RICHMOND — People are releasing long-buried carbon dioxide by burning fuels such as coal and oil. ∙ That’s bad for much of the planet, scientists say, helping cause global warming. But it’s great for ragweed.
Plants like carbon dioxide, or CO2. It’s their food. But ragweed, the bane of many fall allergy sufferers, really likes it.
Exposed to elevated levels of CO2, ragweed grows faster than most other plants in the eastern U.S., said Manuel Lerdau, a University of Virginia biologist.
Also, it appears that CO2 helps ragweed produce chemicals that ward off insects that normally eat the troublemaking weeds, Lerdau said.
“We are just going to have more ragweed,” he said. “It’s just going to do better and better in a higher-CO2 world.”
Ragweed pollen is one of the most serious causes of seasonal allergies. The peak ragweed season is late summer and early fall — that is, now.
A bespectacled man with dark hair graying at the temples, Lerdau studies the relationship between ragweed and carbon dioxide.
As CO2 levels rise, Lerdau said, “ragweed will produce more pollen — which is bad for us, but good for the ragweed. And also the ragweed plants themselves will grow bigger and thus produce more pollen. And they’ll get eaten less, and thus they’ll be able to produce more pollen.”
Lerdau said there is strong evidence that ragweed already is responding to rising CO2 levels. “I would say it’s beyond a reasonable doubt but not certain.”
In addition, nitrogen pollution that spews from cars’ tailpipes and other sources falls to the ground as a sort of fertilizer that ragweed loves, Lerdau said. “People are doing everything they can do to help ragweed.”
For many plants, more carbon dioxide means more pollen, said Chris Gough, a Virginia Commonwealth University forest ecologist.
“There have been a number of studies that demonstrate if you put a plant in an artificially enriched carbon dioxide environment, pollen production increases,” Gough said.
Much of the research has centered on ragweed and loblolly pines, both of which are common in Central Virginia, Gough said.
Carbon dioxide concentrations in the atmosphere have risen more than 40 percent from the early stages of the industrial era in the mid-1800s to about 400 parts per million today, Gough said.
Concentrations could approach 1,000 parts per million by the end of this century, Gough said.
Heather Throop, a New Mexico State University biologist, said, “For people with serious allergies or asthma, [ragweed pollen] is quite a serious threat, and a major annoyance to a lot of people.”
As our climate warms, Throop said, “you potentially could have a much longer allergy season.”
Lerdau pointed out lush stands of common ragweed during a recent walk near the Rivanna River. Some plants were more than 5 feet tall, and they sported tiny yellow flowers that the average passer-by wouldn’t notice.
“It’s a very unimpressive flower,” Lerdau said.
Ragweed may get its name from its rough, serrated leaves, or possibly from the coarse texture of the leaves.
As Lerdau examined a ragweed plant, it dropped a yellow powder on his hands. “That’s hundreds or thousands of pollen grains on my finger. Each one is too small to see.”
Another plant, goldenrod, sports showy yellow flowers this time of year, and many people falsely accuse it of causing fall allergy agony.
Ragweed pollen is spread by wind, so it is light enough to blow away — and into your eyes and nose. Goldenrod pollen, however, is heavier and sticks to flowers while awaiting insects like bees and butterflies to spread it.
“If you want an animal to pollinate you, you don’t want your pollen to blow away,” Lerdau said.
Ragweed is common across most of North America. It is an invasive species in Europe and Asia.
While rising carbon dioxide levels can make ragweed more troublesome, won’t more of the gas be good for plants such as farm crops?
It’s not that simple, because higher CO2 levels will be accompanied by rising temperatures as our climate changes, Lerdau said.
If you are in the southern part of the U.S. wheat range — Kansas or perhaps farther south — “you may well lose wheat as a viable crop. But if you are in Saskatchewan, you are pretty happy about this.”
Also, warming winter temperatures are allowing insects that prey on corn to move north, Lerdau said.
As he poked about in the ragweed, Lerdau sniffled badly.
“I’m very allergic,” he said. “I’m going home to wash my hands very carefully.”