Climate change has already worsened pollen seasons in the United States and Canada, a new study of nearly three decades of data has revealed. Pollen seasons are getting longer and more intense.
The amount of time that people in North America are exposed to pollen, as well as the amount of pollen, has “increased significantly” in the past few decades, according to the authors.
The researchers analyzed different pollen metrics between 1990 and 2018 from 60 monitoring stations in North America. Pollen seasons started 20 days earlier and lasted up to eight days longer, the scientists found.
There was also an increase in pollen count or concentration, with an increase of 20.9% between 1990 and 2018 – and an increase of 21.5% only in the spring.
“It is a clear example that climate change is here and now.”
Regional differences
The biggest and most consistent increases occurred in Texas and the midwestern United States, the study found – something that surprised Anderegg. He expected to see a greater increase in pollen in more northern states.
He said the reason for this is not “entirely clear and will require more research to unravel. One hypothesis may be that the plant species that are found there are particularly sensitive to heating and the production of more pollen”.
The study looked at different factors for this change, including changes in temperature, precipitation, frost days and carbon dioxide concentrations, and found that an increase in average annual temperatures was the strongest factor.
Using computer models of the Earth’s climate, the researchers also calculated the extent to which man-made climate change increased pollen concentrations and made pollen seasons longer.
“These computer models simulate a world without man-made climate change and a world with man-made climate change (that is, the real world),” Anderegg said in an email.
“By combining the observed connection between pollen and temperature with these two different scenarios, we can estimate how much human-caused climate change is influencing pollen trends.”
Longer pollen seasons
The researchers concluded that it was a “strong driver” in terms of early pollen start and longer season durations. However, it was a more modest motivator when it came to higher concentrations of pollen.
The contribution of man-made climate change was greater during the period from 2003 to 2018 compared to the longer period from 1990 to 2018, scientists said, which probably reflects the cumulative effect of climate change and a greater number of pollen monitoring during the shortest period.
The researchers also noted that the human environmental impact was more modest when looking at the figures for an entire year compared to the spring season, with some decreases in the pollen count in the summer, indicating that the life cycles of some plant species have changed.