Flowering phenology of herbarium specimens reveals century‐scale shifts in allergenic pollen seasonality

Societal Impact Statement Pollen allergies are a primary cause of seasonal allergic rhinitis, which affects over 500 million people globally, damaging quality of life and incurring significant costs in medical treatment and reduced work productivity. Many countries monitor pollen to inform allergy sufferers about seasonal pollen levels. In Norway, airborne pollen has been monitored for 45 years. We extend this timeline using museum plant specimen collections to estimate 150 years of Norwegian pollen seasonality. We found that in many areas, pollen seasons are starting earlier and lasting longer today than in the past, providing important context for interpreting pollen observations and improving forecasts. Summary Pollen allergies affect a large portion of the population, adversely impacting their health and quality of life. Climate change is expected to worsen both the incidence and severity of pollen allergies and to change the timing and length of pollen seasons. In Norway, there has been continuous monitoring of airborne pollen since 1980, but this does not fully capture the ‘allergy epidemics’ that have been reported since the late 19th century. We use airborne pollen monitoring data in combination with a long‐term record of angiosperm flowering phenology, estimated from digitised herbarium specimens using machine learning, to predict pollen seasonality going back 150 years. We find an advance in the start of the pollen season and an increase in season duration over this period across multiple allergenic pollen types and regions of Norway.