Ragweed, which flourishes along roadsides and in disturbed habitats throughout North America, produces one of the most common allergens. A study by Harvard researchers found that ragweed grown in an atmosphere with double the current carbon dioxide levels produced 61 percent more pollen than normal. Such a doubling of atmospheric carbon dioxide is expected to occur between 2050 and 2100. “The side effects of carbon dioxide, as well as its impact on heat budget and the water cycle, have to be taken very seriously,” said Paul Epstein, Harvard Medical School instructor in medicine and associate director of the Center for Health and the Global Environment at HMS. “I believe this study can help us understand the true costs of burning fossil fuels.” The report appeared in the March 2002 Annals of Allergy, Asthma, and Immunology. Fakhri Bazzaz, Mallinckrodt professor of biology at Harvard University, and Peter Wayne, formerly with Harvard University’s Department of Organismic and Evolutionary Biology, and Harvard College student Susannah Foster were coauthors. The study was funded by grants from the National Science Foundation and the Forbairt International Collaboration Programme.