion tonnes per year after 2000, with perhaps a further doubling by 2005. These rapidly increasing losses result partly from more melting during warmer summers, and partly from increased discharge of ice from outlet glaciers into the ocean. In particular, the speeds of three of Greenland's fastest glaciers approximately doubled since 2000, although two of them have partially slowed since. The analysis has been derived by comparing satellite and aircraft laser-altimeter surveys. The graph shows rates at which the ice-sheet mass was estimated 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.

Data sources (corresponding to numbers on rectangles)

1 & 2	Krabill, W., Abdalati, W., Frederick, E., Manizade, S., Martin, C., Sonntag, J., Swift, R., Thomas, R., Wright, W. and Yungel, J. (2000). Greenland Ice Sheet: High-Elevation Balance and Peripheral Thinning. Science, 289, 428-430 Krabill, W., Hanna, E., Huybrechts, P., Abdalati, W., Cappelen, J., Csatho, B., Frederick, E., Manizade, S., Martin, C., Sonntag, J., Swift, R., Thomas, R. and Yungel, J. (2004). Greenland Ice Sheet: increased coastal thinning. Geophysical Research Letters, 31, L24402
3	Thomas, R., Frederick, E., Krabill, W., Manizade, S. and Martin, C. (2006). Progressive increase in ice loss from Greenland. Geophysical Research Letters, 33, L10503
4	Zwally, H.J., Giovinetto, M.B., Li, J., Cornejo, H.G., Beckley, M.A., Brenner, A.C., Saba, J.L. and Yi, D. (2005). Mass changes of the Greenland and Antarctic ice sheets and shelves and contributions to sea level rise: 1992-2002. Journal of Glaciology, 51(175), 509-527
5 - 7	Rignot, E. and Kanagaratnam, P. (2006). Changes in the velocity structure of the Greenland ice sheet. Science, 311(5763), 986-990
8 & 9	Velicogna, I. and Wahr, J. (2005). Greenland mass balance from GRACE. Geophysical Research Letters, 32, L18505 Velicogna, I. and Wahr, J. (2006A.) Acceleration of Greenland ice mass loss in spring, 2004. Nature, 443, 329-331
10	Ramillien, G., Lombard, A., Cazenave, A., Ivins, E., Remy, F. and Biancale, R. (2006). Interannual variations of the mass balance of the Antarctic and Greenland ice sheets from GRACE. Global and Planetary Change, 53, 198-208
11	Chen, J., Wilson, C. and Tapley, B. (2006A). Satellite gravity measurements confirm accelerated melting of Greenland Ice Sheet. Science, 313, 1958
12	Luthke, S., Zwally, H., Abdalati, W., Rowlands, D.D., Ray, R.D., Nerem, R.S., Lemoine, F.G., McCarthy, J.J. and Chinn, D. S. (2006). Recent Greenland mass loss by drainage system from satellite gravity observations. Science, 314(5803), 1286-1289

Source: UNEP: Global outlook for Ice & Snow: Greenland, showing rates of surface-elevation change between the late 1990s and 2003. (June 2007). In UNEP/GRID-Arendal Maps and Graphics Library. Retrieved 01:46, August 28, 2007 from http://maps.grida.no/go/graphic/greenland-showing-
rates-of-surface-elevation-change-between-the-late-1990s-and-2003

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Website owner: Neil White | Last modified 12/05/08

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