No stopping rising sea levels, study says

Only part of the picture
The sea-level prediction is based solely on thermal expansion -- the oceans physically swell as they get warmer. The projection does not take into account fresh water that other scientists expect to melt from glaciers and ice sheets, a process that appears already underway and which could snowball, some experts say.

Greenland's largest glacier, for example, doubled its forward progress toward the sea between 1997 and 2003. It is also thinning rapidly, adding water to the sea more quickly than realized, a study last year found.

Add the probable melting in, and seas could rise 8 inches through 2100 in the best-case scenario, Meehl and his colleagues say.

INTERACTIVE Melting mountains Data shows five areas of concern

The rise could swamp some coastal villages, shrink islands, and make hurricanes and other extreme weather events more catastrophic.

The inevitable change, as Meehl's model has it, is due to two factors:

• The ocean lags far behind the land and the air in temperature changes. This "thermal inertia," as scientists call it, means big changes in the oceans occur over decades and centuries, not years. A warming change seen in the atmosphere in recent decades cannot have fully played out yet in the water.
• Carbon dioxide and other greenhouse gases do not break down into other chemicals rapidly, so what's up there will be there for a long time.

'Impossible to avoid'
A separate paper in the journal, also from NCAR, suggests temperatures and sea levels are bound to rise for the next three centuries even if no more greenhouse gases are added to the air.

"Avoiding these changes requires, eventually, a reduction in emissions to substantially below present levels," said Tom Wigley, author of the second study. "For sea level rise, a substantial long-term commitment may be impossible to avoid."

Unlike the models' assumptions, greenhouse gas emissions continue.

"When and how we stabilize concentrations will dictate, on the time scale of a century or so, how much more warming we will experience," Meehl and his colleagues write in the journal. "But we are already committed to ongoing large sea level rise, even if concentrations of [greenhouse gases] could be stabilized."

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