Climate Change and Evolution of the Pollen Content of the Air in Seven European Countries: the Example of Birch

The aim of this paper is to study the relation between climate change and the earliness of Birch pollination, a tree that produces allergenic pollen, which is of particular importance in north-western Europe. The study allowed us to make projections for the next few years in order to evaluate the evolution of Birch pollination starting date according to spring temperatures. It has been done in ten European locations, Kevo and Turku (Finland), London (United Kingdom), Paris and Lyon (France), Brussels (Belgium), Zürich (Switzerland), Vienna (Austria), Poznan and Cracow (Poland). The study period ranged from six to twenty-eight years depending on the city. Pollens were recorded daily thanks to volumetric Hirst traps, and the data were compared to monthly temperatures from January to May using correlations and multiple regressions. Except in Kevo where the trend is towards colder springs and a later pollination, in the other cities the results showed a spring warming associated with an earlier starting date of pollination: this trend is very well marked in Turku, Zürich, London, Vienna and Brussels, less evident in France while there is no trend in Poland. The results of this work should lead to an updating of the pollen calendars and the pollen forecast models, in the cities where there is a pronounced trend.