Researchers continue to narrow the range of uncertainties in the global carbon budget, but some aspects are yet to be resolved, particularly the global distribution of how carbon from the atmosphere is taken up by plants and the ocean. Sarmiento and Gruber are noted researchers in this field, and their update in Physics Today provides a status report of this ongoing research as of 2002.
They point out that most feedbacks between the global carbon cycle and global warming seem to be positive, which means that warming reduces the capability of natural sinks of carbon to sequester more carbon. The "business as ususal" approach to emissions from burning fossil fuels will produce atmospheric carbon levels far too large to be stabilized even by massive reforestation projects coupled with more speculative projects such as fertilizing polar oceans with iron. The only currently available method to substantially reduce the rise in atmospheric greenhouse gases is to reduce emissions.
Reference
Sarmiento, J. L., and N. Gruber, 2002: Sinks for anthropogenic carbon. Physics Today, August 2002, 30-36.