It has been about 3 million years since the Earth’s atmosphere last had 400 ppm of carbon dioxide

In 2015, I was interviewed by Susan Kucera when she visited Bristol to show her beautiful film on climate change – Breath of Life.  Working with Jeff Bridges, she has created a powerful new film – Living in the Future’s Past – that features those interviews with me and many other scientists, psychologists, politicians and philosophers. My own contributions on climate change reflect on the history of our planet and how that provides perspective for our current unprecedented rate of climate change. To elaborate on that, I am posting some recent press releases on the research that informed my reflections (and yet to be published at the time of interview).

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In particular, I discuss work in collaboration with colleagues at Southampton, showing that the Earth’s current pCO2 level of 400 ppm is higher than it has been for nearly 3 million years (this work also refers to our research on the Pliocene, which I have also written about here). 

A multinational research team, led by scientists at the University of Southampton and the University of Bristol Cabot Institute, has developed new records of past CO2 levels.  These reveal that the CO2 content of the Earth’s atmosphere between 2.8 to 3.3 million years ago, were higher than that of the pre-industrial Earth and likely higher than at any other point over the past two million years – but similar to values reached in the past decade.

The new records are based on geochemical analyses of marine sediments. 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 prior to about 1 million years ago, 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 sediments at the bottom of the sea.”

By studying the relationship between CO2 levels and climate change during a warmer period in Earth’s history, the team 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.  “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 4.5°C warming per CO2 doubling summarised in the latest IPCC report. ”

Professor Dan Lunt, also of the University of Bristol and the Cabot Institute adds: “We compared the temperature response to CO2 change in the warm Pliocene to that during colder times, like the glacial cycles of the last 800 thousand years.  The temperature response was around half that of the colder period, but that difference 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.”

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

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. 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.  This suggests that in the long term, we have already committed to 2 °C warming, and future CO2 increases will only add to that.”

NOTE: Subsequent to this work, we pushed this methodology further back into Earth history, into the Eocene (30 to 50 million years ago).  This was probably the last time the Earth had pCO2 levels similar to what we might reach by the end of the century (>800 ppm).  

Plio-Pleistocene climate sensitivities 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 DOI: 10.1038/nature14145

This work was funded by an NERC grant to Pancost (PI), G Foster, D Schmidt and D Lunt.

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