3. Why does sea level rise occur?

Sea-level rise may occur due to short term / periodic changes (e.g. tides, waves, storm surges and seasonal variations), short-term geologic effects (e.g. earthquakes, subsidence) and longer-term sea level changes caused by mass exchange of water, thermal expansion and longer term geological effects. 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.

A very brief history of sea level:

  • Over the last 140,000 years sea level has varied over a range of more than 120 metres. The most recent large change was an increase of more than 120 metres as the last ice age ended.
  • Sea level stabilised over the last few thousand years, and there was little change between about 1AD and 1800AD.
  • Sea level began to rise again in the 19th century and accelerated again in the early 20thCentury.
  • Satellite altimeter measurements show a rate of sea-level rise of about 3 mm/year since the early 1990s – a further increase in the rate is projected.

There are many factors which can produce short-term (a few minutes to 14 months) changes in sea level such as tides, waves, storm surges, earthquakes and seasonal variations. Table 1: Short term and periodic changes of sea level (http://en.wikipedia.org/wiki/Sea_level_rise)
Short-term (periodic) causes Time scale (P = period) Vertical effect
Periodic sea level changes
Diurnal and semidiurnal astronomical tides 12-24 h P  0.2-10- m
Spring tides Twice a month *
Rotational variations (Chandler wobble) 14 month P *
Meteorological and oceanographic fluctuations
Atmospheric pressure Hours to months  – 0.7 to 1.3 m
Winds (storm surges) 1-5 days Up to 5 m
Evaporation and precipitation (may also follow long-term pattern) Days to weeks
Ocean surface topography (changes in water density and currents) Days to weeks Up to 1 m
El Nino southern oscillation 6 months every 5-10 yr Up to 0.6 m
Seasonal variations
Seasonal water balance among oceans (Atlantic. Pacific. Indian)  * *
Seasonal variations in slope of water surface  * *
River runoff floods 2 months lm
water density changes (temperature and salinity) 6 months 0.2 m
Seasonal Seiches
Seiches (standing waves) Minutes to hours Up to 2 m
Tsunamis (generate catastrophic long-period waves) Hours Up to 10 m
Abrupt change in land level Minutes Up to 10 m
* Effects change locally. Source: http://en.wikipedia.org/wiki/Sea_level_rise

A number of geological processes contribute to short-term changes in measured sea level. A few examples are:

  • earthquakes and other small-scale geological events
  • sinking of land (subsidence) through compaction of sediments and/or withdrawal of ground water
  • sinking of land through withdrawal of oil

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.

Ocean thermal expansion is one of the main contributors to long-term sea level change, as well as being part of regional and short-term changes. Water expands as it warms and shrinks as it cools.

From 1961 to 2003, the upper 700 metres of the global oceans absorbed about 3.6 x 1021 Joules per year, increasing global mean sea level (GMSL) by about 22 millimetres. This is equivalent to contributing about 0.52 mm/year to GMSL, and also to an air-sea flux of 0.36 Watts per square metre over the ocean area considered (65°S to 65°N). This contribution to GMSL is about one third of the total GMSL trend (1.6 mm/year) over this period.

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


Figure 1: The top panel of the above plot show changes in the heat content of the top 700 metres of the ocean from 1960 to 2007. The bottom panel shows the change in thermosteric sea level. (http://www.cmar.csiro.au/sealevel)

One of the main long-term geological contributions to sea level is Glacial Isostatic Adjustment (GIA) – which is also known as Post-Glacial rebound (PGR). The ice sheets of recent ice ages compressed the earth’s mantle as they advanced causing the mantle to subside and also to squeeze material out in front of the ice sheet, producing what is known as the “forebulge”. Since the ice sheets retreated the mantle has been slowly returning towards its original configuration. This has produced the following effects:

  • regions which were under the ice sheets (e.g. much of northern Eurasia and North America) are rising – in some cases by up to 7mm/year.
  •  regions which were on the forebulge (e.g. the east coast of the U.S.) are sinking, typically at rates of 1mm/year or slightly more.
  •  regions further away are moving vertically at smaller rates as part of the overall adjustment that this causes. For example, Australia is rising at ~0.3-0.4 mm/year.

These effects contribute to changes in measured sea level through the vertical movements at tide gauge sites and also through the change in volume of the ocean basins as this long-term geological adjustment goes on.

Plate tectonics also contributes, but this is, generally, a much smaller effect.

Thermal expansion is producing about half of the current 3mm/year increase in global sea level. This contribution has increased from around 0.5 mm/year over the second half of the 20th century to around 1.6 mm/year over the last 12-14 years. This contribution is expected to continue at least at this level over the next century or more due to greenhouse-gas-induced warming of the atmosphere and ocean.

Due to the very patchy and sparse (especially as we go back in time) body of ocean temperature data that is available to estimate longer-term contributions, the contribution over most of the 20th century is hard to estimate reliably.

A major contribution to recent sea level rise is from the melting of glaciers, and contributions from the Greenland (both surface melting and iceberg calving and the Antarctic ice sheet (iceberg calving only). This is believed to produce about one third or more of the current 3mm/year annual increase in global sea level. The contribution from the ice sheets is poorly understood at the moment and is an active area of research. The melting of the Greenland ice sheet alone could increase global mean sea level by around 7 metres. This would probably take around 1,000 years, but it is believed that ice melt from Greenland could still contribute significantly to sea level rise over the next 50-100 years.