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Hoodlum Weeds on a 'Fevered' Planet

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The pollens released into the air by flowering plants and trees that cause hay fever and provoke asthma attacks are changing in response to altered climate patterns. Pollen seasons now start up to a month earlier and last weeks longer because of warmer temperatures. Hotter weather coupled with higher concentrations of carbon dioxide in the air prompt allergens like ragweed to grow larger and produce more pollen. What's even more disturbing is that the carbon-enriched pollen is more noxious and damaging because it contains more of the chemicals that cause allergic reactions, according to studies done by a U.S. Department of Agriculture research team led by Lewis Ziska.

The plant physiologist has spent much of his career looking at how climate change will affect one of humanity's more fearsome enemies: weeds. At his lab at the USDA campus in suburban Maryland, Ziska wanted to know how their growth would be affected by the increased concentrations of atmospheric carbon dioxide and higher temperatures that climate models are projecting in the next 50 years. However, with severe budget cuts, erecting the costly facilities to create airtight, climate-controlled growth chambers, which are essentially aluminum boxes about the size of a meat locker, was out of the question.

But he hit upon a novel solution that capitalized on the intense heat-island effect of neighboring Baltimore, where pavement, asphalt and buildings radiated the solar energy they absorbed and exhaust fumes from cars and big rigs and ships in the harbor clogged the air with smog. Downtown temperatures registered about 3 to 4 degrees warmer than in the suburbs, and the CO2 concentrations were 440 to 450 p.p.m (parts per million), which was considerably higher than the 399 p.p.m. of 2013 but almost precisely mirrored the conditions that moderate climate-change scenarios were predicting for the planet as a whole by the middle of this century. "We were looking for a cheap way of getting at future climate," said Ziska. "We figured the urban area might be a pretty good surrogate for that."

Ziska planted weeds on a barren patch of abandoned land next to the city's inner harbor to see how plants would fare in the hotter, CO2-enriched environment. For comparison, he planted weeds at two other sites: at an organic farm in western Maryland, where conditions approximated the current climate, and in a park in a suburb of Baltimore, which represented circumstances between these two extremes. The seeds and the soil composition of plant beds were the same so that the only variables were CO2 concentrations and heat.

Turbocharged Ragweed


Over the next five growing seasons, the downtown weeds -- turbocharged by the warmer temperatures and higher levels of carbon dioxide -- behaved like they were on steroids, soaring up to 20 feet, quadruple the size of their brethren on the farm. Even worse, the faster-growing, CO2-enhanced weeds seemed to change their chemical makeup, so not only did they generate four times the pollen count of their rural cousins, but what they produced was more allergenic and contained more of the protein responsible for inciting allergic reactions. While carbon dioxide is a basic greenhouse gas, it's also a food source for plants, which convert it into sugars and carbohydrates. But not all plants respond the same way, and noxious weeds (as well as vines like poison ivy or kudzu) respond much more strongly to the change in CO2 than other types of plants do, Ziska said. As a consequence, we see not only more growth but more virulent chemicals within the plants.

A more recent 2011 study that Ziska collaborated on with 20 other mostly U.S.-based researchers had even more bad news for the nation's 50 million hay-fever sufferers. In an analysis of U.S. and Canadian data on ragweed and daily temperatures at 10 different latitudes, they found that pollen seasons were 27 days longer than in 1995 in the northernmost areas, where temperatures are rising the fastest in response to climate change, and the lengthening of the pollen season was directly correlated to the hotter weather because the frost is melting earlier in the year.

Putting all these pieces together -- longer pollen seasons and more powerful weeds producing more noxious pollen -- means that climbing temperatures, increased smog and record pollen counts create a combustible mix that is sparking an explosion of new allergy and asthma patients. "When we look at climate change, we think of it as this esoteric thing that will happen to our grandkids, but the fact is this is already occurring," said Ziska, who is deeply frustrated that so few are paying attention to these ecosystem changes. "It's like seeing the train coming and not being able to speak English to warn people."


This post, an adapted excerpt from Linda Marsa's new book, Fevered (Rodale Aug/2013), is part of "The Science of Extinction and Survival, A PLOS BLOGS Conversation on Climate Change." For more, visit PLOS BLOGS Network.