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All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

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Climate and CO₂ levels have always varied together. During past ice ages CO₂ levels were low, and during warm periods CO₂ was higher.

Climate and CO₂ levels have always varied together. During ice ages CO₂ levels were low, and during warm periods CO₂ was higher. In the Eocene (56-34 million years ago) there were no polar ice caps, temperatures were about 10ºC hotter than the 20^th Century, and CO₂ was about 1,500ppm (Westerhold et al. 2020, Rae et al. 2021). During the last Ice Age, CO₂ varied between about 180 and 300ppm as ice sheets waxed and waned with orbital wobbles (Rae et al. 2021). CO₂ was also about that level during the Paleozoic Ice Age, 340-290 million years ago (Foster et al. 2017).

Early attempts to estimate CO₂ for that long ago in Earth’s past were broad-brush and very uncertain (eg Royer 2006), leading to the high CO₂ estimates referred to in the myth. New data and refined techniques have since clarified the picture considerably. The 2006 estimates, for example, averaged data across 10-million-year timesteps, the 2017 data in the figure below used 0.5-million-year timesteps, and newer compilations don’t average across timesteps. At the same time, CO₂ and temperature uncertainties have reduced considerably so that climates from the geological past are now a useful reality check for climate models (Tierney et al. 2020, IPCC 2021, see the intermediate version for more detail).

Data for the Ordovician are still quite uncertain, but they indicate CO₂ was about 2,400ppm and falling before the end-Ordovician glaciation (Pancost et al. 2013). Glaciation at higher CO₂ levels than today was possible at that time for a variety of reasons including a less-bright Sun back then (see the intermediate version). The Jurassic and Cretaceous span 134 million years with several hothouse episodes and several cooler episodes, with CO₂ varying from about 600ppm to about 1500ppm accordingly (Witkowski et al. 2018), but there was no glaciation in that time.

Earth’s long-term climate (over millions of years) is governed by the balance between CO₂ emitted into the atmosphere by volcanoes and CO₂ removed from the atmosphere by weathering of rocks (Joel 2017). This has prevented runaway climates and kept Earth’s climate generally habitable for about 4 billion years, but it can be outpaced by abrupt greenhouse gas releases (e.g. at the end-Permian mass extinction), or removals (e.g. “Snowball Earth” periods).

CO₂ levels for the last 420 million years, showing periods with ice ages. Note this curve is smoothed and too low resolution to show spikes in CO₂, eg at the end-Permian, end-Cretaceous, PETM, etc. Data from Foster et al. nature communications 2017. Late Paleozoic Ice Age per Rolland et al. EPSL 2019. Preindustrial CO₂ 278 ppm, 2021 CO₂ 420ppm (CO2.Earth). Newer data zooming in on the last 66 million years can be found on the intermediate tab.

Updated on 2021-09-01 by howardlee.

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