Why sea level bears watching
Global sea level has risen something like 12 centimeters over the past century and continues rising today. That's a slow rate compared with the meter per century changes associated with rapid buildup or decay of ice age glaciers. But since even a few centimeters further rise means considerably more risk of destructive storm surges along many coastlines, sea level should be closely monitored.
This is the main conclusion to be drawn from a pair of studies reported this year. Beyond that, however, the two research teams involved suggest quite different causes for the sea-level rise.
Robert Etkins and Edward S. Epstein of the US National Oceanic and Atmospheric Administration (NOAA) suggest that warming of the atmosphere has already melted substantial amounts of Antarctic ice. This, they say, is the main cause of sea level rise, although thermal expansion of the upper water layers may be involved to a slight extent.
They also speculate that the mass of melt water spreading over the ocean would change Earth's moment of inertia (the distribution of mass about Earth's spin axis) so as to slow our planet's rotation. Outlining their reasoning in Science last January, they estimated that this could account for about three-fourths of the slowdown over the past 40 years.
There has been considerable public concern that carbon dioxide gas released by burning fossil fuels would warm the planet enough to melt the Antarctic icecap and flood out many coastal cities. Thus the suggestion that such melting may already be under way stirred media comment.
This was premature. As Etkins and Epstein themselves warned, they were only offering a hypothesis based on ''gross approximations.'' Now another research team -- V. Gornitz, S. Lebedeff, and J. Hansen of the Goddard Space Flight Center -- have concluded that most of the sea-level rise is due to thermal expansion after all. Any net melting of the ice sheets is too small to be detected.
They explain in a paper also published in Science that there is clear evidence for a 12-centimeter sea-level rise over the past century. Of this, 10 centimeters is due to such short-term influences as atmospheric warming, while 2 centimeters is due to longer term causes.
However, while the Antarctic ice may not yet be melting, the Goddard scientists warn that thermal expansion alone could raise the sea level another 20 to 30 centimeters over the next 70 years if the predicted global carbon-dioxide warming occurs. Such a higher sea level would put new stresses on the Antarctic ice, and these in turn could accelerate ice-sheet collapse.
They too stress the speculative nature of their reasoning. But they join the NOAA scientists in urging a detailed monitoring of sea level and satellite measurements of the bulk of Antarctic ice.
The bottom line seems to be that there is no need to panic about possible collapse of Antarctic glaciers. But the sea level is rising and humanity must be wary.