How much is sea level rising?

Sea level variations occur on very different time scales: rapid variations (hours to days) are caused by winds and tides, whereas longer-term changes are related to large-scale climate processes. Additionally, tectonic uplift or sinking of landmasses can change the water level locally, e.g., some regions of Scandinavia are currently experiencing an uplift rate of 1 cm per year due to post-glacial rebound. Sea level changes also vary in space on longer time-scales, depending on the distribution of temperature, wind stress and circulation changes. Effectively, this means that local sea level variations can be much larger (or smaller) than the global average value. In light of the ongoing discussions about hazards related to current and future sea level rise, it is interesting to note that over the last 2000 years, global mean sea level has most likely not varied by more than ±25 cm.

Observations – Sea level rise over the last 100 years
Analyzes of tide gauge records indicate a global mean sea level rise of 1.7 ± 0.5 mm per year in the 20th century. The observations have been corrected with geological models to account for tectonic uplift or sinking. The uncertainty of this global mean value is still relatively large, because only very few long-term tide gauge observations exist, and the corrections that need to be applied exhibit uncertainties themselves. In principle, the observed rise should be equal to the sum of its contributions. This is, at least until 1993, not the case, and mostly likely due insufficient global coverage of tide gauge stations on the one hand, as well as missing direct observations of the contributions on the other. However, since the advent of satellite altimetry in 1993, much more accurate observations of global sea level variations exist: between 1993 and 2003, global mean sea level rose by approximately 3.1 mm per year, which represents a considerable acceleration over previous periods. With the help of many modern observation systems and climate models, the contributions to the observed sea level rise are now relatively well understood (see Table 1 and Figure 1):

Warming of the World Ocean
The increased levels of greenhouse gases in the atmosphere cause the oceans to warm – since warm water occupies more volume than cold water, the water column expands and sea level rises (thermal expansion). Sea level changes due to density variations are termed steric sea level variations.  If the entire water column (up to 4000 m depth) were warmed by 1 degree Celsius, sea level would rise by approximately 50 cm. However, such a homogenous warming within a short time period is unrealistic, because the deep ocean layers only exchange very slowly with the ocean surface (the number given is only meant to provide an order of magnitude). Therefore, deep ocean layers warm very gradually, which on the one hand slows thermal sea level rise, but on the hand will also cause sea level to rise much longer into future, even when the atmospheric warming has long come to a halt.
Temperature observations in the second half of the 20th century show a warming of all ceoan basins, which has led to a thermal expansion of these waters. Since about 1990, this expansion is accelerating, and contributes significantly to the observed total sea level rise (Table 1).

 
Influence of the large ice sheets in Greenland and Antarctica, ice caps and glaciers, and terrestrial water reservoirs
Besides steric changes, sea level varies also due to non-steric volume changes – these relate to mass changes from adding water to the oceans from external water reservoirs. The world’s largest fresh water reservoir is the Antarctic ice sheet. Its volume is currently estimated as 24.7 cubic kilometers; melting of the entire ice sheet would raise sea level approximately 56.6 m. The second largest water reservoir is the Greenland ice sheet. Its volume is currently estimated as 2.9 cubic kilometers, and melting of the entire ice sheet would raise sea level approximately 7.3 m. Until recently, estimates of the Antarctic and Greenland mass balances were highly uncertain, and did not even allow the robust determination of the signs of long term changes. More recently, however, new satellite based observations show a retreat at least of the Greenland ice sheet (Table 1). Whether or not these observations represent long-term changes is not clear due to the relatively short observational time period. Similarly, it is currently not resolved whether the Antarctic ice sheet is also shrinking (in Table 1, the mean of all Antarctic observations points to a net melting, but the associated uncertainties are so large, that a growing ice sheet cannot be ruled out).
Since 1850, many mountain glaciers and ice caps have retreated. This melting directly causes sea level to rise, as the melt water enters the oceans through the continental runoff. Currently, the total volume of the world’s mountain glaciers and ice caps (not including Greenland and Antarctic glaciers) is estimated to lie between 0.05 and 0.13 Mio. cubic kilometers. Melting of the entire volume would raise sea level between 15 and 37 cm. In the 20th century, the retreat of mountain glaciers has substantially contributed to the observed sea level rise (Table 1).
As far as other terrestrial water reservoirs (e.g., ground water, perma frost, dams) are concerned, there is currently no evidence for significant trends that would indicate a sea level contribution. Satellite-based observation systems will help in the years to come to determine any anomalies or trends.

 

 

Contribution [mm/year] 1961-2003 1993-2003
Thermal Expansion 0.42±0.12 1.6±0.5
Glaciers & Ice Caps 0.50±0.18 0.77±0.22
Greenland 0.05±0.12 0.12±0.07
Antarctica 0.14±0.41 0.21±0.35
Sum 1.1±0.5 2.8±0.7
Direct Observations 1.8±0.5 3.1±0.7

Table 1: Individual contributions to global mean sea level rise for the periods 1961-2003, and 1993-2003 (Source: IPCC 2007, Table TS.3.).

 

 

 

 

Figure 1: Individual contributions to global mean sea level rise and uncertainties for the periods 1961-2003, and 1993-2003 (Source: IPCC 2007).

 

 

 

Projected Sea Level Changes in the 21st Century

Projections for the next century depend in principle on the global warming scenario. However, since the oceans exchange relatively slowly with the atmosphere, the thermal expansion over the next 20-30 years is more or less independent of the global warming scenario – the ocean are lagging behind atmospheric temperature changes. Based on different greenhouse emission scenarios for the future, climate models project a global sea level rise between 18 and 59 cm between 2090-2099 (relative to 1980-1990). The largest contribution comes from thermal expansion, followed by melting of mountain glaciers and ice caps (Figure 2). A big uncertainty of future sea level rise is associated with the development of the Greenland and Antarctic ice sheets under global warming. If the currently observed melting trend of the Greenland ice sheet continues or accelerates with rising atmospheric temperatures, sea level would rise by another 10 to 20 cm by 2090-2099. For the Antarctic ice sheet, the uncertainty is even larger: an increase of the ice volume (which would decrease sea level) can currently not be ruled out. This is related to increased precipitation (and storage as snow) at high latitudes under global warming projections.

As mentioned in the introduction, sea level rise can be (and is) very different regionally – an aspect that needs to be considered, e.g., for costal planning. Figures and animations of projected regional sea level changes are provided by DKRZ (animations and figures).

 

 

Figure 2: Projected sea level change in 2090-2099 (relative to 1980-1999) for six different climate change scenarios (B1 to A1F1). Shown are the contributions and uncertainties (5 to 95%). The contributions for the Greenland and Antarctic ice sheets do not include possible future changes of ice sheet dynamics (source: IPCC 2007).

 

 

 

Updated: November 2007
Contact: Felix Landerer

 

References:

 

Intergovernmental Panel on Climate Change (IPCC), 2001, University Press Cambridge,
ISBN 0521 01495 6

 

F. W. Landerer, J. H. Jungclaus and J. Marotzke, 2007: Regional dynamic and steric sea level change in response to the IPCC-A1B scenario, Journal of Physical Oceanography, Vol. 37, No. 2, p. 296-312. 
reprint available

 

Meehl, G.A., T.F. Stocker, W.D. Collins, P. Friedlingstein, A.T. Gaye, J.M. Gregory, A. Kitoh, R. Knutti, J.M. Murphy, A. Noda, S.C.B. Raper, I.G. Watterson, A.J. Weaver and Z.-C. Zhao, 2007: Global Climate Projections. In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.