Why were levels of CO2 much higher in the atmosphere millions of years ago?

Carbon dioxide levels today are higher than at any point in at least the past 800,000 years. Global atmospheric carbon dioxide concentrations (CO2) in parts per million (ppm) for the past 800,000 years. ... Carbon dioxide concentrations are rising mostly because of the fossil fuels that people are burning for energy.

Explanation:

a polymer is a chainlike molecule made up of small molecular units. these small molecular units are called monomers. monomers of polymer can be joined together either naturally or synthetically.  

1. a tropical dry forest carries out more rapid nutrient cycling