Most pessimistic scenarios about climate change predicts a global rise of temperature of several Celsius degrees over the course of the 21st century if no action is taken. While such a shift in temperature has many implications such as rising seal levels (due to the melting of ice near the poles) and a mass extinction event, the Earth has recordedly had eras warmer than today in the past, such as the Cretaceous period. For reminders, Cretaceous is the last geological period before the extinction of dinosaurs, and it had notably higher sea levels and a warmer global surface temperature (4 °C above today's average surface temperature).
Assume now that the average surface temperature will rise as predicted by the aforementioned pessimistic scenarios, and that the seal levels will rise drastically.
How would tomorrow's Earth (in this scenario) compare to the Earth of the Cretaceous period ? Would it be in a similar state (besides temperature and sea levels), and perhaps share similar climates around the world, or would various factors I didn't mention (such as the ratio of oxygen in the air) make it very different ?