Ragweed allergy season in North America has grown two to four weeks longer in recent years because of warmer temperatures and later fall frosts, researchers said.

Northern parts of the United States and Canada have seen the most dramatic rise in allergy season length between 1995 and 2009, said the study to be published in Tuesday's edition of the Proceedings of the National Academy of Sciences.

The city of Saskatoon in Saskatchewan, Canada saw the longest pollen season, adding 27 more days in 2009 compared to 1995. Winnipeg, Manitoba saw a 25-day increase during the same period.

Fargo, North Dakota and Minneapolis, Minnesota each saw allergy seasons extend 16 days. But looking further south, Rogers, Arkansas and Georgetown, Texas saw decreases of several says in their pollen seasons.

The study said the starker changes in the northern latitudes were consistent with the United Nations' Intergovernmental Panel on Climate Change projections of more intense warming in areas closer to the Arctic.

"Latitudinal effects on increasing season length were associated primarily with a delay in first frost of the fall season and lengthening of the frost-free period," the study said.

"Overall, these data indicate a significant increase in the length of the ragweed pollen season by as much as 13-27 days at latitudes above 44 degrees north since 1995."

Scientists used pollen measurements from the US National Allergy bureau and Canada's Aerobiology Research Laboratories, combined with data from US weather stations, Environment Canada and the Canadian National Climate Data and Information Archive.

Ragweed allergies, often called hayfever, strike as many as 30 percent of Americans, typically in warmer seasons. Symptoms range from sneezing and sniffling to severe asthma.

The culprit is a family of plants belonging to the genus Ambrosia, whose flowers send off tiny grains of pollen that the body recognizes as a threat.