What was done
The authors carried out a set of calculations that indicate that, in the absence of a feedback system that produces a rough match between planetary degassing of CO2 and the chemical weathering of silicate minerals on geologic timescales, it is to be expected that "drifts in atmospheric CO2 to such low values that plant growth would become impossible or to such high values that animal life would be eliminated would likely have occurred."  Since neither of these events has ever happened, however, they conclude that such a feedback system must indeed exist.

The authors then proceed to identify this feedback system, suggesting that it is strongly linked to the greenhouse effect, such that if atmospheric CO2 rises too high, global warming occurs together with an intensification of the planet's hydrologic cycle, both of which phenomena speed up the chemical weathering reactions that remove CO2 from the atmosphere, and vice versa.  They note, however, that this concept, which dates back to the early 1980s, has been criticized for several reasons, not the least of which are extensive data sets that suggest that temperature and real-world mineral weathering rates may not be as tightly linked as this hypothesis requires.

Finally, although they continue to adhere to the CO2-climate connection as the most likely mode of operation of the required feedback phenomenon, they describe another candidate mechanism in vivid detail: "CO2 is a key ingredient for plant growth.  The more CO2 in the atmosphere, the less water plants must spend per unit of growth.  Further, in order to be utilized, the enhanced growth capability offered by higher atmospheric CO2 contents must be matched by increased recovery from soils of the other required constituents (i.e., P, K, S,...).  To attain these constituents, plants divert energy to the nourishment of fungi living symbiotically on the tips of their rootlets.  It is possible that these organisms are the dominant agents for the chemical weathering of soils.  Hence, even in the absence of the climatic impacts of extra atmospheric CO2 (our italics), there will very likely be a direct impact.  Given more CO2, plants will intensify their efforts to break down mineral matter in the soil."

What was learned
We learn from this study that the earth and its biosphere interact in a very complex way to maintain the environment of the planet's surface within a range of physical and chemical characteristics that make it suitable for the continued existence of life in the face of certain perturbations that might otherwise tend to make the world uninhabitable.  We learn further that the direct climatic effect of atmospheric CO2 need not be as great as has been previously believed for it to function appropriately within this context.

What it means
One consequence of the insights gained from this study is that there is new reason to believe that we may not yet fully comprehend the ultimate climatic consequences of an increase in the air's CO2 content.  The evidence presented herein, for example, suggests that the greenhouse effect of CO2 may be considerably less than what is currently being predicted for earth's future, as it need not be as great as has previously been believed to explain certain features of earth's past.