A drop in CO2 levels prompted ancient climate change

A new study has found that declining levels of carbon dioxide (CO2) in the atmosphere one million years ago significantly altered the Earth’s climate.

An international research team led by the University of Southampton compared new data on the chemistry of the Earth’s crust with a model of the climate system. They showed that the expansion of continental ice sheets coincided with a series of events that lowered atmospheric CO2 during intervals of time when the planet experienced extreme cold.

The researchers demonstrated that the plummeting CO2 levels triggered the Mid-Pleistocene Transition (MPT), which lasted around 400,000 years. The MPT had long lasting effects on the rate at which the Earth shifted between periods of freezing glacial intervals to warm interglacial climates.

The ice-age cycles are influenced by Earth’s rotation and also by the gravitational pull of other planets. The sequences became abruptly unpredictable around one million years ago during the MPT.

“We know from bubbles of the ancient atmosphere trapped in Antarctic ice cores that changes in atmospheric CO2 accompanied the more recent ice age cycles, “ explained co-lead author Dr. Tom Chalk. “CO2 was low when it was cold during the glacials and it was higher during the warm interglacials – in this way it acted as a key amplifier of the relatively minor climate forcing from the orbital cycles. Unfortunately, the ice core records only stretch back to around 800,000 years ago and so do not go over this key transition interval. In order to better understand the cause of the MPT, we needed a way to reconstruct CO2 further back in time.”

The research team examined ancient fossils of tiny marine plankton because the boron isotopic composition of their microscopic shells holds records of environmental conditions millions of years ago.

“From these boron isotope measurements we were able to recover a snapshot of the variability in atmospheric CO2 around 1.1 million years ago,” said study co-author Gavin Foster. “We were able to show, for the first time that, just as in the ice core record, CO2 and climate varied in tandem. There were two main differences however: firstly, during the glacials before the MPT, CO2 did not drop as low as it did in the ice core record after the MPT, remaining about 20-40 parts per million (ppm) higher. Secondly, the climate system was also more sensitive to changing CO2 after the MPT than before.”

The study, which is published in the Proceedings of the National Academy of Sciences, highlights the intricate relationship between climate change, glacier ice mass, and the polar ocean factors that influence natural CO2 change.

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By Chrissy Sexton, Earth.com Staff Writer