For 1993 to 2008, the sea level rose at 3.4 mm/year (0.13 inches/year) according to satellite measurements. The rate has been increasing. For the period 1961-2003, for example, the best estimate for the sea level rise was 1.7 mm/year. Although slow, the rise in sea levels threatens low-lying communities. Flooding during extreme weather is increased and the water table is more likely to be contaminated with salt water.
Environmental Impacts of Sea Level Rise
There is already clear evidence that some countries are being affected by environmental degradation, even though climate change impacts are currently small. In October 2011, Tuvalu (population 11,000) ran out of fresh drinking water and emergency supplies were donated by Australia, New Zealand and USA. Some of the pressure on drinking supply though, has certainly arisen from increased population demand.
In the developed world, people are being faced with the loss of beachfront property. Nonetheless, vulnerable countries feel victimised by the USA because the USA has not ratified the Kyoto Protocol, the international agreement aimed at combating global warming.
The Three Components of Sea Level Rise
The sea level changes on all timescales due to weather systems, tidal forces and so on. Here we consider only the long term (multi-decadal) changes.
1.Global sea level rises due to simple thermal expansion of the oceans.
Since about 1970, the global atmospheric temperature has risen about 0.7C (1.3F) which coincides with a mean thermal expansion for the oceans of about 4 cm (1.6").
2. The melting of land based ice.
This has contributed another 6 cm to global sea level rise. Note that the melting of floating ice does not contribute to sea level rise, by Archimedes principle of physics. This states that weight of liquid (water) displaced by a floating body (ice) equals the weight of the body.
3. The changing shape of land under the geoid.
The geoid is the surface of constant gravitational force which liquids tend towards. The Earth's crust behaves like a spring forced down under the weight of ice and snow. As the ice melts, millions of tons are released, and the land surface rebounds slightly upwards (and downwards in the opposite hemisphere to compensate).
Measured relative to the moving land surface, sea level would seem to rise if the land went downwards. This is a smaller effect than the other two causes of sea level rise, and is less well known. The effect is up to a few cm (Johansson et al., 2002), but very variable from place to place. Global sea level is unaffected.
Sea Level Simulations
Climate models are, in principle, ideal tools for the simulation of sea level rise, since they are now sophisticated enough to include at least a simple cryosphere (ice and snow volume) and a more complex ocean coupled to the atmosphere. So, as greenhouse gases increase and the atmospheric temperature rises, the heat will pass to the oceans and thermal expansion will continue.
Unfortunately, despite the complexity of climate models, they have not produced satisfactory simulations of past changes. Comparison with observations indicate that the typical model from the 2007 IPCC assessment predicted only half the sea level rise observed. The observed sea level rise was best simulated by the most extreme of those models included. See the figure from the Copenhagen Diagnosis.
It might be speculated that the reason for the large underestimation in sea level rise is related to the underestimation in Arctic sea ice melt. Models which allow the ice to interact more dynamically with the Earth's surface may allow more melting.
The Future Sea Level
Sea level is one of the most serious consequences of climate change. The population is about 3 times more dense near the coasts than in the whole of the landmass. Also, as noted above, island countries can be devastated by only modest changes in sea level rise. In view of the noted discrepancies between climate prediction and observations, future estimates of sea level rise may be unreliable.
Nonetheless, it seems reasonable to use the most extreme values previously simulated as an indicator of what would likely occur in practice. This would give a number in the area of 1m (3 feet) for sea level rise by the end of the century, putting many small countries under pressure.
It becomes speculative about what might happen well into the future. A certain amount of further climate change is already committed in the climate system, since it will take centuries for the deep oceans to warm up. Estimates suggest that by the time the climate reaches a stable state, the total sea level rise will be near 5m (17 feet) in 500 years' time.
Additional Reference
J. M. Johansson et al. (2002), Continuous GPS measurements of postglacial adjustment in Fennoscandia 1. Geodetic results, Journal of Geophysical Research, 107, 2157, doi:10.1029/2001JB000400 http://www.agu.org/pubs/crossref/2002/2001JB000400.shtml
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