At 33.5 Ma, around the time of the Eocene-Oligocene boundary, Earth’s climate took a sudden shift towards cooler conditions, coinciding with the onset of glaciation in the Northern Hemisphere and growth of Antarctic ice cover. Studies of a variety of proxies, including the density of pores or stomata on plant leaves, suggests that the transition resulted from a halving of atmospheric CO₂ content from more than 1000 ppm in the Early Eocene to ~560 ppm in the Oligocene. So, even at twice the pre-industrial level greenhouse warming was compatible with high-latitude frigidity. Ocean-floor sediments from a site close to the Arctic Circle in the Norwegian-Greenland Sea yield pollen and spore records that chart vegetation change from 50 to 30 Ma (Eldrett, J.S et al. 2009. Increased seasonality through the Eocene to Oligocene transition in high northern latitudes. Nature, v. 459, p. 969-973. The proxy data suggest that in the period preceding the decisive global climate change conditions became increasingly seasonal, with greater differences between winter and summer temperatures. This was largely due to increasingly cold winters, a more constant summer temperature suggesting that any land ice on Greenland was of the valley type rather than an all-covering ice sheet.