Bad news for—achoo!—those who sniffle, er suffer their way through ragweed—sniff, snort, itch—season: A team of researchers has found that increased warming, particularly in the northern half of North America, has added weeks to the fall pollen season.

It's enough to make you grab a tissue: Minneapolis has tacked 16 days to the ragweed pollen season since 1995; LaCrosse, Wisc. has added 13 days, Winnipeg and Saskatoon in Canada have added 25 and 27 days, respectively.

The new research, published Monday in the journal Proceedings of the National Academy of Sciences, finds the longer pollen seasons correlate with the disproportionate warming happening around the planet and attributed to greenhouse gas emissions.

Upper latitudes are warming faster than mid-latitudes, and the pollen season is lengthening in proportion. Scientists and health officials found no appreciable warming in Texas, Arkansas or Oklahoma.

"It's not just theoretical," said Lewis Ziska, the study's lead author and a plant physiologist with the U.S. Department of Agriculture's crop system and global change laboratory. "We are seeing a signal based on what in fact the [U.N. Intergovernmental Panel on Climate Change] is predicting."

The impact goes far beyond mere sniffles and inconvenience. Some 50 million Americans have allergies, according to the Asthma and Allergy Foundation of America. Of those, 35 million suffer nasal allergies, known broadly as hay fever, said Mike Tringale, the association's vice president.

For 75 percent of those 35 million, ragweed is the primary allergen, he added.

And in many cases, allergies can trigger a bout of asthma, or make it worse.

Dr. Nancy Ott, a physician with Southdale Pediatrics in Edina, Minn., has seen "a lot more desperate calls" over the past four to six years. "These longer seasons can be a problem" particularly for those with asthma, she said. "I try to get patients in early, make sure they have a red 'X' on Feb. 28 or whenever the pollen season starts."

The findings correlate with analysis last year by the National Wildlife Federation that found ragweed growth rates and pollen counts increased with global warming. In one study, accelerating spring's arrival by 30 days prompted a 54 percent increase in ragweed pollen production.

The danger with a lengthening season—and perhaps a more intense one—is pollen's potential to overwhelm immune systems that, up till now, have withstood the onslaught, Tringale said.

Much as water in a bathtub is not a problem until it starts to overflow, pollen for many is not an irritant until it crosses a particular threshold, he said.

"With the longer season, with the creeping breadth of the geographic footprint of the season, and with more powerful plants producing more pollen, it's a triple threat," he added. "Now you've got yourself a much wider population that could potentially be affected that might not have been affected before.'