We have good evidence going back 800,000 years, from ice cores and other sources, that the global temperature for that period has remained with about a 1°C range and CO2 concentrations between 180 and 300ppm. But farther back, in the age of the dinosaurs there is evidence that the temperatures were quite a bit warmer and CO2 could have been as high as 1000ppm.
Several things become apparent. This level of CO2, long before the industrial revolution, can scarcely be blamed on the human species, and is considered to be a result of heavy volcanic and other geological activity. This warming, however, was very slow in coming - over tens of thousands, maybe millions ofyears, not the rapid warming we are seeing today. Plants and animals survived and did well because they had the ability to evolve and adapt to slowly changing conditions.
At these temperatures, there were no ice caps. Sea levels during portions of the dinosaur period was more than 200 feet higher than today which would drown all coastal areas and low lying areas including Florida and Texas – anything currently at less than 200 feet above sealevel.
Just because we are pretty sure that some types of life survived, and did well, that warm period is of little comfort to us and modern civilization.
The only rapid warming like we are seeing right now, that we know of, is called the “Permian Mass Extinction,” 251.9 million years ago. This was triggered by a rare event called a “Large Igneous Province,” massive volcanic eruptions in Siberia that emitted huge amounts of CO2 and methane at speeds similar to what humans are emitting now.
This compares to the speed of warming we are currently seeing. Just because it was natural doesn’t mean it was benevolent; it caused extinction of around 90% of all species, quite a bit worse than the event that wiped out the dinosaurs. The extinctions of ecosystems continued for about 61,000 years and it took over 10 million years for the planet to recover.
Imitating this kind of natural occurrence with anthropogenic dumping of massive amounts of GHGs in the atmosphere is not in our best interests. Our current world wide warming may seem slow to a human, but in geological time this is the climate equivalent of the Daytona 500.