If history is a guide, sea levels could rise much higher than previously predicted, a new study said Thursday.
The study, published in the US journal Science, found that sea surface temperatures today were like those during the last interglaciation period when sea levels were between six and nine meters above their present height.
"The trend is worrisome," said the study, led by Jeremy Hoffman of the Oregon State University.
The last interglaciation, which occurred 129,000 to 116,000 years ago, is previously thought to have been about as warm or a bit warmer than today.
As a result, the period has been used as a reference to validate global climate models and understand sea level response to a warming climate.
In the new study, Hoffman and colleagues examined marine sediment core records from 83 sites to understand sea surface temperatures at that time.
Each core site was compared to data sets from 1870-1889 and 1995-2014, respectively.
Their analysis revealed that, at the start of the last interglaciation, or 129,000 years ago, the global ocean surface temperature was already similar to the 1870-1889 average.
However, the peak global ocean surface temperature, which was reached 125,000 years ago, was "indistinguishable" from the 1995-2014 average.
The study suggests that in the long term, sea level may rise continuously in response to the warming people are causing, said Professor Andrew Watson of Britain's University of Exeter, who was not involved in the study.
"The good news is that with luck it will continue to rise slowly, so that we have time to adapt, but the bad news is that eventually all our present coastal city locations will be inundated."
Previously, the US Intergovernmental Panel on Climate Change has predicted a sea level rise of up to one meter by 2100, if carbon emissions are not constrained.