The articles I have read agree that the earth has been warmer in the past and had higher carbon levels.  That doesn't mean that global warming (which changes the climate all over the world in different ways, not all places will see an increase in temperature) is not being caused by man right now.  Also, even if it has happened in the geological past, it doesn't mean that causing it to happen right now is a good idea for humanity or the species that inhabit earth now.