a really good resource on the subject.
... and a link
to a recent article about how CO2 and methane levels are higher than they have been for 650,000 years.
For the doubters - that doesn't mean they were higher than now 650,001 years ago, but this is how long we have definite evidence. It could be a longer time than this, but scientists being cautious are only stating what they know rather than extrapolating.