From NASA Earth Observatory:
“The amount of carbon that is being held in the land surface by trees, and how it has changed over time through disturbance and subsequent regrowth, is the least understood aspect of the global carbon cycle,” said Ralph Dubayah, a remote sensing scientist at the University of Maryland. “Until we know how much carbon there currently is in Earth’s forests, and how that has changed over the last 20 to 30 years, we will have a difficult time predicting how much more carbon forests will absorb in the future, and what role they’ll play in helping to mitigate or accelerate atmospheric carbon dioxide concentrations.”