From the link:
Whether they are natives or aliens, increases in plant pollen will cause more allergic reactions in sensitive people, and speaking as one who gets frequent sneeze attacks from pollen this is not good news.
Researchers from the American Academy of Allergy, Asthma & Immunology say in the September issue of the group's Journal of Allergy and Clinical Immunology, that they have decisively linked climate change to "longer pollen seasons, greater exposure and increased disease burden for late summer weeds, such as ragweed."
Scientists have found that increased carbon dioxide has resulted in pollen production increases of 60 percent to 90 percent in some ragweed varieties.
"We are starting to see a trend of increased plant size and the amount of pollen and other leaves and material that they produce," said Dr. Leonard Bielory, director of the Asthma and Allergy Research Center at New Jersey Medical School.