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.