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The oceans may have been higher than they are now between 4000 and 8000 years ago. Understanding how the ancient climate led to those high seas could improve projections of how climate change will affect sea level in this century.

There are three points in Earth鈥檚 recent geologic history where the planet was warm enough to somewhat resemble today鈥檚 climate, says at Columbia University in New York. The most recent of these was around the middle of the Holocene Epoch, which began just under 12,000 years ago.

Such climate 鈥渁nalogues鈥� can help calibrate models that show how rising temperatures will affect other geophysical processes. However, there remains a lot of uncertainty about temperature, melting ice and sea level during the Holocene, says Creel.

He and his colleagues modelled changes in global average sea levels during the Holocene using the most up-to-date geological estimates of past relative sea levels along coasts, as well as improved models of the volume of Antarctic ice at the time based on new measurements of rock isotopes. They also accounted for expanding warmer waters, melting of the Greenland ice sheet, melting of mountain glaciers and the rebounding of continents freed from the weight of the ice.

They found a high probability that global average sea levels were higher than they were at the start of the industrial revolution in 1850 at some point between 4000 and 8000 years ago, possibly by as much as 1.5 metres. Creel says the results are preliminary, but this may be connected to high polar temperatures in the mid-Holocene warm period, which coincided with the Antarctic ice sheet shrinking to smaller than its present extent. Since 1850, global average sea levels have risen by just over 20 centimetres.

Antarctic ice shrinkage appeared to lag around 250 years behind these higher temperatures.聽 at Bryn Mawr College in Pennsylvania says the finding could imply that current models underestimate future sea level rise. 鈥淭he better we can understand those dynamics in the past, the better we can project into the future,鈥� she says.

Human settlements would have been around during the time of those past high seas. But Creel says this does not suggest we can easily adapt today to what are currently the fastest rates of sea-level rise in at least 5000 years. According to projections from the Intergovernmental Panel on Climate Change (IPCC), average sea levels will surpass even this new high estimate by 2080.

Ancient climates can help put this rapid change in context, but the comparison is imperfect, says Creel. 鈥淭hese are partial analogues, because we鈥檝e never put the earth through an experiment quite like this,鈥� he says.