SEA levels could rise by more than a metre by 2100, significantly more than previously predicted, according to Australian research presented at an international climate change conference.
The findings warn that reductions in emissions need to be "urgent and significant", or coastal flooding seen in Australia once every 100 years today, could occur several times a year by 2100.
About 600 million people, or 10 per cent of the world's population, living in coastal areas could also be hit by flooding.
The new estimate appears to significantly worsen the predictions presented two years ago in a UN Intergovernmental Panel on Climate Change report, which projected a sea-level rise of between 18 and 59 centimetres by 2100.
However, John Church, of the Centre for Australian Weather and Climate Research in Tasmania, told the summit in Copenhagen that sea levels were already rising beyond that projected rate.
"Unless we undertake urgent and significant mitigation actions, the climate could cross a threshold during the 21st century, committing the world to a sea-level rise of metres," he said.
Dr Church, lead speaker in the conference's sea-level session, said the increase in projected sea levels was due to changes in the polar ice sheets that were not included in earlier calculations. "The oceans are continuing to warm and expand, the melting of mountain glacier has increased and the ice sheets of Greenland and Antarctica are also contributing to sea-level rise," he said.
Co-chairman of the session, Konrad Steffen, of the University of Colorado, said the projected rise of up to one metre by 2100 was a global average.
There would be regional differences, depending on where the source of ice loss occurred, he said.
Meanwhile, leading climate change expert Michael Raupach told the conference that the stabilising influence carbon sinks have had on rising carbon emissions was weakening, and could even be reversed.
Without carbon sinks, which absorb carbon dioxide from the atmosphere, climate change would occur twice as fast as predicted.
Canberra-based Dr Raupach, a CSIRO scientist and co-chair of the independent research network Global Carbon Project, said land and sea sinks were not keeping up with the rise in emissions.
"The predictions show that in the later part of this century, there is a chance that the land sink will reverse and become a source (of carbon), and that would be a dramatic reversal of the sink," he said.
This would lead to the effects of climate change accelerating about 1½ times as fast as predicted.
However, Dr Raupach said the oceans were expected to remain a sink.
He said land sinks were vulnerable to shifts in atmospheric composition, temperature, drought, rainfall, fire frequency and deforestation - all of which can slow or reverse sinks.
"This kind of vulnerability might cause an extra, say, 20 to 30 per cent of warming over this century," he said.