A new report explores the sneezier and wheezier side of global warming.

PHOTO: ALLAN FOSTER

On top of economic losses of $14.5 billion, 4 million missed workdays, and $700 million in lost productivity each year, allergies make people (millions of them) feel miserable. And a new report out today says climate change could make pollen pack an even greater punch.

A new analysis by NRDC (disclosure) shows that 109 million Americans live in counties with high levels of both ragweed and ground-level ozone, increasing their risks for asthma and allergic attacks. Climate change is likely to exacerbate symptoms of both conditions, in part because of its relationship to ragweed, one of the great villains of the botanical world. Ragweed stimulates allergic reactions and can also narrow the airwaysof asthmatics, triggering wheezing symptoms.

Here are five ways that climate change and ragweed are conspiring against us. (Hat-tip to Colleen Reid of the University of California, Berkeley, and Janet Gamble of the U.S. Environmental Protection Agency, who published an excellent review of these issues in 2009.)

MORE POLLEN

Ragweed reproduces by releasing pollen-encased sperm cells into the wind. Inhaling pollen is, for starters, a little bit gross—you are basically caught in the middle of plant copulation. But it also triggers a range of deeply unpleasant symptoms. Ragweed pollencauses sneezing, coughing, itchiness, swelling, and oral tingling.

Climate change will significantly increase the amount of ragweed pollen in the air. In 2002 a team of University of Oklahoma researchers tested the effects of temperature on ragweed growth by cultivating two plots using an infrared heater on one of them. The heater increased air temperatures by about one degree Celsius, and the soil by slightly more than two degrees. Over a two-year period, the warmed ragweed produced 84 percent more pollen than the unheated control group. That’s enough to turn a low-pollen day into a high one in many American cities.

Even if you don’t believe that increased atmospheric carbon concentration will warm the atmosphere—and if you don’t, what’s wrong with you?—you should still worry about future pollen production. Since plants breathe carbon dioxide, higher concentrations of the gas boost plant growth. (The carbon dioxide “fertilization effect” is why many experts think climate change could briefly increase agricultural productivity in some regions.)

The increases of atmospheric carbon dioxide expected in the next few decades will cause ragweed pollen production to surge. Harvard researchers proved this by growing plants with a CO2concentration of 350 parts per million in the air (slightly lower than the current atmospheric level) and 700 ppm (the level expected in the late 21st century). Doubling the CO2 concentration increased ragweed pollen production by 61 percent. A series of other experiments produced similar results.

It’s important to note that these two effects—heat and atmospheric CO2levels—are independent of each other. The CO2 study held temperature constant, and the temperature study held CO2 constant. No one knows exactly how the two variables will work when combined, but it’s likely that they’ll amplify each other to some degree.

EARLIER POLLEN

Humans have long suspected that plants and animals can detect more subtle changes in weather than our blunted senses can. People think dogs behave differently just before a thunderstorm strikes, and Americans have the bizarre habit of asking a groundhog when spring will arrive. There’s limited evidence to support these bits of weather-based folk wisdom, but global warming has already caused measurable changes in the behavior of plants and animals. One of the clearest effects is the earlier blooming of wild plants.

In 2003 Stanford University biologist Terry Root published an influential article in the journal Nature on how wildlife are responding to rising temperatures over the long-term. By aggregating existing data, Root showed that trees have shifted forward their seasonal changes, such as spring blossoming, by an average of three days over the past 50 years. Other plants, such as grasses and weeds, are now emerging and blooming five days earlier than they did a half-century ago.

These effects are likely due to a combination of higher temperatures and carbon fertilization. A 2003 study showed that ragweed flowers earlier near cities, which have higher concentrations of carbon dioxide in the air and tend to be a few degrees warmer than rural environs, thanks to the heat island effect.

As climate change advances, the sneezing season will likely get earlier every year.

MORE POTENT POLLEN

Ragweed pollen contains a protein called Amb a 1, which is the chemical that causes your immune system to overreach. (There are other allergens in ragweed, but Amb a 1 is the primary culprit.) The more Amb a 1 you inhale, the more extreme your symptoms are likely to be.

A 2005 study showed that the expected rise in atmospheric CO2 levels would increase the concentration of Amb a 1 in ragweed pollen by 60 percent by the end of this century. Other studies have shown that the human immune system responds more dramatically to allergens in plants grown at higher temperatures, which suggests that global warming will intensify allergic reactions.

NEW POLLEN HABITATS

This one is a bit more speculative. Climate change is already altering the historic range of many plants (and the U.S. Department of Agriculture has been updating its growing zones accordingly). As global warming progresses, trees that prefer cool climates, such as spruce, will shift northward, while large swaths of the southeastern United States will transform from forest to grasslands. This habitat shift may not change the total number of ragweed plants growing in the States, but it could expose new people to the allergen.

Why does that matter? Because some researchers believe that newly introduced allergens can trigger more reactions in people than those that have been around a while. So a rise in new allergic responses would more than offset the relief experienced by communities that ragweed has left.

In the early 1980s, ragweed pollen moved into a Milanese suburb (for reasons having nothing to do with climate change). Researchers noticed a surge in allergic reactions, most notably among people who had no family history of allergies. New patients were also older than traditional first-time allergy sufferers, suggesting that the introduction of a foreign allergen had a unique power to afflict the previously unexposed population.

Doctors aren’t sure whether the climate-induced shift of ragweed would trigger an increase in allergies in the United States, but the evidence is worrying.

RAGWEED HURRICANES

As a kid, I loved to kick the seed heads off of dandelions—until my mother told me I was doing the weed a favor by spreading its seeds all over the lawn. Thunderstorms and hurricanes essentially do the same thing to ragweed: High winds blow the pollen all over the place.

Several studies, from England to New Orleans to Australia, have found that doctor visits for allergic reactions tend to increase in the aftermath of extreme weather. Will climate change exacerbate the problem? We don’t know. Climatologists expect more powerful hurricanes in the coming decades, but the winds associated with today’s hurricanes are plenty strong to spread ragweed pollen. Perhaps hurricanes with a wider span will subject more people to these effects, but the more important question is whether there will be more hurricanes—something climatologists argue about.

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See the list below for NRDC's ranking of where pollen and elevated ozone levels are teaming up to give asthma and allergy sufferers a good reason to stay indoors. For more explanation of the analysis, see the full report.

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