What was done
Based on estimates of the magnitude and spatial distribution of fossil fuel CO2 emissions, observations of spatial patterns of atmospheric CO2 concentration, two different estimates of the sea-air flux of CO2, and the results of two atmospheric transport models, the authors calculated the net surface exchange of CO2 over three large regions of the globe.

What was learned
The best constrained region of the planet for which the results of the study are considered to be fairly robust was North America north of 15°N latitude.  For this portion of the globe, it was calculated that, between 1988 and 1992, there was a net terrestrial uptake of CO2 of magnitude 1.7 ± 0.5 petagrams of carbon per year.  Most of this uptake (70 to 100%) was estimated to occur in the broadleaf-forest region of the continent located between 15 and 51°N latitude.

What it means
If correct, the implications of this paper are profound, for the calculated CO2 sink strength of North America would be sufficient to yearly remove from the atmosphere all of the CO2 annually released to it by fossil fuel consumption in both the United States and Canada; and it would therefore call into question any claim that either the United States or Canada is contributing to the ongoing rise in the air's CO2 content.  In addition, the authors suggest that the historical rise in the atmosphere's CO2 concentration, via its aerial fertilization effect, may be one of the major causes of this phenomenon.