What About Sea-Level Rise?

Sea-level rise is one of the most feared aspects of global warming for island nations like Tuvalu, and for inhabitants of other low-lying parts of the planet. Yet keeping tabs on global mean sea level is, if anything, an even more complicated problem than monitoring the Earth's temperature – and again provides scope for disagreement and controversy among scientists.

Today, sea-levels are recorded by coastal tide gauges relative to a fixed benchmark on land. Averaged over a period of time (a year, say, to remove short-term effects due to waves, tides, weather conditions, etc.), the result is the local ‘mean sea level’. The difficulty in interpreting changes in mean sea level at a particular locality is that the land moves up and down as well.

Tuvalu (Photo Credit Unknown)

"As an island nation comprised mostly of low-lying small islands with large coastal areas, the FSM is vulnerable to the adverse impact of climate change. Exposed to the effects of extreme weather events, our livelihood and traditions as island people, in fact our whole civilization, are under greater threat than ever before. The sad irony is that those of us who have little to contribute to the causes of climate change and sea level rise are the first in line to suffer the consequences."
Redley Killion, Vice President, Federated States of Micronesia

These vertical land movements can result from human activities, or more generally from natural causes – including tectonic processes (e.g. earthquakes) and very slow adjustments to major changes in ice-loading. For instance, the UK is still adjusting to the melting of ice at the end of the last glacial period; Scotland is rising a few mm a year and the south of England is sinking at a similar rate.
With this in mind, you can begin to see why it might be difficult to establish how global mean sea level (sea level averaged across the globe) has varied over the past century due solely to changes in the total volume of water in the oceans.
It is this so-called ‘eustatic’ sea-level change that is linked to the climate-related factors identified above: thermal expansion of seawater and melting of land ice. All the historical records from tide gauges around the world measure only relative sea level. Not only is the spatial distribution of high-quality long-term records decidedly patchy, but individual records must also be adjusted for local land movements.

‘Global Warming’. An OpenLearn chunk used/reworked by permission of The Open University copyright © (2007).’ http://creativecommons.org/licenses/by-nc-sa/2.0/uk/