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How did the CO2 concentration in Earth’s atmosphere once decrease?

by David Armstrong McKay and Paul Glantz

During the Cenozoic era – i.e. 66 million years ago until today – the Earth’s climate has shifted from a hot ‘Greenhouse’ state with no polar ice sheets to the cool ‘Icehouse’ state with polar ice sheets that we’re in today. This is revealed by palaeorecords from ancient single-celled marine organisms such as ‘benthic foraminifera’ that provide a geochemical record of past temperatures, ice sheets, and CO2 levels [Zachos et al., 2001b, 2008]. During this time atmospheric CO2 has gradually declined from ~1000 ppm in the Eocene to below ~300 ppm during the late Miocene [Pearson and Palmer, 2000; Royer et al., 2001; Beerling and Royer, 2011]. This decline has been hypothesised to be the result of either declining volcanic emissions as the rate at which new ocean crust forms has slowed, increased rates of silicate rock weathering (a chemical reaction which draws down CO2) due to the tectonic uplift of the Himalayas, or increased burial of carbon in seafloor sediment as a result of either greater plankton abundance in the ocean or carbon being better preserved in sediments [Berner, 1991; Larson, 1991; Raymo and Ruddiman, 1992; Raymo, 1994; Derry and France-Lanord, 1996; France-Lanord and Derry, 1997; Kump and Arthur, 1997; Royer et al., 2004a; Kent and Muttoni, 2008, 2013; Lefebvre et al., 2013].

More recently (the last ~1-2.5 million years), ice core and geochemical data show that CO2 levels have varied between ~180 ppm in cold glacials (a.k.a. “ice ages”) and ~280-300 ppm in warmer interglacials [Sigman et al., 2010; Lisiecki & Raymo, 2005]. The drivers of these changes can be explained by the theory by Milutin Milanković, who found that significant changes in solar radiation received by the Earth have occurred during this period as a result of gradual changes in the Earth’s orbit (known as Milankovitch cycles). This in turn affected the uptake and release of CO2 between the atmosphere and oceans (as more CO2 can dissolve in to colder water, so a cooler planet means more CO2 is taken up by the oceans, and vice versa). Thus, natural changes in the temperature during this period probably determined changes in atmospheric CO2 (as well as in CH4) and the greenhouse effect, which acted as an amplifying feedback to those changes. Today though, it is the release of greenhouse gases from humans burning fossil fuels that are warming both the atmosphere and the ocean. Around a quarter of man-made CO2 has been taken up by the ocean so far, acting as a stabilising feedback on warming, but the rate of this uptake will decrease as the oceans get warmer making even more anthropogenic CO2 stay in the atmosphere.

Acknowledgement

Thank you to Emil V. Nilsson for starting the question and Magnus Carlbring for linking it to Researchers’ Desk.

Reference

Armstrong Mckay, David (2015) Investigating the drivers of perturbations to the Cenozoic carbon-climate system. University of Southampton, Ocean & Earth Science, Doctoral Thesis, 185pp. https://eprints.soton.ac.uk/384565/