London, Jan 6: Past greenhouse gas concentrations, similar to present levels of about 400 parts per million, were also linked to sea levels rising at least nine metres higher than current levels over the past 40 million years, according to a study.
The new study by National Oceanography Centre, Southampton (UK), determined the ‘natural equilibrium’ sea level for CO2 concentrations ranging between ice-age values of 180 parts per million and ice-free values of more than 1,000 parts per million.
It takes many centuries for such an equilibrium to be reached. Therefore, while the study does not predict any sea level value for the coming century, it does illustrate what sea level might be expected if climate were stabilized at a certain CO2 level for several centuries, the journal Proceedings of the National Academy of Sciences reported.
Gavin Foster, from Ocean and Earth Science at the University of Southampton, study co-author, who is based at the centre, said: “A specific case of interest is one in which CO2 levels are kept 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 compiled more than 2,000 pairs of CO2 and sea level data points, spanning critical periods within the last 40 million years, according to a National Oceanography Centre statement.
Some of these had climates warmer than present, some similar, and some colder.
They also included periods during which global temperatures were increasing, as well as periods during which temperatures were decreasing.
“This way, we cover a wide variety of climate states, which puts us in the best position to detect systematic relationships and to have the potential for looking at future climate developments,” said co-author Eelco Rohling, also from Ocean and Earth Science at the University of Southampton.
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.