London, Jul 12 (IANS): Scientists have identified what actually triggered the rapid rise of sea levels in the past, using climate and ice sheet models.

The process, named 'saddle-collapse', was found to cause two rapid sea level rise events: the Meltwater pulse 1a around 14,600 years ago and the '8,200 year' event.

Using a climate model, Lauren Gregoire of University of Bristol's School of Geographical Sciences and colleagues unearthed the series of events that led to saddle-collapse, in which domes of ice over North America became separated, triggering rapid melting and the opening of an ice-free corridor, the journal Nature reports.

Gregoire, who led the study, said: "We didn't expect our model to produce such a rapid sea level rise. We got really excited when we realised that the events we simulated corresponded to real events!"

Ice domes were up to three km thick, formed in regions of high snowfall and higher topography, such as the Rocky Mountains. Together with the saddles -- lower valleys of ice between the domes -- these made up the ice sheet, according to a Bristol statement.

Towards the end of the last ice age, at the time of mammoths and primitive humans, the climate naturally warmed. This started to melt ice at increasingly high elevations, eventually reaching and melting the saddle area between the ice domes.

This triggered a vicious circle in which the melting saddle would lower, reach warmer altitudes and melt even more rapidly until the saddle had completely melted.

In just 500 years, the saddles disappeared and only the ice domes remained.

The melted ice flowed into the oceans, leading to rapid sea level rises of nine metres in 500 years during the Meltwater pulse event 14,600 years ago and 2.5 metres in the second event 8,200 years ago.