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Ragweed is invading Europe, and climate change will make it worse

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Researchers warn of growing health risks as invasive species creeps across the continent

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Ragweed pollen has made life miserable for allergy sufferers in Europe, and it's likely going to get much worse over the coming years, according to a study published today. By 2050, airborne pollen loads from the invasive species are projected to increase by a factor of four — largely due to climate change — pointing to what the paper's authors describe as an "urgent" need for action.

The study, published in the journal Nature Climate Change, combines two models to predict the effects of climate change, land use, and seed dispersal on ragweed pollen levels across Europe. Using data on ragweed distribution collected from 1986 to 2005, the models show that pollen concentrations will increase significantly across Europe by 2050, with one-third of the increase due to seed dispersal, and the remaining two-thirds due to changes in climate and land use that will create more habitable terrain. Pollen concentrations are projected to increase substantially in north-central Europe, northern France, and the southern UK, where levels are currently negligible. In areas where pollen levels are already high, concentrations are projected to increase by up to a factor of two.

A troubling outlook for allergy sufferers

Common ragweed (Ambrosia artemisiifolia) is a resilient, flowering plant native to North America, but it has spread across parts of Asia, South America, and much of Europe, where it was first introduced in the late 19th century. Since the 1940s, the highly allergenic plant has become entrenched across much of central and eastern Europe, and its pollen can travel long distances, causing itchy eyes, runny noses, and respiratory problems in susceptible individuals. It has also been shown to impact agriculture, with crop yields in some cases declining by more than 50 percent. A 2013 report from the EU estimated the combined economic losses from agriculture, work productivity, and health costs at €4.5 billion (nearly $5 billion).

The findings published today paint a grim picture for the coming decades. Previous studies have suggested that climate change will allow ragweed to spread further north under warmer temperatures, while others modeled the spread of its pollen. But the paper published today is the first to use an integrated model to predict how its pollen could spread over the coming years under different climate scenarios, accounting for seed dispersal, pollen production, and pollen spread driven by wind.

Ambrosia artemisiifolia L. plant at the flowering growth stage. (Inra-Dijon)

"That's one of the really new things — that they've been able to take all these things into account," says Carsten Skjøth, an aerobiologist at the University of Worcester, who was not involved in today's study. "Many of these previous climate change studies don't really take into account changes in local vegetation, because it's difficult. But they have done this."

The Balkans, Ukraine, and northern Italy could see pollen counts reach as high as 22,000 grains per cubic meter, according to the models. Today, counts in these regions can reach as high as 13,000 grains per cubic meter, but even counts of 2,000 grains are considered high for people susceptible to allergies. The rate at which seeds disperse will influence the magnitude of this increase over time, but the authors note that even if ragweed distribution were to remain unchanged, climate change would increase pollen loads across much of the continent. They speculate that this is likely due to rising CO2 levels that facilitate pollen release and drier conditions that are more conducive to airborne pollen transport.

Daniel Chapman, a plant community ecologist at the Center for Ecology and Hydrology in Edinburgh, says that the projections should be taken with "a pinch of salt" because data on the distribution of ragweed plants across Europe is imprecise, and that the calibrations the authors used to account for this may not hold true over the coming years. But he also points to observed changes in the plant's biology that may also contribute to its spread into new areas.

"It's spreading all over the world. It's not just a European problem."

"We know that ragweed is rapidly evolving as it's spreading through Europe," says Chapman, who was not involved in the study. "Because of this rapid evolution, ragweed won't necessarily behave the same way in the future as it does now. It can behave differently in different places and adapt to local conditions, and it's very challenging to incorporate that into the model."

The study's authors acknowledge that the uncertainties surrounding the distribution of ragweed plants and the rate at which seeds may disperse, but experts agree that their overall predictions — increased pollen concentrations and newly invaded areas — are likely to hold true. Mitigating this scenario presents more difficult challenges. The plant is very resilient, often growing back after it's been removed, and its seeds and pollen can survive for decades. It also tends to grow along roadsides, where removal can be difficult, and treating it with traditional herbicides can impact nearby flora and fauna. And although the European Union has taken measures to limit its spread, experts say stronger action is needed — both within Europe, and beyond its borders.

"It is urgent to put in place European coordinated and integrated management programs," including "eradication programs and public awareness campaigns," says Lynda Hamaoui-Laguel, a researcher at the Laboratoire des Sciences du Climat et de l'Environnement (LSCE) in France and one of the study's authors.

"It's spreading all over the world," Skjøth adds. "It's not just a European problem."