We're in a period in between ice ages at the moment, and the last time the world was in this state the sea-surface temperatures were about the same as they are today, a new study finds. But sea levels were a lot higher then, suggesting that we could see a prolonged period of rising sea levels.

The last inter-glacial period happened 129 to 116 thousand years ago and was in many ways very like the present Holocene period. Researchers sampled sediment cores from the sea bed at 83 sites with historical records from the pre-industrial era, publishing the results in a paper published in the journal Science.

They found that sea surfaces temperatures at the start of the last interglacial were similar to the pre-industrial temperatures. But by about 4,000 years into the interglacial, the temperatures had risen by about 0.5C, to the 1995-2014 average.

Warmer sea temperatures are a considerable factor in melting ice sheets and glaciers in the polar regions.

"The study suggests that, in the long term, sea levels will rise six metres at least in response to the warming we are causing," said Andrew Watson, a climate researcher the University of Exeter in the UK who was not involved in the research, said in a statement.

"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," he said.

The rate at which sea levels are likely to rise is not known, and could be over a scale of centuries to millennia.

Louise Sime, head of palaeoclimate research at the British Antarctic Survey, told the Guardian: "The rates of ice sheet loss are really difficult to predict. Estimates are anything from 200 to 7,000 years."

Sea wall
It's not known at what rate sea levels will rise to those seen in the last interglacial period Ben Salter / Flickr