How have carbon dioxide levels changed over earth's history?

Answer 1

Billions of years ago we surmise CO2 levels were fairly high. With the advent of marine photosynthetic organisms, the cyanobacteria (aka blue-green algae), atmospheric carbon dioxide was broken apart and oxygen released. Atmospheric oxygen levels gradually rose over the course of a billion years, achieving their highest peaks during the carboniferous.

In the past billion years CO2 appears to have reached its highest level during the cambrian, 500 to 600 million years ago, topping out at 7 to 8 thousand parts per million (ppm). This translates to 3/4 of 1%, or 0.75%. Although the sun was cooler, earth was very warm back then. CO2 declined to the present day value between 330 and 270 million years ago, then rose back up to 2000 ppm at the beginning of the Triassic, to nearly 3000 ppm in the middle of the Jurassic. By the end of the cretaceous it had once again declined to a value close to the present day value.

The rise and fall is associated with ice ages, which appear to be closely correlated to orbital cycles. For the past 20 million years CO2 never rose above 280 ppm, and for the past 800,000 years the range has varied from only 250 ppm to 280 ppm, changing up or down about 5 ppm per thousand years.

Beginning in 1700 CO2 was 280 ppm, and in a very brief 200 year period rose clear up to 290 ppm. By 1950 it had risen another 10 ppm, to 300 ppm. Today we are at nearly 400 ppm (2013), and will pass 500 ppm before 2050 at the current rate of increase.

Note the rate of change, 100 ppm/50 years is 2 ppm per year, contrasted with preindustrial rates of 5 ppm/1000 years, or 0.005. 2/0.005 = 400, indicating CO2 is climbing 400 times faster than it ever has before. This rate of increase is consistent with the human consumption of fossil fuels, which emits 20 billion tons of the gas per year. Another 10 billion tons of CO2 comes from cement manufacture, deforestation, and other activities.