Richmond moved down from No. 10 last year but maintains a spot in the Top 15 because of high pollen scores. Climate change (see how these cities will be totally different) is affecting ground level ozone, causing longer ragweed growing seasons. “Global warming has caused changes in our climate by adding more carbon dioxide, which pollen-producing grass and trees thrive on, to the environment and more moisture as well, according to Dr. Borczuk.