So yes, there will be more ragweed pollen, and somewhat more pollen from all plant species.

FORBES: Global Warmists Allergic To The Truth During Pollen Season

Such traits -- whether of viruses, ragweed pollen or cancer -- are called antigens.

FORBES: In search of antigens

The spring allergy capitals are based on tree pollen counts, while the fall cities are ranked on ragweed pollen counts from September through November of the previous years.

CNN: Worst U.S. cities for allergy sufferers

Although the South suffers from weed and mold spore allergies in the fall, Gerald Kress, an executive with SDI Health, which operates Pollen.com, says the North and Northeast gets hit particularly hard by ragweed -- and 2009 is shaping up to be a banner year for ragweed pollen.

CNN: Southern deluge may help fall allergy sufferers

Dr Epstein offers a novel twist: he thinks that rising carbon dioxide concentrations may be promoting the growth of ragweed, pollen and fungus.

ECONOMIST: Climate change

But in the past researchers have focused on allergens in the air, Silverberg said, such as pollen or ragweed, instead of other environmental exposures.

CNN: Where do allergies come from?

Warmer temperatures, we are told, mean longer growing seasons and longer pollen seasons for nuisance plants such as ragweed.

FORBES: Global Warmists Allergic To The Truth During Pollen Season

Trees start scattering their pollen in early spring, followed by grasses, and then ragweed kicks in during the summer and runs through fall.

CNN: Predicting allergy season eludes experts