Amid all the fantastic aspects of spring, tree pollen is the microscopic bully that often makes life miserable for allergy sufferers. This year, abnormally warm weather has boosted average tree pollen counts to near-record levels in the D.C. area.
Spring has awakened unusually early this year, with Washington’s average temperature the sixth-warmest on record year to date. Plant and tree life, including the famed cherry blossoms at the Tidal Basin, are speeding into spring mode.
Spring’s early arrival and the fast start to allergy season have become common in recent years because of rising temperatures driven by human-caused climate change.
Sneezin’ season
The first tree pollen grains of 2024 were counted less than 10 days into the year. By late January, counts were above average and climbing. Other than a slowdown or two during short-lived cold spells, pollen season has roared to life in late winter — a common theme in recent years.
Moderate tree pollen levels were first observed Feb. 8, about a week earlier than the norm; Feb. 22 marked the first high-pollen day. A pollen count of at least 15 grains per cubic meter of air is considered moderate; high starts at 90 grains.
During the fourth week of February, the average count was 522 grains compared with an average of 75, Susan Kosisky, director of aerobiological reporting and evaluation at the U.S. Army Centralized Allergen Extract Lab, said in an email.
February’s final week and the first week of March both had the second-highest average counts on record, Kosisky said.
There has yet to be a count over 1,500 grains this year, which is considered “very high,” but the average first date of this — on April 8 — has been moving up in recent years. In every year since 2020, it has occurred before this date.
Last year, the first very high count occurred Feb. 23 — the earliest on record — while it was March 6 the year before.
Why so soon?
Most of this early blooming is tied to temperature, especially in winter and early spring. It was so mild this year that some tree species did not go fully dormant as they are intended.
This winter season saw the fewest overnight freezes on record, while every month from December to February was 2 to 4 degrees warmer than normal. In late January, the temperature surged to a record 80 degrees, and February saw highs in the 60s on five days.
“A string of very warm days (60s to 70s especially) will cause the trees to stir and release their pollen as early as mid-January in our region,” Kosisky said.
Washington’s average temperature has risen about 0.3 degrees per decade since 1900 during the winter and only slightly less quickly in March.
Both the rise in temperatures and increasing levels of carbon dioxide in the atmosphere from the burning of fossil fuels can increase pollen levels. A study published in 2020 found that pollen season has expanded by several weeks and increased by double-digit percentages since the late 1900s throughout North America.
Plenty more pollen to come
The average date for peak tree pollen in the Washington region is April 14, or still about a month off.
Last year, despite a very warm winter and a major pollen spike in late February, the peak occurred April 13; different tree species release their pollen at different times, and there can be multiple peaks some years. From 2020 to 2022, the peak had already happened by now. When there’s a peak early in the season, sometimes there’s less pollen later on, as trees only have so much to expel.
“Generally, if we have a high production of tree pollen in March, April will indeed be lower,” Kosisky said.
After a major explosion of pollen during the warmest March on record in 2012, counts remained on the lower side for the rest of the season, Kosisky said.
The average peak value is 2,516 grains, or about twice as high as this year’s peak so far. Last year, the value topped out at 3,319 grains. The peak has surpassed 4,000 twice in the past two decades; it reached 4,539 grains April 25, 2009, and 4,147 grains April 6, 2010.
It’s too soon to know whether the fast start to this year and a possible early peak will mean a tamer second half of the season — typically tree pollen tends to fade by early May.
“Forecasting future pollen counts for the season is a challenge due to the variability from year to year (and week to week) in temperatures, relative humidity and precipitation,” Kosisky said. “Tree species may also have inherent cycling patterns where there are on and off years for higher amounts of pollen that are released.”
