Our planet’s atmosphere is thought to have changed gradually and over a very wide range of CO2 concentrations throughout history. From ancient atmospheric gases trapped in ice bubbles, we have strong evidence indicating that atmospheric CO2 values reached minimum concentrations of approximately 180 parts per million during the Last Glacial Maximum, which was only 15,000 years ago. At the other extreme, calculations suggest that some 500 million years ago the atmospheric CO2 concentrations may have been about 4000 to 5000 parts per million
