Many allergy sufferers dread the first warm days of spring, when the air fills with pollen from blooming flowers and trees. As the climate gets hotter, that season of dread is getting longer.

Hotter temperatures could dramatically worsen allergy season, according to new research, bringing on the spring bloom as many as 40 days sooner, if greenhouse gas emissions remain high. In the fall, weeds and grasses could keep releasing pollen up to 19 days later.

Rising temperatures will also cause some plants, such as oak and cedar, to release more pollen overall, meaning higher rates of allergy attacks and asthma. Around 30% of the world's people have pollen-related allergies.

"This is another unintended consequence of climate change that hasn't been explored that much," says Allison Steiner, a professor of atmospheric sciences at the University of Michigan and an author of the study. "It has a big impact on human health."

Springtime flowers are blooming sooner

Many trees and other plants use temperature as a signal, waiting for warmer spring days to time their bloom. In the fall, many weeds produce pollen until there's a winter chill. The tiny grains fill the air, some carried by insects and others simply wafting in the breeze to pollinate nearby flowers. For humans, it can mean allergy attacks, asthma and emergency room visits.

Steiner and her colleagues looked at a range of trees, grasses and weeds and calculated how hotter temperatures could affect them by the end of the century. They found the total amount of pollen could grow 16% to 40% under a scenario of high greenhouse gas emissions. Even if humans cut their emissions, the researchers still found that allergy season would get worse.

"Temperature plays a big role," Steiner says. "Trees and grasses and weeds are essentially responding to these climate changes and putting out more pollen."

That effect could be particularly bad in the Pacific Northwest, where alder trees are expected to bloom sooner. Later-season plants could also get an earlier start, which means they'd overlap more with other species, a major downside for people sensitive to multiple pollens. Northern states are expected to see the biggest changes in allergy season, because temperatures are rising faster there.

Some plants also could get a boost from higher levels of carbon dioxide, which acts like a fertilizer, causing plants to grow larger and release more pollen. Steiner says that effect is more uncertain, since there are limits to how much plants are affected by higher carbon dioxide.

Allergy season has already gotten worse

Other studies have shown that people with allergies already have something to complain about. In North America, pollen season became 20 days longer between 1990 and 2018, with pollen concentrations 20% higher, according to one study.

"We're already experiencing the effects of climate change with every breath we take in the spring," says William Anderegg, an associate professor of biology at the University of Utah. "Acting on climate change really does matter for people's health."

Seasonal allergies are more than just a nuisance. One study found the medical costs add up to more than $3 billion per year.

"Pollen has major health consequences for a huge number of people," Anderegg says. "Millions of children struggle with asthma that pollen can affect. And there are a lot of nonintuitive effects — things like worker productivity on the job. It can affect kids' learning in schools and their performance on tests."

Climate change could also cause unexpected allergies for some. As temperatures get hotter, plants are moving and growing in new locations. Ragweed is expected to migrate farther north as the environment becomes more suitable.

This means that not just the timing of allergy season will shift, but so too where it's happening.