Do we have any evidence of global warming in the past?
Yes, there is a great deal of evidence for global warming events in the past. The following lists two of the more well-known ones:
Mid-Cretaceous Period—During this period (between about 120 and 90 million years ago), new ocean crust was produced at about twice the normal rate. Large volcanic plateaus were forming in the ocean basins, ocean temperatures were very high, and there was a peak in worldwide petroleum formation. Just as startling was the sea level, which was about 330 to 660 feet (100 to 200 meters) higher than at present. The reasons for the high temperatures were probably numerous, including the release of carbon dioxide (a greenhouse gas) by volcanic eruptions, creating a “supergreenhouse” effect. This led to temperatures about 20 to 22°F (10 to 12°C) above our current average global temperatures. Interestingly enough, it is thought that the large volume of basalts that erupted on the ocean floor displaced a great deal of ocean water, causing sea levels to rise. And with the rise in sea level and temperatures, organisms flourished, eventually providing material necessary for petroleum formation.
Eocene Period—During this time (between about 55 to 38 million years ago), temperatures also increased, with tropical vegetation reaching about 45 to 55 degrees north and south of the equator, or about 15 degrees higher than today. Based on rock samples, it appears that the Earth had between 2 and 6 times the amount of carbon dioxide we have today. Scientists believe this global warming was caused by continental collisions, events that released large amounts of this greenhouse gas into the atmosphere. (This also shows how the rock cycle and tectonic processes can affect atmospheric conditions.)