Wisconsin weather: Allergies worsen with warming climate, here's why

MILWAUKEE - If you feel like you're sniffling and sneezing more than usual, it may be because allergy seasons are not only getting longer, they're also getting more intense. 

Fall allergies

Big picture view:

According to Climate Central, there are three distinct plant-based allergy seasons in North America: 

• Spring: Tree pollen
• Early summer: Grass pollen
• Late summer and fall: Grass and weed pollen

Ragweed is the main cause of fall allergies and can be found in most states, thriving in both rural and urban areas. A 2003 study suggests that the urban heat island effect can even help ragweed grow faster and produce more pollen in cities.

Warmer climate, more pollen

Why you should care:

Seasonal allergies can already last from early spring through late fall, but as the climate warms, there are more freeze-free days. Fewer freeze-free days means plants have more time to grow and release pollen.

"Allergy seasons are not only getting longer on the front end and the back end, but they're also getting more intense," said Climate Central's Shel Winkley. "The same thing that is causing us to have more warmth is also feeding the plants, which means that more (pollen) is going into your face and then more is coming, grossly, out of your face."

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Longer annual growing seasons and pollen seasons are due both to an earlier spring warm-up and a later fall cool-down.

"It's just a reminder that cold has a place in this world. It definitely has a place for our plants," Winkley said. "This is one of those negative impacts of seeing more warmer days than experiencing the cold that we expect to experience."

By the numbers:

A recent Climate Central analysis found the annual freeze-free growing season lengthened in 87% of U.S. cities analyzed, extending 21 days longer on average from 1970 to 2025. Peer-reviewed studies have found that human-caused warming has already led North American pollen seasons to lengthen by 20 days on average from 1990 to 2018.

"We now experience about a week less of freezing temperatures in the City of Milwaukee," said Winkley. "That plays into that signal for the plants to say, OK, it's time for a night-night. It's time to stop producing all this junk in the air, and let's go dormant for a couple of months."

What about mold?

Dig deeper:

Mold can cause fall allergies, too. Molds grow on soil and dead plants, releasing tiny spores throughout the year but tending to peak in late summer and fall.

For people who have both pollen and mold allergies, this means that allergies can last for much of the year. Although outdoor mold isn’t as well-studied as pollen, climate change is likely affecting how both allergens impact people with allergies and asthma.