There is as much carbon dioxide in the atmosphere today as there was 4 million years ago.

Prior to the industrial revolution, CO₂ concentration was 280 parts per million. Since that time, humans have emitted 1.5 trillion tons of carbon dioxide into the atmosphere. In May 2020, its level reached 417 parts per million, in May 2021 - 419, in May 2022 - 421. The current amount of CO₂ is comparable to what was on Earth 4.1-4.5 million years ago. Then forests grew on the site of the Arctic tundra, and the sea level was 5-25 meters higher than it is now. This would be enough to flood most modern coastal cities.