What was done
The authors collected seeds from the common ragweed plant (Ambrosia artemisiifolia L.) and vernalized them by sowing them in containers kept in a refrigerator maintained at 4°C, after which they transferred one third of the seeded containers at 15-day intervals to glasshouse modules maintained at atmospheric CO2 concentrations of either 380 or 700 ppm, where they were allowed to germinate (also at 15-day intervals, with the middle germination date approximating that of plants currently growing naturally in the vicinity of Boston, Massachusetts, USA), and where they remained under well-watered and fertilized conditions until they senesced and were harvested, at which time assessments of plant and allergenic pollen biomass were made.

What was learned
As best we can determine from the graphical representations of Rogers et al.'s data, the end-of-season CO2-induced increase in aboveground plant biomass was about 16% for the date of emergence typical of the present, while the corresponding increase in pollen production was about 32%. However, for the 15-day earlier date of emergence, which was chosen to represent "anticipated advances of [biological] spring several decades into the future," based upon projected rates of future global warming, the end-of-season CO2-induced change in aboveground plant biomass was only about +3%, while the end-of-season CO2-induced change in pollen production was actually a negative 3%. The most meaningful way of viewing the results, therefore, is to determine the change in pollen production that would occur in going from today's atmospheric CO2 concentration and date-of-onset of biological spring (380 ppm, middle date of germination) to the elevated CO2 concentration and earlier date-of-onset of biological spring explored by Rogers et al. (700 ppm, 15-day earlier date of germination); and when this is done, the production of allergenic pollen is seen to rise by a less-than-whopping 1-2%, which is obviously totally insignificant.

What it means
Based on the results of this study, we can all breathe easier (pun intended), knowing that the individual consequences of (1) the aerial fertilization effect (caused by the ongoing rise in the air's CO2 concentration) and (2) the earlier occurrence of biological spring (caused by concomitant global warming) essentially cancel each other out, leaving season-long allergenic ragweed pollen production about the same as it is today.