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Evidence from warm past confirms recent IPCC estimates of climate sensitivity - research published in Nature

5 February 2015

Evidence from warm past confirms recent IPCC estimates of climate sensitivity - Research by Professor Rich Pancost and Dr Marcus Badger published in Nature

New evidence shows that the levels of atmospheric CO2 millions of years ago support recent climate change predictions from the Intergovernmental Panel on Climate Change (IPCC).

A multinational research team, led by scientists at the University of Southampton and the University of Bristol’s Cabot Institute, has developed new records of past CO2 levels.  These reveal that the CO2 content of the Earth’s atmosphere between 2.3 to 3.3 million years ago, during the Pliocene, was similar to that of today but likely higher than at any other point over the past several million years.

The new records are based on geochemical analyses of marine sediments, in particular careful measurement of the isotopic composition of boron preserved in marine fossils. These have been measured using techniques developed at Bristol and Southampton over the past decade. The Bristol team includes Professor Richard Pancost from the University of Bristol, Director of the Cabot Institute and the Primary Investigator of the wider grant under which this research was conducted, as well as Dr Marcus Badger, Professor Dan Lunt and Professor Daniela Schmidt.  Professor Pancost explains: “We cannot directly measure the CO2 levels on Earth during these warmer climates and so we instead use proxies.  In the case of our project, funded by the NERC, we used a combination of approaches based on the chemical signatures of organisms preserved in ancient sediments.”

By studying the relationship between CO2 levels and climate change during a warmer period in Earth’s history, the scientists have been able to estimate how the climate will respond to increasing levels of carbon dioxide, a parameter known as ‘climate sensitivity’.

The findings, which have been published in Nature, fall in line with estimates in the most recent IPCC report. Crucially, the records also show how climate sensitivity can vary over the long term. “Today the Earth is still adjusting to the recent rapid rise of CO2 caused by human activities, whereas the longer-term Pliocene records document the full response of CO2-related warming,” says Southampton’s Dr Gavin Foster, co-lead author of the study.  “Our estimates of climate sensitivity lie well within the range of 1.5 to 4.5ºC warming per CO2 doubling summarised in the latest IPCC report.  This suggests that the research community has a sound understanding of what the climate will be like as we move toward a Pliocene-like warmer future caused by human greenhouse gas emissions.”  During the Pliocene the Earth was warmer by around 2°C than it is today and atmospheric CO2 levels were around 350-400 parts per million (ppm), similar to the levels reached in recent years.  

“Our new records also reveal an important change at around 2.8 million years ago, when levels rapidly dropped to values of about 280 ppm, similar to those seen before the industrial revolution,” says lead author of the study Dr Miguel Martínez-Botí, also from Southampton. “This caused a dramatic global cooling that initiated the ice-age cycles that have dominated Earth’s climate ever since.” 

The new records also allowed the research team to assess whether climate sensitivity was different in warmer times, like the Pliocene, than in colder times, like the glacial cycles of the last 800 thousand years.  Professor Dan Lunt, also of the University of Bristol and the Cabot Institute says: “The temperature response to CO2 change during the Pliocene was around half that of the colder period. That difference, however, can be largely resolved by considering the growth and retreat of large continental ice sheets during more recent glacial cycles. These ice sheets reflect a lot of sunlight and their growth consequently amplifies the impact of CO2 changes, but they were smaller and less variable during the warm Pliocene.” 

Professor Pancost added: “When we account for the influence of the ice sheets, we can confirm that the Earth’s climate changed with a similar sensitivity to overall forcing during both warmer and colder climates.”

Further information

Paper -  Plio-Pleistocene climate sensitivity evaluated using high-resolution CO2 records by M.A. Martínez-Botí, G.L. Foster, T. B. Chalk, E.J. Rohling, P.F. Sexton, D.J. Lunt, R.D. Pancost, M.P.S. Badger & D.N. Schmidt 518, pp 49-54. DOI: 10.1038/nature14145 

This research was featured on the BBC news website.

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