PR: What the geological evidence of the past tells us about modern sea-level rise

12 December 2013

Researchers from the University of Southampton and the Australian National University report that sea-level rise since the industrial revolution has been fast by natural standards and – at current rates – may reach 80cm above the modern level by 2100 and 2.5 metres by 2200.

The team used geological evidence of the past few million years to derive a background pattern of natural sea-level rise. This was compared with historical tide-gauge and satellite observations of sea-level change for the ‘global warming’ period, since the industrial revolution. The study, which was funded by the Natural Environment Research Council (iGlass consortium) and Australian Research Council (Laureate Fellowship), is published in the journal Scientific Reports.

Lead author Professor Eelco Rohling, from the Australian National University and formerly of the University of Southampton, says: “Our natural background pattern from geological evidence should not be confused with a model-based prediction. It instead uses data to illustrate how fast sea level might change if only normal, natural processes were at work. There is no speculation about any new mechanisms that might develop due to man-made global warming. Put simply, we consider purely what nature has done before, and therefore could do again.”

Co-author Dr Gavin Foster, a Reader in Ocean and Earth Science at the University of Southampton, who is based at the National Oceanography Centre, Southampton (NOCS), explains: “Geological data showed that sea level would likely rise by nine metres or more as the climate system adjusts to today’s greenhouse effect. But the timescale for this was unclear. So we studied past rates and timescales of sea-level rise, and used these to determine the natural background pattern.”

Co-author Dr Ivan Haigh, lecturer in coastal oceanography at the University of Southampton and also based at NOCS, adds: “Historical observations show a rising sea level from about 1800 as sea water warmed up and melt water from glaciers and ice fields flowed into the oceans. Around 2000, sea level was rising by about three mm per year. That may sound slow, but it produces a significant change over time.”

The natural background pattern allowed the team to see whether recent sea-level changes are exceptional or within the normal range, and whether they are faster, equal, or slower than natural changes.

Professor Rohling concludes: “For the first time, we can see that the modern sea-level rise is quite fast by natural standards. Based on our natural background pattern, only about half the observed sea-level rise would be expected.

“Although fast, the observed rise still is (just) within the ‘natural range’. While we are within this range, our current understanding of ice-mass loss is adequate. Continued monitoring of future sea-level rise will show if and when it goes outside the natural range. If that happens, then this means that our current understanding falls short, potentially with severe consequences.”

When was CO2 last at 400 ppm? And what was the climate like?

Climate change clues from tiny marine algae – ancient and modern

Microscopic ocean algae called coccolithophores are providing clues about the impact of climate change both now and many millions of years ago. The study found that their response to environmental change varies between species, in terms of how quickly they grow.

Coccolithophores, a type of plankton, are not only widespread in the modern ocean but they are also prolific in the fossil record because their tiny calcium carbonate shells are preserved on the seafloor after death – the vast chalk cliffs of Dover, for example, are almost entirely made of fossilised coccolithophores.

Fossil and modern coccolithophore cells of species Toweius pertusus and Coccolithus pelagicus. Courtesy of Paul Bown,UCL

The fate of coccolithophores under changing environmental conditions is of interest because of their important role in the marine ecosystem and carbon cycle. Because of their calcite shells, these organisms are potentially sensitive to ocean acidification, which occurs when rising atmospheric carbon dioxide (CO2) is absorbed by the ocean, increasing its acidity.

There are many different species of coccolithophore and in an article, published in Nature Geoscience this week, the scientists report that they responded in different ways to a rapid climate warming event that occurred 56 million years ago, the Palaeocene-Eocene Thermal Maximum (PETM).

The study, involving researchers from the University of Southampton, the National Oceanography Centre and University College London, found that the species Toweius pertusus continued to reproduce relatively quickly despite rapidly changing environmental conditions. This would have provided a competitive advantage and is perhaps why closely-related modern-day species considered to be its descendants, (such as Emiliana huxleyi) still thrive today.

In contrast, the species Coccolithus pelagicus grew more slowly during the period of greatest warmth and this inability to maintain high growth rates may explain why its descendants are less abundant and less widespread in the modern ocean.

“This work provides us with a whole new way of looking at living and fossil coccolithophores,” said lead author Dr Samantha Gibbs, Senior Research Fellow at University of Southampton Ocean and Earth Science.

By comparing immaculately preserved and complete fossil cells with modern coccolithophore cells, the researchers could interpret how different species responded to the sudden increase in environmental change at the PETM, when atmospheric CO2 levels increased rapidly and the oceans became more acidic.

“We use knowledge of how coccolithophores build their calcite skeletons in the modern ocean to interpret how climate change 56 million years ago affected the growth of these microscopic plankton,” said co-author Dr Alex Poulton, a Research Fellow at the National Oceanography Centre.

“This is a significant step forward and allows us to view fossils as cells rather than dead ‘rocks’. Through this we can begin to understand the environmental controls on oceanic calcification, as well as the potential effects of climate change and ocean acidification.”

Reference: Gibbs S.J., Poulton A.J., Bown P.R., Daniels C.J., Hopkins J. Young J.R., Jones H.L., Thiemann G.J., O’Dea S.A., Newsam C. (2013) Species-specific growth response of coccolithophores to Palaeocene–Eocene environmental change. Nature Geoscience doi: 10.1038/NGEO1719

The study was primarily supported by the UK Ocean Acidification Research Programme, which is jointly funded by the Natural Environment Research Council (NERC), the Department of Environment, Food and Rural Affairs (Defra) and the Department of Energy and Climate Change (DECC).

Integrated Ocean Drilling Program, Expedition 342. Watch Episode One

Descent into the Icehouse  Phil Sexton  is one of the participant scientists in the Integrated Ocean Drilling Program Expedition 342 that will drill down through the seabed near Newfoundland, close to the legendary Titanic wreckage site.  The two halves of the wreck lie between the volcanic seamounts of the Southeast Newfoundland Ridge because there the southward-flowing surface waters of the cold Labrador Sea carry icebergs to their intersection with the warm tongue of the Gulf Stream.

The mission of IODP Expedition 342 is to recover sediments that tell the story of climate change, ocean currents and glaciations over millions of years. The drill sites, not far from the Titanic’s resting place, are positioned to monitor the strength and chemistry of deepwater formation in the Atlantic as well as outflows from the Arctic basins through Baffin Bay and the Norwegian seaway.

The Newfoundland ridges are mantled with some of the oldest sediment drifts known in the deep sea and range in age from the Late Cretaceous to Paleogene. Pliocene–Pleistocene drifts in the northeastern Atlantic commonly have sedimentation rates of 4–20 cm/k.y. and therefore can be used to study rates of abrupt climate change  The main drilling target is an interval in the geologic past when the Earth was a lot warmer than today. The sediments of the Newfoundland ridges contain enough detail to teach us precisely what happened, when, and why.

More information about the Expedition