Off balance: the CO2 and sea level seesaw – An interview with Dr Gavin Foster

 The study “Relationship between sea level and climate forcing by CO₂ on geological timescales” by Dr Gavin Foster and Professor Eelco Rohling which was this week published in the  Proceedings of the National Academy of Sciences  had quite an impact on the science websites and blogs.

In an interview in Katy Edgington (ScienceOmega.com), Gavin “expounded on the link between CO₂ and the seas, and how the correlation exhibited in this study could influence our forecasts for the future.”

While sea level change is arguably one of the most long-lasting and significant impacts of anthropogenic climate change, long-range predictions of the change that can be expected as the oceans warm and the continental ice sheets melt are quite uncertain.

“This is largely because the complex processes involved in the melting of the continental ice sheets are difficult to incorporate into climate models,” Dr Foster pointed out. “We currently rely on semi-empirical methods to describe their behaviour; methods which remain as yet untested.”

@ Matthew Humphreys

Dr Foster and his colleagues approached the problem in a slightly different way, by delving into the rich archive of the geological past to find examples of warmer worlds that could provide an insight into how the Earth may behave in a warmer future. Focusing on the relatively recent past – the last 40 million years – they minimised the impact of changes in continental configuration, for example.

“The main advantage of looking at the geological past, and what makes it worthwhile, is that it represents a reality – a state which we can be sure the Earth system once occupied,” explained Dr Foster. “It inherently includes all feedbacks involved in the system whether we currently know about them or not; this is not the case with modelling, of course, which draws directly on the state of our current knowledge.”

The inclusion of data from points at which the global temperature was increasing and decreasing allowed Dr Foster and co-author Professor Eelco Rohling – also from Ocean and Earth Science at the University of Southampton – the best chance of spotting trends and relationships.

“As many people are aware, CO₂ is a potent greenhouse gas responsible for a significant portion of the greenhouse effect,” stated Dr Foster. “On geological timescales, the concentration of CO₂ in the atmosphere is determined by subtle imbalances in the amount of CO₂ coming out of volcanoes and the amount being removed by silicate weathering.”

Over the last 40 million years CO₂ concentration has changed quite dramatically, from 1200 parts per million (ppm) 40 million years ago to 180 ppm during the last glacial maximum.

“We show here that ice volume and sea level, and hence global temperature, have changed in concert,” stated Dr Foster. “Correlation does not prove causation of course, but we have known from fundamental physics for over 100 years that if you change atmospheric CO₂ concentration you change global temperature by ~1ºC per doubling. This basic response is amplified by other processes that operate in the atmosphere and the result is something like a 2–5ºC global temperature change for a doubling of CO₂.”

New study documents the natural relationship between CO2 concentrations and sea level

National Oceanography Centre, Southampton (UK)

By comparing reconstructions of atmospheric CO2 concentrations and sea level over the past 40 million years, 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 nine metres above current levels.

The study 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 whilst 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.

Lead author Dr Gavin Foster, from Ocean and Earth Science at the University of Southampton which 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 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, 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, with 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.”

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 behaviour 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, towards its natural equilibrium position at around 24 +7/-15 metres, at 68 per cent confidence. 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.

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 The study “Relationship between sea level and climate forcing by CO₂ on geological timescales” is published this week online ahead of print in Proceedings of the National Academy of Sciences of the United States of America