Spring seemed to come early in much of the United States this year, and nearly everyone with seasonal allergies noticed. As climate change causes winters to be warmer and plants begin blooming earlier, studies have shown that the pollen that causes allergy symptoms has been arriving earlier than in past decades.
Here’s a breakdown of how warmer winters are leading to a longer plant-growing season and how it’s affecting millions of Americans with hay fever.
A warm, wet winter in the eastern United States
“February continued the unusually mild start to 2023, with much of the eastern U.S. seeing record or near-record warm temperatures,” according to the National Oceanic and Atmospheric Administration (NOAA).
“The average temperature across the contiguous U.S. last month was 36.5 degrees F, 2.7 degrees above the 20th-century average, ranking in the warmest third of the 129-year climate record,” the agency said in a report released last week.
Virginia had its warmest February on record. Eight other states east of the Mississippi River had their second-warmest February ever, and three had their third-warmest.
However, there was an exception: Six western states — Arizona, California, Colorado, Idaho, Nevada, and Oregon — experienced colder-than-normal February temperatures this year.
Although there is still another week of winter on the astronomical calendar, which is based on the Earth’s position in relation to the Sun, “meteorological winter” refers to the coldest three months of the year, and it is considered December-February.
NOAA reported last week that the meteorological winter’s average temperature was 2.7 degrees warmer than the 20th Century average, making it the 17th warmest meteorological winter on record. Massachusetts had its warmest winter ever, and seven states in the Northeast, Appalachia and the Upper Midwest had their second-warmest winter on-record. Another 21 states had one of their 10 warmest winters.
Total average winter precipitation for the December-February meteorological winter of 2022-2023 has so far been 0.90 of an inch above the historical average. Wisconsin had its wettest winter ever, and another six states had one of their 10 wettest winters. Studies have found that more rain during a plant’s growing season leads to earlier and faster growth.
An early bloom in the eastern half of the country
“Observers are reporting very early leaf outs of the common lilac in Pennsylvania, the brilliant yellow blossoms of forsythia in Maine and American witch hazel in New York,” USA Today reported late last week. A study of records from over 140 years at the Missouri Botanical Garden found that violets are responding to increased rainfall and higher temperatures by budding earlier, the newspaper reported.
“I’m sitting outside on March 7 and all my daffodils are blooming, and it’s ridiculous,” Lois Krauss, a local environmental activist in Westfield, N.J., told Yahoo News.
The National Phenology Network, which tracks the arrival of spring by following the blooming of plant species that are common nationwide and typically among the first to sprout leaves, such as honeysuckles and lilacs, reported in late February that leaves were sprouting the earliest they ever had in parts of the eastern U.S. In New York City, buds appeared 32 days before the historical average.
“Spring leaf out continues to spread north, arriving several days to weeks earlier than average (the period of 1991-2020) in much of the Southeast, lower Midwest, and mid-Atlantic. Kansas City, MO is 9 days early, Nantucket, MA is 35 days early,” the National Phenology Network reported on Monday. The group added that “spring bloom has also arrived in southern states, days to weeks early in the Southeast,” including 22 days early in Norfolk, Va.
“It’s a little unsettling, it’s certainly something that is out of the bounds of when we’d normally expect spring,” Teresa Crimmins, the National Phenology Network’s director and an environmental scientist at the University of Arizona, told the Guardian, regarding the early blooms. “It perhaps isn’t surprising, given the trajectory our planet is on, but it is surprising when you live through it.”
However, this is not uniformly true nationwide. “The West is a mix of early and late,” the group noted. “Southwest UT is days to over a week late and Portland, OR is 2 days late… Spring bloom is 10 days late in Las Vegas, NV.”
A new report from Climate Central, a climate research nonprofit, analyzed temperature data for 203 U.S. cities since 1970, to measure the length of the plant-growing season — the stretch between the last freeze in or before spring and the first of the following fall or winter. The group found 85% of the cities have longer growing seasons than they did in 1970. On average, the freeze-free season grew the most in the West, 27 days, followed by the Southeast (16 days), Northeast (15 days), South (14 days), and Central U.S. (13 days).
