Global climate change is expected to increase pollen allergies world-wide in two ways. (1) By extending the growing season in temperate climates, it provides plants with more time to produce pollen. (2) More CO2 in the atmosphere means bigger plants (remember the basic photosynthesis equation? CO2 + Water (in the presence of sunlight and chlorophyll) = sugars + O2. Just think of sugars as more plant mass, and you get the picture. Of course, it isn't exactly that simple but then, nothing in ecology is.
If you are interested, you can read this blog post by the Union of Concerned Scientists.
and here is a link to primary research about the expand in pollen season for Ragweed in NA.
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