Globigerinoides ruber | Image by H.J. Spero

image source  Smithsonian national museum of natural history


This colourful foraminifera, a Globigerinoides ruber (G. ruber), was found about 3 meters below the ocean surface near the coast of Puerto Rico.  The dark shell at the centre is surrounded by symbiotic algae (the tiny yellow dots) and spines that will fall off when it dies, leaving the shell alone to be preserved in the fossil record.  In fact, some ancient ancestors of this G. ruber enabled scientists to double the CO2 proxy record beyond the current 1 million-year age limit for ice cores.

Ice core records provide a high-resolution proxy record of past atmospheres and climates.  Scientists combine many types of proxy records to reconstruct environmental conditions from thousands to many millions of years ago.  Other examples of proxy evidence include sediments in lakes and oceans, and micro fossils.  Multiple records are integrated with observations of the contemporary earth sysem to reconstruct past atmospheres and climates.  This scientific work expands our understanding of how the earth system works and it helps predict future climates.

Of significance to the composition of past atmospheres, Bärbel Hönisch and other scientists (2009) analyzed a sediment core taken from the bottom of the Atlantic ocean west of Sierra Leone.  From the ancient shells of G. ruber in the sediment core, they developed a high-resolution record of oxygen isotopes (δ18O).  This provided evidene showing, among other things, that atmospheric CO2 levels fluctuated between 213 ppm (+30/-28) and 283 ppm (+30/-32) during the period from 900,000 years ago to 2.1 million years ago.  This is similar to the range of 172 to 300 ppm established by the ice core record.

Hönisch and her co-investigators did not have air bubbles that were older than 800,000 years.  The relied on less direct, geological evidence, to enhance and extend the resolution of knowledge about oceanic and atmospheric CO2 levels by more than a milllion years into 'deep time.'

As for climate states between 2.6 and 5.3 million years ago, during the Pliocene Epoch, atmospheric CO2 levels appear comparable to today, and models suggest that global temperatures were 3°C to 4°C warmer than pre-industrial climates (Yhang et al., 2014).  In 2020, a study centred on CO2 in vascular plants from terrestrial sediments found that CO2 has remained below present-day levels for at least the past 7 million years, and potentially for the past 23 million years (Cui et al., 2020).

Scientists have studied the history of this planet since its formation 4.54 billion years ago.  Evidence suggests that it has likely been millions of years since atmospheric CO2 and global temperatures were higher than they are today (Hönisch et al., 2009; Yhang et al., 2013; Zhang et al, 2014).  As we learn about the past, the facts of warmer climates so long ago has less immediate relevance for our species that emerged just 200,000 years ago.  It has less relevance for the stable conditions in which civilization developed over the past 12,000 years.


"If humanity wishes to preserve a planet similar to that on which civilization developed and to which life on Earth is adapted, paleoclimate evidence and ongoing climate change suggest that CO2 will need to be reduced from its current 385 ppm to at most 350 ppm."

~ James Hansen et al. (2008)




Science Daily Jun 2020 | Today's atmospheric CO2 levels greater than 23 million-year record 

Geology 2020 | A 23 m.y. record of low atmospheric CO2

Science Daily 2009  [Release] CO2 higher today than last 2.1 million years

Earth Institute Columbia U. 2009  CO2 higher today than last 2.1 million years

National Geographic 2009  CO2 levels highest in two million years

AGW Observer  Papers on atmospheric CO2 from proxies [until 2011]

USGS 2007  Divisions of geologic time



Cui, Y., Schubert, B.A., Jahren, A.H. (2020). A 23 m.y. record of low atmospheric CO2. Geology.  doi: [pdf]

 Hansen, J., Sato, M., Kharecha, P., Beerling, D., Berner, R., Masson-Delmotte, V., . . . Zachos, J. C. (2008). Target atmospheric CO2: Where should humanity aim? [arXiv:0804.1126,]. Open Science Atmospheric Journal, 2, 217-231. doi:10.2174/1874282300802010217 [source + .pdf]

Hönisch, B., Hemming, N. G., Archer, D., Siddall, M., & McManus, J. F. (2009). Atmospheric carbon dioxide concentration across the Mid-Pleistocene Transition. Science, 324(5934), 1551-1554. doi:10.1126/science.1171477 [source + researchgate .pdf]

Zhang, Y. G., Pagani, M., Liu, Z., Bohaty, S. M., & DeConto, R. (2013). A 40-million-year history of atmospheric CO2. Philosophical Transactions of the Royal Society of London A: Mathematical, Physical and Engineering Sciences, 371(2001), 1-20. doi:10.1098/rsta.2013.0096 [source + .pdf]

Zhang, Y. G., Pagani, M., & Liu, Z. (2014). A 12-million-year temperature history of the tropical Pacific Ocean. Science, 344(6179), 84-87. doi:10.1126/science.1246172  [source + columbia u .pdf]