Myth: CO2 levels have been much higher in the past

We’ve had higher levels of CO₂ in the past, and life has survived quite successfully. So, there is no need to worry now. 

Fact:

It’s true that CO₂ levels were much higher hundreds of millions of years ago, but the amount of solar energy reaching the Earth back then was also significantly less (possibly only 70 percent of what it is now). This is because stars like our sun get hotter as they age and emit more radiation. Another reason is because at that time the continents were joined together near the equator and were reflecting more solar energy back into space. These two factors cancelled out much of the impact of the CO₂ level. If CO₂ were to return to the level of the distant past, the result would be catastrophic, as so much more solar energy is reaching the planet now.

It should also be noted that the last time CO₂ levels were as high as they are now (427 parts per million) was during the Pliocene period, around about 3 million years ago. At that time, sea levels were 10 metres higher than now, humans had not yet evolved and we certainly didn’t need to feed 8 billion people.