3.3 Long-term changes – Mass exchange

On time scales of months and longer, sea level changes as a result of both changes in ocean mass (addition of water to the ocean from the land) and expansion/contraction of the ocean water as it warms/cools.

Exchange of water between “reservoirs” is an important contribution to sea level change. A significant part of this is through the hydrological cycle, where water evaporates from the ocean, resides in the atmosphere, then returns to the ocean either directly or via reservoirs (snow, ice, lakes, rivers, groundwater etc). There are both annual variations as well as longer-term variations. For example, extraction of water from underground aquifers can increase the mass of the ocean whereas the storage of water in dams can decrease the mass of the ocean.

Figure 1 Causes of sea level change (source: IPCC Third Assessment Report, 2001)

Contributions from non-polar Glaciers

A major contribution to sea level change is from the changing mass of glaciers, and the ice sheets. At the time of the last glacial maximum (140000 years ago) when sea level was more than 120 m below present level, there were major ice sheets in North American and northern Europe and Asia.

Several different estimates are proposed for the contribution of non-polar Glaciers by scientists. 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. Another group of scientists, 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).

Figure 2: The time series of glacier contributions to global sea level (top panel) and the cumulative effect (bottom panel) from 1961-2004. (source: 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)

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. (http://www.cmar.csiro.au/sealevel).

Figure 1 Antarctica surface elevation change (cm/year) Source: UNEP: Global outlook for Ice & Snow

Figure 1 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 mass budget estimates.

Figure 1: Greenland surface elevation change (cm/year) Source: UNEP: Global outlook for Ice & Snow

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.