More evidence for rising sea levels. New study documents the natural relationship between CO2 concentrations and sea level

Lead author Dr Gavin Foster and co-author Professor Eelco Rohling, of University of Southampton Ocean and Earth Science have compared reconstructions of atmospheric CO₂ concentrations and sea levels over the past 40 million years. They found that CO₂ levels similar to the present (almost 400 parts per million) were associated with sea levels at least nine metres above current levels.

Dr Gavin Foster says: "A specific case of interest is one in which CO₂ levels are at 400 to 450 parts per million, because that is the requirement for the often mentioned target of a maximum of two degrees global warming."

The researchers found that the natural relationship displays a strong rise in sea level for CO₂ increase from 180 to 400 parts per million, peaking at CO₂ levels close to present-day values, with sea level at 24 +7/-15 metres above the present, at 68 per cent confidence limits.

According to the study, sea level stays more or less constant for CO₂ changes between 400 and 650 parts per million. At CO₂ levels above 650 parts per million, the researchers again saw large changes in sea level.  An ice-free planet, with sea level 65 metres above the present, occurred in the past when CO₂ levels were around 1,200 parts per million.

Professor Rohling adds: "Sea level rises to these high values will take many centuries, or even millennia, but the implications from the geological record are clear. In Intergovernmental Panel on Climate Change terms, 400 ppm CO₂ is associated with a likely sea level rise of at least nine metres above the present. Previous research indicates that such rises above present sea level may occur at rates of roughly one metre per century."

Based on these results, which document how the Earth system has operated in the past, future stabilisation of CO₂ at 400-450 parts per million is unlikely to be sufficient to avoid a significant steady long-term sea-level rise.

 The study is published online ahead of print in Proceedings of the National Academy of Sciences of the United States of America.