[dhs12345]

I read closer to 1000 to 1. Basically a drop in the bucket.

[palmer]

Not really, the human contribution is about 9 Gt of carbon per year. About 10% of that is shrinking forests so that might a fudged number. But the rest is from fossil fuel use and cement making which are well measured.

For the natural contributions the estimates are more difficult. The ocean cycle is about 50 Gt per year in both directions, see www.opl.ucsb.edu/tommy/pubs/hawaii01/bishop.pdf (A petagram is the same a a gigatonne) Vegetation takes up 150-175 Gt of carbon per year mostly in the northern hemisphere spring. They release most of that in the fall. See http://wattsupwiththat.com/2011/09/29/plants-gobbling-up-co2-45-more-than-thought/ The total ratio (ocean plus land vegetation) is about 30 to 1, but it's very rough.