Climate Change Is Exacerbating North American Pollen Seasons

By Scott Baltic

March 09, 2021

NEW YORK (Reuters Health) - Over the past three decades, anthropogenic climate change has contributed to making pollen seasons in North America both longer and more severe, researchers in the U.S. report.

Across the continent, pollen seasons advanced by 20 days and lengthened by eight days on average, and pollen concentrations rose by 21%, the team reports in PNAS, noting that the trends were "strongly coupled to observed warming."

They estimate that human forcing of the climate system contributed about half of the trend in pollen seasons and roughly 8% of the trend in pollen concentrations.

"Our results indicate that human-caused climate change has already worsened North American pollen seasons, and climate-driven pollen trends are likely to further exacerbate respiratory health impacts in coming decades," Dr. William R. L. Anderegg of the School of Biological Sciences at the University of Utah, in Salt Lake City, and colleagues write.

The study is the first to demonstrate a correlation between warmer temperatures and increasing pollen levels on a continental scale and to formally attribute this to human-caused climate change, Dr. Anderegg told Reuters Health by email.

The link between a warming climate and worsening pollen seasons, he and his colleagues say, lies in the fact that "pollen concentrations are often highly temperature-sensitive."

Using long-term pollen records from 60 North American cities, the study found significant trends over time in several common pollen metrics, including daily pollen extremes, pollen season start date and length, and seasonal and annual total pollen load. Overall, these trends "revealed a substantial intensification of pollen seasons in North America" between 1990 and 2018.

The largest and most consistent increases in annual and spring pollen loads were seen in Texas and the U.S. Midwest. Tree pollen (as opposed to grass or weed pollen) showed the largest increases.

The authors note that in addition to the known health consequences of high pollen loads - such as allergies, asthma, hampered school performance and increased emergency-department visits - a European study published in 2020 indicated that pollen can suppress innate antiviral immunity, whether or not an individual is allergic to pollen.

A variety of factors that the study did not focus on may also be involved in the exacerbation of pollen seasons, said Dr. Anderegg, including drought, land use and larger ecological changes, such as spread of invasive species or shifting vegetation patterns.

SOURCE: PNAS, online February 16, 2021.