There is evidence for high concentrations between 200 and 150 million years ago of over 3,000 ppm, and between 600 and 400 million years ago of over 6,000 ppm. In more recent times, 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 plants, which increased greatly in abundance between 7 and 5 million years ago. Based on an analysis of fossil leaves, Wagner et al. argued that atmospheric concentrations during the last 7,000–10,000 year period were significantly higher than 300 ppm and contained substantial variations that may be correlated to climate variations. Others have disputed such claims, suggesting they are more likely to reflect calibration problems than actual changes in . Relevant to this dispute is the observation that Greenland ice cores often report higher and more variable values than similar measurements in Antarctica. However, the groups responsible for such measurements (e.g. H.J. Smith et al.) believe the variations in Greenland cores result from "in situ" decomposition of calcium carbonate dust found in the ice. When dust concentrations in Greenland cores are low, as they nearly always are in Antarctic cores, the researchers report good agreement between measurements of Antarctic and Greenland concentrations.