text: Atmospheric  concentration continued to fall after about 60 million years ago. About 34 million years ago, the time of the Eocene–Oligocene extinction event and when the Antarctic ice sheet started to take its current form,  was about 760 ppm, and there is geochemical evidence that concentrations were less than 300 ppm by about 20 million years ago. Decreasing  concentration, with a tipping point of 600 ppm, was the primary agent forcing Antarctic glaciation. Low  concentrations may have been the stimulus that favored the evolution of C4 carbon fixation| plants, which increased greatly in abundance between 7 and 5 million years ago.