By BBC News Online's Damian Carrington

Global warming can lead to a dramatic fall in sea level, says a US geologist.

This suggestion is the opposite of the generally-expected effect of rising temperature. And while it is unlikely to happen in the near future, Dr John Bratton of the US Geological Survey says the process behind it could offset the sea level rises which are predicted to flood low-lying areas of the world.

It could also explain mysterious plunges in sea level in warmer periods in the Earth's geological past.

Ice cool

The sea level drops could be caused by the melting of "clathrates". These are sea-floor crystals of water ice and gases such as methane. When the crystals melt, the gas bubbles away and other gases trapped in the ocean sediments below could also be released.

In the worst circumstances, the "hole" left behind could result in a sea level drop of 25 metres but Dr Bratton told BBC News Online that his conservative estimates suggest a drop of up to 1.5m.

"Any temperature rise will start to melt clathrate," he says. "The apparent massive hydrate melting about 60 million years ago was triggered by an increase in bottom water temperatures of about four degrees centigrade.

"Therefore, it appears that the process could get going with an increase of even one or two degrees, especially in the polar regions where gas hydrate is abundant."

Balancing act

This is quite possible but any actual drop in sea level would be countered by the simultaneous melting of the Earth's polar ice caps. The rising temperatures would also cause ocean waters to expand.

Dr Bratton says the predicted drop resulting from clathrate melting "is of the same order of magnitude as those associated with thermal expansion of the oceans, melting of non-polar ice and melting of the West Antarctic ice sheet."

If correct, this is good news for threatened coastal areas but Dr Bratton warns that the release of methane, a greenhouse gas, to the atmosphere could itself have a significant effect in driving further climate warming.

Positive feedback

Asked if this is a worry, Dr Bratton says: "Yes, definitely, although not everyone agrees the effect would be that significant relative to anthropogenic forcing by carbon dioxide emissions.

"Almost everyone agrees that hydrates melt when climate warms. The debate is now about whether hydrates may actually drive natural climate warming or whether they just go along for the ride."

Dr Bratton's research is published in the journal Geology.