The June 2020 carbon dioxide (CO2) concentration of 416 parts per million, is a level not believed seen since the mid-Pliocene warm period approximately 3 million years ago. The Pliocene period (5.33–2.58 million years ago) was studied using stratigraphic record to understand Earth’s history.
Geological proxies, like fossilized flora and fauna, indicate that temperatures during the Pliocene were about 3°F. to 5°F. (2.7°C. to 4.0°C.) hotter than those in the recent preindustrial era. Coastal rocks and sediments can also help identify the Pliocene’s elevated sea levels, from a time when polar ice extents were much smaller than today.
Hearty et al. 2020 studied the stratigraphic record to determine how high the ocean rose the last time CO2 levels exceeded 400 ppm. They studied ancient coastal deposits along the western and southern coastlines of the Republic of South Africa.
That coast was stable during the Pliocene, and later Pleistocene period. Elevations measured in the field must be corrected to account for past vertical land motion. The authors found 17 sites where geologic records of past sea levels have been preserved, and marine shells collected from those deposits were analyzed using strontium dating. They used high-accuracy GPS data to measure the 3D coordinates of key geological features to within a few centimeters.
Some of the 17 surveyed sites provided reliable age data, establishing that average sea levels during the Pliocene were between 15 and 30 meters higher than the present waterline. There were two prominent topographic zones, one about 25–35 meters above modern sea level , from about 4.5 million to 3 million years ago. The other zone was about 15–20 meters above modern sea level, from about about 1 million years ago, during the Pleistocene.
The researchers also point out that their data set can serve as a baseline against which to compare future studies of global mean sea level.
Paleoceanography and Paleoclimatology,
https://doi.org/10.1029/2019PA003835
https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2019PA003835