Sea levels could keep rising higher than previously projected — for the next couple of millenia — even if we manage to keep atmospheric carbon dioxide levels from growing any further, according to new research published this week.

Researchers from the National Oceanography Centre, Southampton (NOCS), the University of Bristol and the University of Tübingen, Germany, reached their conclusion after creating a continuous reconstruction of sea level fluctuations over the last 520,000 and comparing it with data on global climate and carbon dioxide.

Little is known about the total amount of possible sea-level rise in equilibrium with a given amount of global warming. This is because the melting of ice sheets is slow, even when temperature rises rapidly. As a consequence, current predictions of sea-level rise for the next century consider only the amount of ice sheet melt that will occur until that time. The total amount of ice sheet melting that will occur over millennia, given the current climate trends, remains poorly understood.

The new research shows a systematic equilibrium relationship between global temperature and carbon dioxide concentrations and sea-level changes over the last five glacial cycles. By projecting that relationship to today’s carbon dioxide levels, the researchers calculated a sea level rise of 25 metres above today’s levels, plus or minus 5 metres.

“We emphasise that such equilibration of sea level would take several thousands of years,” write researchers Michal Kucera and Mark Siddall. “But one still has to worry about the large difference between the inferred high equilibrium sea level and the level where sea level actually stands today. Recent geological history shows that times with similarly strong disequilibria commonly saw pulses of very rapid sea-level adjustment, at rates of 1 to 2 metres per century or higher.”

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