The last time atmospheric CO2 was at 400 parts per million was during the ancient Pliocene Era, three to five million years ago, and humans didn’t exist.
- Global average temperatures were 3 to 4 degrees C warmer than today (5.4 to 7.2 degrees F).
- Polar temperatures were as much as 10 degrees C warmer than today (18 degrees F).
- The Arctic was ice free.
- Sea level was between five and 40 meters higher (16 to 130 feet) than today.
- Coral reefs suffered mass die-offs.
And much more: As Robert Monroe notes: “The extreme speed at which carbon dioxide concentrations are increasing is unprecedented. An increase of 10 parts per million might have needed 1,000 years or more to come to pass during ancient climate change events. Now the planet is poised to reach the 1,000 ppm level in only 100 years if emissions trajectories remain at their present level.”
Here are some scientific links for those wanting to know more: