Studies and analysis of samples taken in the core of polar ice layers show that over the last 800,000 years the level of concentration of atmospheric CO2 has ranged between highs (of about 300 parts per million (ppm)) and lows (of about 170 ppm)! The highs of 300 ppm were reached on several occasions – 130,000 thousand years ago, 240,000 years ago and 330,000 years ago.
