Climate change has made allergy season longer and more intense, researchers find

By the end of the century, allergy season could start 40 days earlier and last 19 days longer due to climate change — potentially increasing the pollen count by 250% and making allergies far more intense, new research shows.

According to a study published in Nature Communications last week, as temperatures get hotter, trees, grasses, and weeds are responding by putting out more pollen earlier in the year.

And the correlation between climate change and earlier allergy seasons aligns with what researchers in Nevada are seeing in their own monitoring.

Warmer weather earlier in the year signals trees and other plants to begin flowering, meaning plants could pollinate much earlier and for a longer period of time.

Climate scientists at the University of Michigan Department of Climate and Space Sciences and Engineering who authored the study used a predictive model to measure how much pollen levels would likely increase due to climate change over the next 50 years.

Studies linking warming temperatures and worsening allergies have been published in the past, however, UM researchers note that their research also breaks down the individual types of pollen and tree sources by region, analyzing a variety of plant sources across the U.S. including oak, cedar or ragweed.

“Pollen emission is related to plant types growing in different regions. Pine trees are big pollen emitters in Nevada regions, Oak is also emitting a lot of pollen and their pollen emission will increase with climate change,” Yingxiao Zhang, the study’s lead author.

Fellow author Allison Steiner said the modeling developed by their team could eventually allow for allergy season predictions targeted to different geographical regions.

“We’re hoping to include our pollen emissions model within a national air quality forecasting system to provide improved and climate-sensitive forecasts to the public,” Steiner said.

The consequence of climate change on pollen and corresponding health effects has long concerned health researchers. One study found that between 1995 and 2011, warmer temperatures in the United States caused the pollen season to be 11 to 27 days longer. The study warned that longer pollen seasons could trigger more severe asthma attacks among the 10 million Americans with allergic asthma.

In the future, different tree pollen varieties that once varied in timing will eventually overlap with each other, leading to overall higher concentrations that threaten public health, researchers found. Pollen has been shown to exacerbate asthma symptoms in children and adults. Children with asthma will be exposed to pollen for a longer time and with more intense allergy seasons, said researchers. 

“Considering children are more sensitive to pollen than adults, higher pollen concentration will lead to more severe symptoms for children,” said Zhang.

Mulberry madness

“We now have longer periods of higher temperatures, so yes allergies can come earlier,” said Asma Tahir, director of the pollen monitoring program at UNLV. “We have noticed the summer time is longer, spring is shorter and it’s creeping back .”

However, she notes the pollen monitoring program only has records for the last six to seven years. Every year is different, said Tahir, while some years do have early starts to allergy season others don’t. 

The timing of when tree pollen is released plays a role in how long allergens will affect Nevadans, she said. For example, in Nevada ash trees typically pollinate first in January, then mulberry or pine in March, followed by other species over the course of a few months.

This year in Las Vegas, allergy inducing mulberry pollen started in early March, the typical start date from year to year, said Tahir.

Wind-driven pollen, which plays an important role in plant fertilization, is also closely tied to temperature and precipitation changes, say researchers. Strong winds brought on by high-pressure patterns in the Southwest can boost pollen’s reach.

“Every place is different,” said Tahir. “In Las Vegas the difference that I’ve seen that is impacting the duration is also the wind, not just the temperature.”

Tahir said in the past two weeks mulberry trees have started blooming and while some days have low pollen counts other days have extremely high counts, all depending on how windy the day is.

“It’s interesting and kind of strange that one day earlier we were only counting 35 mulberry pollen on one slide and on the next day were counting 1,200 on one slide,” Tahir said. 

Lack of precipitation also affects the timing and duration of pollen season in Southern Nevada, said Tahir. Rain keeps pollen down and prevents it from being carried by wind, but with drier soil pollen becomes a more frequent issue. 

Likewise when snow makes it to the valley floor plants fail to pollinate, pushing allergy season later into the year, Tahir said.

In 2019 Las Vegas received about a half-inch of snow, but that thin patch managed to push spring pollen back to late March resulting in a shorter allergy season, said Tahir. But as temperatures rise snow in the valley will become even more infrequent.

Currently, the southwest is facing a 1,200 year long megadrought, and those conditions could increase ragweed pollen emissions in Nevada, said Zhang.

In Southern Nevada, a warmer autumn can also make ragweed, a major fall allergen, release pollen up to 19 days longer. The study noted that as temperatures get warmer and drought plagues the Southwest, pollen from plants like ragweed are projected to be higher across the region.

In 1991, Nevada officials decided to ban high pollen producing male mulberry trees and fruit bearing olive trees, in an effort to alleviate allergies.

“Most of these trees are not native trees. They were brought here in the early 60’s when the city was developing for shade,” Tahir said. “In areas of the city that were developed in the 60s and 70s, that’s where mulberry and olive trees are.”

Tahir noted that allergy inducing trees, like mulberry and olive trees, are more present in older parts of Las Vegas, where lower income families tend to live. Newer housing divisions built after 1991. like Summerlin, do not face the same swell of allergens. 

The program tests near two schools located in the older part of the city, Sunrise Acres Elementary School on North Eastern and East Charleston, and Jerome Mack Middle School on South Lamb Boulevard and East Sahara Avenue. Both are Title 1 schools and both are surrounded by mulberry and olive trees and produce high pollen counts, said Tahir.

Allergen testing near Palo Verde High School in Summerlin on the other hand shows very limited pollen counts.

“The number is very different from what I will see in the older part of Las Vegas as compared to newer divisions because there are no mulberry and olive trees,” Tahir said.