Sorry, I should of said methane, not overall greenhouse emissions. But according to a quite reliable source (
http://www.maf.govt.nz/mafnet/rural-...eengas-14.htm), humans contribute, on a global scale, around 0.59% of total methane output, and cows contribute 1.8%, making my figures overestimations. Methane is only 23% as strong as C02 in causing global warming though, so lets consider this chart (
http://en.wikipedia.org/wiki/Image:P...n_Dioxide.png). According to this chart, which is based on the GEOCARB, COPSE, and Rothmann models, under the GEOCARB and COPSE models, C02 550 million years ago was significantly higher than today. But it was around this period of time that life flourished, and the Cambrian explosion occured. So, C02 could actually be an extremely good thing for Terra's biosphere. Even the not so optimistic Rothmann model does not show a significant increase in C02 during the Holocene era.