Warm past points to sea level rises

5 February 2015

Scientists have for the first time constructed a detailed record of carbon dioxide fluctuations from the Pliocene epoch, around three million years ago, when the Earth was most recently warmer than today.

The team of scientists from the UK and Australia used the new data to estimate the sensitivity of climate to increasing levels of carbon dioxide.

Co-author Professor Eelco Rohling from the Research School of Earth Sciences at The Australian National University (ANU) said the pattern supports the predictions of the Intergovernmental Panel on Climate Change (IPCC) for a warmer world, and points to sea level rises in the tens of metres.

“We have needed more data for warmer periods, and now we have it,” he said.

“We are headed towards a climate like the Pliocene, where there is a mean warming of around 2.5 to 3 degrees.

“The scary bit is that Pliocene sea level stood at least nine meters higher than today, as we found in previous studies. Our current study finds that modern climate is as sensitive to carbon dioxide change as the Pliocene climate. Therefore, we may expect ice-sheet retreat and sea-level rise to continue toward similar values over centuries to come.” he said.

During the Pliocene, atmospheric carbon dioxide levels were around 350 – 400 parts per million, similar to the level reached in recent years.

The international team of scientists deduced the carbon dioxide levels by studying the levels of the element boron in microfossils from the ocean floor. Changes in ocean acidity affect the ratio of boron isotopes that various marine organisms take up in their shells.

New techniques to measure the boron levels in the fossils have enabled scientists to build a detailed record of the carbon dioxide fluctuations in the Pliocene epoch.

“Today the Earth is still adjusting to the recent rapid rise of carbon dioxide caused by human activities, whereas the longer-term, Pliocene record documents the full response of carbon-dioxide related warming,” said joint lead researcher Dr Gavin Foster from the University of Southampton.

Sea level rises will depend on the delayed response of the ice sheets to the sudden rise in carbon dioxide, Professor Rohling said.

“This is a game that plays over centuries; it does not stop at the much-debated year 2100,” he said.

“It’s inevitable. We’re not going to get away from this. It could take the Earth 200,000 – 300,000 years to clean up the carbon we’ve emitted.”

The research is published in Nature.