January 3, 2013
Greenhouse Gas Associated With High Sea Levels Over Past 40 Million Years
April Flowers for redOrbit.com - Your Universe Online
Researchers based at the National Oceanography Centre, Southampton, have found that greenhouse gas concentrations similar to the present — almost 400 parts per million - were systematically associated with sea levels at least 30 feet above current levels by comparing reconstructions of atmospheric CO2 concentrations and sea level over the past 40 million years.
The study, published online in the Proceedings of the National Academy of Sciences (PNAS), determined that the "natural equilibrium" sea level for CO2 concentrations ranging between ice-age values — 180 parts per million — to ice free values of more than 1,000 parts per million.
The equilibrium is reached over many centuries, so while the study does not predict any sea level value for the coming century, it does demonstrate what sea level might be expected if the climate was stabilized at a certain CO2 level for several centuries.
Dr Gavin Foster, from Ocean and Earth Science at the University of Southampton, 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 research team compiled more than 2,000 pairs of CO2 and sea level data points over critical periods during the last 40 million years. Some of these critical periods were warmer than present, some similar, and some colder. Periods during which global temperatures were increasing were included, along with 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 Professor Eelco Rohling, also from Ocean and Earth Science at the University of Southampton.
The natural relationship, the team found, displays a strong rise in sea level CO2 increase from 180 to 400 parts per million, peaking at CO2 levels close to present-day values. This includes sea level at 24 +7/-15 metres above the present, at 68 per cent confidence limits.
"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."
For CO2 changes between 400 and 650 parts per million sea level stays more or less constant, according to the study, 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 near 215 feet 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 [3.6 F], that is with CO2 stabilized at 400 to 450 parts per million, sea level is set to steadily rise for many centuries, towards its natural equilibrium position at around 24 +7/-15 meters, at 68 per cent confidence. In Intergovernmental Panel on Climate Change (IPCC) terms, this is a likely rise of at least [30 feet] above the present. Previous research indicates that such rises above present sea level may occur at rates of roughly one meter per century."
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 based on the results of this study, which document how the Earth system has operated in the past.