What was done
The authors used a complex global vegetation model to predict how increasing atmospheric CO2 concentrations and global change parameters, including air temperature and nitrogen deposition, might affect vegetative composition and carbon sequestration in biomass and soils located north of 50°N latitude for the years 1860 to 2100.

What was learned
Model results predicted that forests (coniferous and temperature/mixed forests) would expand their areas, both north and south, by about 50%, mainly at the expense of tundra.  This expansion resulted from the direct effects of rising CO2 levels and warming on tree photosynthesis and growth, along with the indirect effects of increased nitrogen deposition and improved water relations.

Model results also suggested that the land area north of 50°N latitude is currently accumulating about 0.4 Pg of carbon per year.  In addition, it was predicted this degree of carbon sequestration could possibly increase to 1.0 Pg of carbon per year by 2050 and persist at that level through the year 2100.  In determining the primary cause of this growing carbon sink, the authors stated that "this sink was due mainly to an increase in forest productivity and biomass in response to increasing atmospheric CO2, temperature, and nitrogen deposition."  However, it was shown that this sink was dependent upon an increase in forest net primary productivity, which only occurred when the model was run with an increasing atmospheric CO2 concentration.  Indeed, when the model was run with a constant atmospheric CO2 concentration of 280 ppm, forest net primary productivity declined and forests actually became carbon sources, rather than carbon sinks.

What it means
According to this model, as the CO2 content of the air continues to rise and air temperatures become slightly warmer, regardless of the cause, forests in the Northern Hemisphere should significantly expand their areas and contribute to a growing carbon sink in this region.  In fact, the strength of this carbon sink, which increases with rising CO2 concentrations, is already one-third of that required to balance the global carbon cycle.  Thus, if these results can be extrapolated to forested regions of the Southern Hemisphere and elsewhere, it is conceivable that earth's forests may actually be able to sequester all anthropogenic carbon emissions of the future.