“Because of climate change, we’re now seeing an earlier and longer growing season for plants, which of course make pollen, which is the enemy of many Americans that suffer from pollen allergies – and mold allergies as well,” Lauren Casey, a meteorologist with Climate Central, told CNN. “Pollen can also trigger an asthma attack, which of course is much more serious for people that suffer from asthma.”
Warmer weather, heavier rains and early blooms are consistent with climate change
Although weather will always vary from year to year, temperatures have been consistently rising, by an average of 0.14 degrees Fahrenheit since 1880 per decade, due to climate change, according to the federal government. Average global temperatures have risen 2 degrees Fahrenheit since the Industrial Revolution, as a result of increased concentrations of greenhouse gases such as carbon dioxide in the atmosphere.
“Winter is getting warmer,” Matthew Barlow, a professor of climate science at the University of Massachusetts Lowell, told Yahoo News. As a result, the duration of cold weather is getting shorter on both ends.
Climate change is also leading to more precipitation. “As average temperatures at the Earth’s surface rise more evaporation occurs, which, in turn, increases overall precipitation,” the Environmental Protection Agency (EPA) explains. “Therefore, a warming climate is expected to increase precipitation in many areas. Since the beginning of the 20th Century, precipitation in the contiguous 48 states has increased at an average rate of 0.20 inches per decade, according to the EPA.
It’s not just climate change contributing to the early sprouting. The carbon dioxide emissions that cause climate change also spur earlier and faster plant growth. Since plants take in carbon dioxide when performing photosynthesis, higher atmospheric concentrations of carbon dioxide rev up that process.
“Carbon dioxide (CO2), in addition to being the principal global warming gas, can also be thought of as plant food — it’s the source of carbon needed to make sugars during photosynthesis,” explained a 2016 research paper in the journal Environmental Health Perspectives. “When exposed to warmer temperatures and higher levels of CO2, plants grow more vigorously and produce more pollen than they otherwise would.”
Earlier, faster plant growth causes an earlier, more severe allergy season
Seasonal allergies, also known as “hay fever,” are caused by allergic reactions to plant pollen and mold spores in the air. As higher concentrations of CO2, higher temperatures and heavier precipitation contribute to earlier and faster plant growth, allergy season is extended and exacerbated.
For example, Atlanta “saw pollen counts rise to ‘extremely high’ levels on March 6, the earliest in 30 years,” Forbes recently reported.
The Washington Post reported in mid-February that “unusually high winter temperatures have resulted in a historically early and intense tree pollen explosion,” in the nation’s capital. D.C.’s first high tree pollen count came on Feb. 8, marking the third-earliest recorded high pollen count, and the third such occurrence since 2017.
A number of studies in recent years have identified longer and more intense pollen seasons as a consequence of climate change.
QA 2021 paper published in the Proceedings of the National Academy of Sciences between 1990 and 2018 from 60 pollen count stations across the United States and Canada, found a 21% increase in pollen concentrations and a 10-day longer annual average pollen season over the course of the 38-year period.
“Trend data suggest that the prevalence of asthma, including forms of the disease triggered by pollen, mold, and other allergenic substances, is on the rise,” a 2016 paper in the journal Environmental Health Perspectives reported.
For many, allergies are just a minor nuisance, but for asthma sufferers it can be deadly, as allergies are a major cause of asthma attacks.
Eighty-one million Americans, including roughly 26% of adults and 19% of children, had an asthma diagnosis as of 2021, according to the Asthma and Allergy Foundation of America.
Although some treatments for allergies exist, including antihistamine medications and allergy shots, none are 100% effective and some are quite expensive or time-intensive. Ultimately, environmental and public health advocacy organizations argue for reducing greenhouse gas emissions to limit the severity of climate change.
"We're already experiencing the effects of climate change with every breath we take in the spring," William Anderegg, an associate professor of biology at the University of Utah, told NPR last year. "Acting on climate change really does matter for people's health."