Analysis of 40-million year record calibrates CO2 concentrations with historic sea levels
By Summit Voice
SUMMIT COUNTY — Even if atmospheric carbon dioxide concentrations were to be stabilzed at today’s levels of about 400 parts per million, sea levels would gradually increase by about 30 during the next few centuries, according to researchers who calibrated CO2 levels against sea level for the past 40 million years.
The study sought to pinpoint 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.
Since it can take centuries to reach that equilibrium, the study doesn’t make any short-term forecasts about sea level rise, but the results suggests what sea levels might be if atmospheric CO2 is stabilized at certain levels.
“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,” said lead author, Dr. Gavin Foster, from Ocean and Earth Science at the University of Southampton.
The researchers compiled more than two thousand pairs of CO2 and sea level data points, spanning critical periods within the last 40 million years. 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 Professor Eelco Rohling, from the 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.
“This strong relationship reflects the climatic sensitivity of the great ice sheets of the ice ages,” said Dr Foster. “It continues above the present level because of the apparently similar sensitivity of the Greenland and West Antarctic ice sheets, plus possibly some coastal parts of East Antarctica.”
According to the study, sea level stays more or less constant for CO2 changes between 400 and 650 parts per million and it is only for CO2 levels above 650 parts per million that the researchers again saw a strong sea level response for a given CO2 change.
“This trend reflects the behavior of the large East Antarctic ice sheet in response to climate changes at these very high CO2 levels. An ice-free planet, with sea level 65 metres above the present, occurred in the past when CO2 levels were around 1200 parts per million.”
Professor Rohling said, “Sea level rises to these high values will take many centuries, or even millennia, but the implications from the geological record are clear. For a future climate with maximum warming of about two degrees Centigrade, that is with CO2 stabilized at 400 to 450 parts per million, sea level is set to steadily rise for many centuries … In Intergovernmental Panel on Climate Change terms, this is a likely 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 stabilization 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 this week online ahead of print in Proceedings of the National Academy of Sciences of the United States of America (PNAS manuscript # 2012-16073R).