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Why the Pliocene period is important in the upcoming IPCC report

Critical to our understanding of the Earth system, especially in order to predict future anthropogenic climate change, is a full comprehension of how the Earth reacts to higher atmospheric CO2 conditions. One of the best ways to look at what the Earth was like under higher CO2 is to look at times in Earth history when atmospheric CO2 was naturally higher than it is today. The perfect period of geological history is the Pliocene, which spans from 5.3 – 2.6 million years ago. During this time we have good evidence that the Earth was 2-3 degrees warmer than today, but other things, such as the position of the continents and the distribution of plants over the surface, was very similar to today.

There is therefore a significant community of oceanographers and climate modellers studying the Pliocene, many of whom were in Bristol last week for the 2nd Workshop on Pliocene climate, and one of the main points of discussion was the exact value of CO2 for the Pliocene.

80 top scientists from 12 countries gathered for the 2nd Workshop on Pliocene climate on 9-10 September 2013 at the University of Bristol
The imminent release of the first volume of the 5th assessments of the IPCC is also expected to include sections on Pliocene climate.

Today we published a paper in Philosophical Transactions of the Royal Society A which therefore represents an important contribution to the debate. Several records of Pliocene CO2 do exist, but their low temporal resolution makes interpretation difficult. There has also been some controversy about what these records mean, as some show surprisingly high variability, given what we understand about Pliocene climate.

We sampled a deep ocean core taken by the Ocean Drilling Program in the Carribean Sea. Cores such as this record the ancient envrionment as sediment collects over time like the progressive pages in a book, and by analysing the chemical composition of the layers a history of the Earth System can be discovered. The approach that Badger et al take is to use the carbon isotopic fractionation of photosynthetic algae, which has been shown to vary with atmospheric CO2.

What this study revealed is that atmospheric CO2 was actually quite low, at around 300 ppm for much of the warm period. What was also revealed was that CO2 was relatively stable, in contrast to previous work. This implies that in the Pliocene the Earth must have been quite sensitive to CO2, as small changes in atmospheric CO2 drove changes in climate. The study of Badger et al doesn't explicitly reconstruct climate sensitivity but it does have important implications for future change.

The paper is published in a special volume of Philosophical Transactions of the Royal Society A, edited by Bristol scientists Dan Lunt, Rich Pancost, Andy Ridgewell and Harry Elderfield of Cambridge University. The volume is the result of the Warm Climates of the Past – A lesson for the Future? meeting which took place at the Royal Society in October 2011. The volume can be accessed here: http://bit.ly/PTA2001

Marcus Badger

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