Sea levels were much higher millions of years ago

Myth:

3 million years ago, sea levels were 10 metres higher than they are today, yet CO₂ levels were roughly the same as now. So, why aren’t sea levels 10 metres higher in the 21^st century? This is proof that they are unrelated to CO₂. 

Fact:

Sea levels have a long lag time following temperature increases. It may take hundreds of years before they reach their peak, but the evidence suggests that this is the direction they are headed. At current rates of increase, scientists expect a rise of up to one metre by the end of the century, with more coming after that. 

Changes in global sea levels, going back 542 million years. The present is at the extreme left on the horizontal axis. [Source: Wikimedia Commons]

Sources:

K.G. Miller et al, “The Phanerozoic record of global sea-level change,” Science, no. 310, 2005.

C. Murray-Wallace and C. Woodroffe, Quaternary Sea-level Changes, Cambridge University Press, 2014

Jonathan Nott, How climate and sea level have changed over the long-term past, National Climate Change Adaptation Research Facility, Gold Coast, 2016. https://nccarf.edu.au/wp-content/uploads/2019/04/How-climate-and-sea-level-have-changed-over-the-long-term-past-_-CoastAdapt.pdf

Fiona Hibbert, Eelco Rohling and Katharine Grant, “Scientists looked at sea levels 125,000 years in the past. The results are terrifying,” The Conversation, 7/11/19

“Paleoclimate,” IPCC AR4 WG1, 2007. https://www.ipcc.ch/site/assets/uploads/2018/02/ar4-wg1-chapter6-1.pdf