Pollen and climate change. Birch and grasses in metropolitan France

There is now evidence that the climate is changing. This trend has the potential to affect human health. In particular, global warming is expected to alter pollination and increase the incidence of allergic airway diseases. The present study was conducted at 6 or 7 sampling sites in metropolitan France with the aim of detecting possible shifts over the last 24 to 32 years in the timing, duration, and intensity of the pollen seasons of two very allergenic taxa, birch and grasses. The most convincing results were obtained for Betula. On the one hand, the beginning of the birch pollen season advanced by 0.02 to 0.69 days/year until about 2002, after which this trend reversed, in such a way that the pollination period now starts as late as it did thirty years ago, if not later; the most likely explanation is that milder winters delay dormancy release and nullify the advance of spring phenology induced by warmer spring weather. On the other hand, all the sites studied showed major increases in the quantity of pollen released, which is positively correlated with the temperature of the twelve months from July (year N-1) to June (year N). Although the results for Poaceae are often inconsistent from one monitoring station to another, they nevertheless revealed almost universal downward trends in airborne pollen concentrations, as confirmed by negative correlation coefficients with the annual temperature. In conclusion, the most salient finding concerns the wide discrepancy between the effects on the pollen seasons already observed in France and the mainstream narrative concerning pollen response to climate change. (C) 2019 Elsevier Masson SAS. All rights reserved.