Carbon dioxide and climate - the plot thickens

8 June 1999

Levels of atmospheric carbon dioxide 43 million years ago were not much higher than they are today, and were only about a third higher than they were before man started burning vast amounts of fossil fuels, researchers from Imperial College, London, and Bristol University, report in Science this week (11 June). As the Earth at that time was relatively warm and had no polar ice caps, this finding appears to challenge the prevailing dogma that carbon dioxide is the key driver of climate change over long periods.

Using a novel technique, the researchers obtained a best estimate for carbon dioxide levels 43 million years ago of 385 parts per million (ppm), slightly higher than todays figure of 360 ppm. This result is far below earlier estimates which suggested that carbon dioxide levels were up to six times present day values. There are two ways to interpret these new findings; one may give comfort to those who do not believe rising carbon dioxide levels are a problem, but the other suggests we are about to embark on a period of massive climate change.

One of the co-authors of the article, Professor Martin Palmer of Imperial College, says, Our research indicates that plate tectonic movement of the continents and consequent reorganisation of ocean currents may have been responsible for the big climate changes of the last 43 million years. Antarctica was largely ice free as it was kept warm by a tropical heat supply. But as continents drifted, and gateways between them opened and closed, warm waters were forced to circulate in different patterns around the globe. This allowed circumpolar currents of cold water to form around Antarctica. Cut off from warm waters Antarctica cooled and ice sheets built up, acting as a trigger to cool other parts of the Earth. As plate tectonic movements occur over millions of years and are totally outside our control, there is little point in worrying about their role in future climate change.

However, an alternative explanation of our data is that the Earths climate is far more sensitive to atmospheric carbon dioxide levels than we have previously thought. If a fall in carbon dioxide from 385 ppm to preindustrial values of 280 ppm was sufficient to drive us from the warm world of 43 million years ago to the icehouse world of the recent past, then the current rapid rise in carbon dioxide may see us soon heading back to a greenhouse world. Clearly, melting even a small portion of the polar ice caps would be disastrous for low-lying areas.

Of course, it is likely that plate tectonics and carbon dioxide levels both play a role in climate change, so we hope to carry out further studies to examine their relative importance in more detail. This will first involve closer calibrations of our measuring technique and then we hope to put it into use to measure the carbon dioxide levels from the time of the dinosaurs, the Cretaceous period, when it was much warmer than today, and from which we would expect to observe high carbon dioxide levels.

To reach their estimate of carbon dioxide levels of 43 million years ago, Professor Palmer and Dr Pearson of Bristol University analysed the shells of plankton which had been recovered from cores of marine sediment from the tropical Pacific Ocean.

They analysed these samples for the isotope composition of boron, which is an indicator of the pH of the surface seawater in which the plankton formed their calcium carbonate shells. As pH is closely dependent on atmospheric levels of carbon dioxide, the boron isotope composition can be taken as an indicator of atmospheric carbon dioxide levels.

Notes to editors:

1. The article Middle Eocene Seawater pH and Atmospheric CO2 Concentrations is published in Science (11 June 1999). The authors are Paul Pearson (University of Bristol) and Martin Palmer (Imperial College).

2. The research was funded by the UK's Natural Environment Research Council

3. For more information please contact:
Professor Martin Palmer

TH Huxley School of Environment, Earth Sciences and Engineering
Imperial College of Science, Technology and Medicine
Tel: 0171 594 7359 Fax: 0171 594 6403
E-mail: martin.palmer@imperial.ac.uk

4. Imperial College of Science, Technology and Medicine has the largest income (£309m) and the largest research income (£117m) of any university institution in the UK. It has 8823 full time students, 30 per cent of whom are postgraduate and 35 per cent non-British nationalities. Imperial College was rated second overall in both The Times Good University Guide (23 April 1999) and the Financial Times Guide to Britains top 100 universities (1 April).