A fundamental aspect of climate change is the potential shifts in flowering phenology and pollen initiation associated with milder winters and warmer seasonal air temperature. 

Here we report that duration of the ragweed (Ambrosia spp.) pollen season has been increasing in recent decades as a function of latitude in North America. Latitudinal effects on increasing season length were associated primarily with a delay in first frost of the fall season and lengthening of the frost free period. 

Overall, these data indicate a significant increase in the length of the ragweed pollen season by as much as 13–27 d at latitudes above ∼44°N since 1995. This is consistent with recent Intergovernmental Panel on Climate Change projections regarding enhanced warming as a function of latitude.

For much of geographic North America, there are three distinct plant-based aero-allergen seasons; tree pollen in the spring; grass pollen in the early summer, and, weed pollen, including ragweed (Ambrosia spp.) in the summer and fall. Pollen from the genus Ambrosia which includes A. artemisiifolia (short or common ragweed), A. trifida (giant ragweed), A. psilostachya (western ragweed), and A. bidentata (lanceleaf ragweed) has long been acknowledged to be a significant cause of allergic disease

Recent warming by latitude associated with increased length of ragweed pollen season in central North America. Lewis Ziska, et al. PNAS | March 8, 2011 | vol. 108 | no. 10

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