Climate change has worsened pollen seasons in the United States and Canada, a new study of nearly three decades of data has revealed. The pollen seasons are getting longer and more intense.
The amount of people exposed to pollen in North America as well as the amount of pollen, according to the authors, has ‘increased significantly’ in recent decades.
The researchers looked at different pollen statistics between 1990 and 2018 from 60 monitoring stations in North America. Pollen times start up to 20 days earlier and last up to eight days longer, the scientists found.
There was also an increase in pollen count or concentration of pollen, with an increase of 20.9% between 1990 and 2018 – and an increase of 21.5% for the spring season alone.
“This is a clear example that climate change is here and now.”
Regional differences
The largest and most steady increases were in Texas and the Middle East, the study found – something that surprised Anderegg. He expected larger pollen to increase in more northern states.
He said the reason for this was “not entirely clear and that it will unravel more research. One hypothesis may be that the plant species found there are particularly sensitive to warming and more pollen.”
The study looked at various drivers for this change, including temperature changes, rainfall, 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 human climate change increased pollen concentrations and made pollen times longer.
“These computer models simulate a world without human-caused climate change and a human-caused world-changing world (ie the real world),” Anderegg said in an email.
“By combining the observed relationship between pollen and temperature with these two different scenarios, we can estimate how much human climate change affects the trends of pollen.”
Longer pollen seasons
The researchers concluded that it was a ‘strong driving force’ in terms of the earlier start of the pollen season and longer season lengths. However, it has been a modest driver when it comes to higher pollen concentrations.
The contribution of man-made climate change was stronger during the period 2003 to 2018 compared to the longer period from 1990 to 2018, the scientists said, probably reflecting both the cumulative effect of climate change and a greater number of pollen monitoring stations during the shorter period.
The researchers also noted that the human environmental impact was modest when we look at figures for a whole year compared to the spring season, with some declines in summer pollen numbers suggesting that the life cycles of some plant species have changed.