When did the atmosphere have the most carbon dioxide?

Answer 1

There is geologic evidence for high atmospheric concentrations of CO2 in the distant past--in the Paleozoic. Organisms pulled billions of tons of CO2 out of the atmosphere over millions of years to form the great Permian coal and oil deposits we are mining now. Limestone and formations also represent billions of tons of CO2 being sequestered by small hard shelled marine organisms.

Fifty million years ago a freshwater fern known as azolla grew in Arctic regions. When it died it sank to the sea floor, where it did not rot in the cold water. Over a few hundred thousand years this fern pulled billions of tons of CO2 out of the atmosphere which cooled the earth down enough to cause an ice age. A series of ice ages occurred after that.

So CO2 levels have risen and fallen, but they have not exceeded 280 ppm in the past 300,000 years, nor 300 ppm in the past 20 million years. However, between 1700 and 1900 CO2 shot up from 280 to 290 ppm--incredibly fast. From 1900 to 1950 CO2 rose again to 300 ppm. Today it is close to 400 ppm, and we will pass 500 ppm before 2050.

So over the past 20 million years, last year was the year the atmosphere had the most CO2, which we have been able to say for every year of the 20th and 21st centuries. We will be able to say the same thing next year about this year.

Answer 2

The atmosphere had the most carbon dioxide during the Paleozoic era, around 300-400 million years ago. This period was characterized by high levels of carbon dioxide due to volcanic activity and the absence of significant land-based vegetation to absorb it.