CO2 on an Early Earth - no hinderence for life
Atmospheric carbon dioxide greater 1.4 billion years ago
"How much greenhouse gas was there in the ancient atmosphere? A 1993 model by Jim Kasting of Pennsylvania State University estimates that carbon dioxide (CO2) levels in the Earth's early atmosphere must have been 10 times to as much as 10,000 times today's level, in order to compensate for the young (and fainter) sun. Now, a measurement of the fossil record using a new instrument has confirmed a portion of the model. Atmospheric CO2 level 1.4 billion years ago was at least ten to 200 times greater than today, according to the new research"
http://www.eurekalert.org/pub_releases/2003-09/vt-acd091503.php