Prehistoric Glaciers and Carbon Dioxide

Hello readers, and welcome to the first From Physics to English post in June! Today we’ll be inspecting Northern Hemisphere glaciers and carbon dioxide levels. Much more specifically, we will be looking at these during the Pliocene-Pleistocene Transition (PPT), which happened around 3.2 to 2.5 Ma. “Ma” means ‘million years (ago)’. Another acronym you’ll need for this article is ppm, parts per million: it is the same concept as per cent, but instead of out of one hundred, it is out of one million.
The previously mentioned PPT included a decrease in the levels of atmospheric carbon dioxide (which we are measuring in ppm) and an increase in glaciation in the Northern Hemisphere. This is known from data preserved in the geologic record and the δ18O (read delta-O-18) oxygen isotope ratios found in ice cores (in case you’ve forgotten, an isotope is an atom of an element with a different number of neutrons; in this case the isotopes called, 16O and 18O are involved in the ratio). Most likely, a drop in atmospheric carbon dioxide (the cause of which is a different story with multiple possibilities) created cooler temperatures around the globe, which then gave way to conditions for larger glaciation in the Northern Hemisphere. This is supported by models, which suggest that even a relatively small drop in carbon dioxide would make a noticeable difference. If this is the case, then just how much did the amount of carbon dioxide change?  That is not an easy thing to figure out, as records of carbon dioxide concentration get shaky going back to just 0.8 Ma.
geologic time scale
The geologic time scale. Taken from Portrait of a Planet, page 443.
If you don’t have a record of something, how are you to find out how much it changed and what its effects were likely to have been? Matteo Willeit, Andrey Ganopolski, Reinhard Calov, Alexander Robinson, and Mark Maslin have the answer. They are the authors of today’s featured article, The Role of CO2 Decline for the onset of Northern Hemisphere Glaciation, published in Quaternary Review Papers in 2015. These clever scientists worked backwards towards the answer: using an often used model, they ran simulations of the possible path atmospheric carbon dioxide took those millions of years ago to see the results. Those results were then compared to known δ18O data and sea surface temperature data. This gave a set of most realistic simulations.
So, what did they find? The results showed the Northern Hemisphere gained ice volume in the glacial periods from 3.2 to 2.7 Ma; in between glacial periods, there was not much change. Ice, when at its peaks, partially covered Greenland, Scandinavia, and parts of Canada. After 2.7 Ma, at which there was a definitive intensification in ice volume, ice now covered Greenland, Scandinavia, and Northern America. Greenland specifically went from partial to completely glaciated at this time. Geologically speaking, this was a swift change.
How does this compare to reality? The model misses a sudden cooling event around 2.75 Ma. After about 2.7 Ma, the model captures the variability and cooling the records indicate. In general, it shows the long-term trends well. While imperfect and only modelling the ice sheet in the northern hemisphere, it certainly serves its purpose. Given all the uncertainty, there is, as always in science, more to be done.

 

Bibliography
Marshak, Stephen. Earth: Portrait of a Planet. 3rd ed. W.W. Norton & Company. Print
My geography textbook, used for the image of the geologic timeline and information about oxygen isotope ratios.
Willeit, Matteo, et al. “The role of CO2 decline for the onset of Northern Hemisphere glaciation.” Quaternary Science Reviews 119 (2015): 22-34.
Today’s main article.
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