Impacts of Climate change and seed dispersal on airborne ragweed pollen concentrations in Europe

International audienceCommon ragweed (Ambrosia artemisiifolia) is an invasive weed native to North America producing very allergenic pollen which causes serious health effects like rhinitis, asthma and atopic dermatitis. It was introduced in Europe since the mid-19th century and invaded large areas during the last few decades (Pannonian plain, Northern Italy and South-Eastern France). Furthermore, there is a high potential for ragweed spread in current suitable habitats and future changes in Climate and land use may increase the spread by altering the climatic niche determined by physiological thresholds or affecting cropping patterns. The rate of spread depends also on seed dispersal due to natural or anthropogenic processes and the efficiency of ragweed eradication policies. However, ragweed airborne pollen concentrations depend not only on plant infestation, but also on phenology, pollen production, release, dispersion and atmospheric transport. Here, we present the first integrated modelling framework, based on an explicit representation of plant phenology, pollen production, and release to the atmosphere, to assess future changes in airborne pollen concentration under scenarios of climate and land use changes and seed dispersal. Two model suites are implemented differing in the atmospheric processing and in the driving climate models. The CHIMERE suite uses the Chemistry- Transport Model CHIMERE model, forced by regional climate simulations from the WRF model downscaling of the IPSL-CM5A-MR model. The RegCM suite uses the RegCM4 regional climate model forced by global climate simulations from HadGEM CMIP5. We performed three types of simulations (50 km grid covering Europe), which are hind-cast (2000–2012), historical (1986–2005) and future (2041–2060) simulations. The hindcast simulations, forced by ERAInterim reanalysis, are performed to calibrate and evaluate the modelling chain. The historical simulations are carried out using calibrated ragweed density to serve as a reference simulation for the future. We considered two contrasting RCPs (Representative Concentration Pathways) climate change scenarios including a high-end (RCP 8.5) and moderate (RCP 4.5) climate change scenarios and three seed dispersal scenarios (reference, slow and rapid). We show that airborne pollen concentrations may drastically increase in 2050 by a factor of 4.5 under highend (RCP 8.5) and 4.0 under moderate (RCP 4.5) climate change scenarios. This upsurge is largely dependent on the seed dispersal rate, making this increase vary in a range of factors from 2 to 12 according to the range of formulated assumptions. We estimate that about one third of the projected increases of pollen concentration are due to the on-going seed dispersal within the present niche regardless of climate change. Climate change will be responsible of two thirds of the future pollen loads increase. It will extend the habitat suitability for ragweed in Northern and Eastern Europe and result in higher pollen concentrations in established ragweed areas mostly due to a larger primary production with increasing CO2. Therefore, future increase of airborne pollen concentrations will be caused by the combined effects of climate change and ragweed seed dispersal in current and future suitable areas. Our results indicate that controlling the current European ragweed invasion will become more difficult in the future as the environment will be more favourable for ragweed growth and spread, highlighting the need for the development of effective and regionally co-ordinated eradication programmes