IT is the doomsday scenario which will permanently change the map.

Global sea levels are set to rise twice as fast as they would naturally over the next 200 years according to a new report by Southampton academics, leaving coastal cities and towns at the mercy of the waves.

They have warned that if the rate the water is rising increases, action would be needed to avert a catastrophe. It is the latest blow for people living in seaside communities across the south who are already on flood alert as they are battered by rain, winds and high tides.

The stark warning from University of Southampton researchers comes in their new landmark report exposing the impacts of human activity on world climate change.

The scientists and oceanographers reveal sea level changes have rapidly increased above natural standards since the industrial revolution.

Now they forecast sea levels rising by 80cm (31.44in) by 2100 and by 2.5m (8.2ft) by 2200.

The joint study with the Australian National University used geological evidence from the past few million years to understand the background pattern of natural sea level rises.

This compares with historical tide-gauge and satellite observations of sea level change for the “global warming” period since the industrial revolution.

The background pattern looks only at how fast sea levels might change if only natural process were at work rather than speculating about new mechanisms that might develop due to man-made global warming.

Report co-author Dr Ivan Haigh, a University of South-ampton coastal oceanography lecturer who also works for the Natural Environment Research Council said: “Historical observations show a rising sea level from about 1800 as sea water warmed up and melt water from glaciers and ice fields flowed into the oceans.

“Around 2000, sea level was rising by about three millimetres per year. That may sound slow, but it produces a significant change over time.”

The background pattern allowed the team to see whether recent sea level changes are exceptional or within the normal range, and whether they are faster, equal, or slower than natural changes.

They concluded the sea-level was “just within” natural expectations, but was still quite fast by natural standards.

Lead author Professor Eelco Rohling, a former University of Southampton student now at the Australian National University, said: “Based on our natural background pattern, only about half the observed sea level rise would be expected.

“Continued monitoring of future sea level rise will show if and when it goes outside the natural range. If that happens, then this means that our current understanding falls short, potentially with severe consequences.”

The study was funded by the Natural Environment Research Council and the Australian Research Council and is published in the online journal Scientific Reports.