A very broad stroke picture of developments in the atmosphere over its first few billion years of existence has the carbon dioxide level plummeting maybe as much as 1,000-fold due mostly to weathering. Volcanic activity, material bombardment, tectonic creeping of landmasses, continual subductions, churning magma, chemically reactive oceans, and violent weather raining down corrosive chemicals everywhere was pretty much the picture, although evidence of the accuracy of this picture of the young planet is extremely indirect. The 2007 Intergovernmental Panel on Climate Change published a compilation of atmospheric carbon dioxide levels interpreted as representing changes, the Phanerozoic Eon, which proceeds through the Paleozoic, Mesozoic, and Cenozoic Eras. Chemically reactive, oxygen can be toxic, a characterization that seems inconceivable at first. An interesting aspect of the early Paleozoic Era is the finding of fossil charcoal residues. These indicate that wildfires became possible about 420 Mya when terrestrial plants evolved and atmospheric oxygen surpassed 13%.